PaperMC/Spigot-Server-Patches/0375-Async-Chunk-Loading-and-Generation.patch

From 662563828cc23dfd798fb73fac09c21be18cce82 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 21 Jul 2018 16:55:04 -0400
Subject: [PATCH] Async Chunk Loading and Generation

This brings back parity to 1.12 and older versions in that any
chunk requested as part of the PlayerChunkMap can be loaded
asynchronously, since the chunk isn't needed "immediately".

The previous system used by CraftBukkit has been completely abandoned, as
mojang has put more concurrency checks into the process.

The new process is no longer lock free, but tries to maintain locks as
short as possible.

But with 1.13, we now have Chunk Conversions too. A main issue about
keeping just loading parity to 1.12 is that standard loads now
are treated as generation level events, to run the converter on
another thread.

However mojangs code was pretty bad here and doesn't actually provide
any concurrency...

Mojangs code is still not thread safe, and can only operate on
one world per thread safely, but this is still a major improvement
to get world generation off of the main thread for exploration.

This change brings Chunk Requests triggered by the Chunk Map to be
lazily loaded asynchronously.

Standard chunk loads can load in parallel across a shared executor.

However, chunk conversions and generations must only run one per world
at a time, so we have a single thread executor for those operations
per world, that all of those requests get scheduled to.

getChunkAt method is now thread safe, but has not been tested in
use by other threads for generations, but should be safe to do.

However, we are not encouraging plugins to go getting chunks async,
as while looking the chunk up may be safe, absolutely nothing about
reading or writing to the chunk will be safe, so plugins still
should not be touching chunks asynchronously!

diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
index 886e97fec5..454ac2c091 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -375,4 +375,57 @@ public class PaperConfig {
             }
         }
     }
+
+    public static boolean asyncChunks = false;
+    public static boolean asyncChunkGeneration = true;
+    public static boolean asyncChunkGenThreadPerWorld = true;
+    public static int asyncChunkLoadThreads = -1;
+    private static void asyncChunks() {
+        if (version < 15) {
+            boolean enabled = config.getBoolean("settings.async-chunks", true);
+            ConfigurationSection section = config.createSection("settings.async-chunks");
+            section.set("enable", enabled);
+            section.set("load-threads", -1);
+            section.set("generation", true);
+            section.set("thread-per-world-generation", true);
+        }
+
+        asyncChunks = getBoolean("settings.async-chunks.enable", true);
+        asyncChunkGeneration = getBoolean("settings.async-chunks.generation", true);
+        asyncChunkGenThreadPerWorld = getBoolean("settings.async-chunks.thread-per-world-generation", true);
+        asyncChunkLoadThreads = getInt("settings.async-chunks.load-threads", -1);
+        if (asyncChunkLoadThreads <= 0) {
+            asyncChunkLoadThreads = (int) Math.min(Integer.getInteger("paper.maxchunkthreads", 8), Runtime.getRuntime().availableProcessors() * 1.5);
+        }
+
+        // Let Shared Host set some limits
+        String sharedHostEnvGen = System.getenv("PAPER_ASYNC_CHUNKS_SHARED_HOST_GEN");
+        String sharedHostEnvLoad = System.getenv("PAPER_ASYNC_CHUNKS_SHARED_HOST_LOAD");
+        if ("1".equals(sharedHostEnvGen)) {
+            log("Async Chunks - Generation: Your host has requested to use a single thread world generation");
+            asyncChunkGenThreadPerWorld = false;
+        } else if ("2".equals(sharedHostEnvGen)) {
+            log("Async Chunks - Generation: Your host has disabled async world generation - You will experience lag from world generation");
+            asyncChunkGeneration = false;
+        }
+
+        if (sharedHostEnvLoad != null) {
+            try {
+                asyncChunkLoadThreads = Math.max(1, Math.min(asyncChunkLoadThreads, Integer.parseInt(sharedHostEnvLoad)));
+            } catch (NumberFormatException ignored) {}
+        }
+
+        if (!asyncChunks) {
+            log("Async Chunks: Disabled - Chunks will be managed synchronosuly, and will cause tremendous lag.");
+        } else {
+            log("Async Chunks: Enabled - Chunks will be loaded much faster, without lag.");
+            if (!asyncChunkGeneration) {
+                log("Async Chunks - Generation: Disabled - Chunks will be generated synchronosuly, and will cause tremendous lag.");
+            } else if (asyncChunkGenThreadPerWorld) {
+                log("Async Chunks - Generation: Enabled - Chunks will be generated much faster, without lag.");
+            } else {
+                log("Async Chunks - Generation: Enabled (Single Thread) - Chunks will be generated much faster, without lag.");
+            }
+        }
+    }
 }
diff --git a/src/main/java/com/destroystokyo/paper/util/PriorityQueuedExecutor.java b/src/main/java/com/destroystokyo/paper/util/PriorityQueuedExecutor.java
new file mode 100644
index 0000000000..5c77b6e8e1
--- /dev/null
+++ b/src/main/java/com/destroystokyo/paper/util/PriorityQueuedExecutor.java
@@ -0,0 +1,281 @@
+package com.destroystokyo.paper.util;
+
+import com.google.common.util.concurrent.ThreadFactoryBuilder;
+import net.minecraft.server.NamedIncrementingThreadFactory;
+
+import javax.annotation.Nonnull;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.AbstractExecutorService;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Supplier;
+
+/**
+ * Implements an Executor Service that allows specifying Task Priority
+ * and bumping of task priority.
+ *
+ * @author aikar
+ */
+@SuppressWarnings({"WeakerAccess", "UnusedReturnValue", "unused"})
+public class PriorityQueuedExecutor extends AbstractExecutorService {
+    private final ConcurrentLinkedQueue<Runnable> high = new ConcurrentLinkedQueue<>();
+    private final ConcurrentLinkedQueue<Runnable> normal = new ConcurrentLinkedQueue<>();
+    private final RejectionHandler handler;
+    private volatile boolean shuttingDown = false;
+    private volatile boolean shuttingDownNow = false;
+    private final List<Thread> threads = new ArrayList<>();
+
+    public PriorityQueuedExecutor(String name) {
+        this(name, Runtime.getRuntime().availableProcessors(), null);
+    }
+
+    public PriorityQueuedExecutor(String name, int threads) {
+        this(name, threads, null);
+    }
+
+    public PriorityQueuedExecutor(String name, int threads, RejectionHandler handler) {
+        ThreadFactory factory = new ThreadFactoryBuilder()
+            .setThreadFactory(new NamedIncrementingThreadFactory(name))
+            .setDaemon(true)
+            .build();
+        for (int i = 0; i < threads; i++) {
+            final Thread thread = factory.newThread(this::processQueues);
+            thread.start();
+            this.threads.add(thread);
+        }
+        if (handler == null) {
+            handler = ABORT_POLICY;
+        }
+        this.handler = handler;
+    }
+
+    public void shutdown() {
+        shuttingDown = true;
+        synchronized (this) {
+            this.notifyAll();
+        }
+    }
+
+    @Nonnull
+    @Override
+    public List<Runnable> shutdownNow() {
+        shuttingDown = true;
+        shuttingDownNow = true;
+        List<Runnable> tasks = new ArrayList<>(high.size() + normal.size());
+        Runnable run;
+        while ((run = getTask()) != null) {
+            tasks.add(run);
+        }
+
+        return tasks;
+    }
+
+    @Override
+    public boolean isShutdown() {
+        return shuttingDown;
+    }
+
+    @Override
+    public boolean isTerminated() {
+        if (!shuttingDown) {
+            return false;
+        }
+        return high.isEmpty() && normal.isEmpty();
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, @Nonnull TimeUnit unit) {
+        synchronized (this) {
+            this.notifyAll();
+        }
+        final long wait = unit.toNanos(timeout);
+        final long max = System.nanoTime() + wait;
+        for (;!threads.isEmpty() && System.nanoTime() < max;) {
+            threads.removeIf(thread -> !thread.isAlive());
+        }
+        return isTerminated();
+    }
+
+
+    public PendingTask<Void> createPendingTask(Runnable task) {
+        return createPendingTask(task, Priority.NORMAL);
+    }
+    public PendingTask<Void> createPendingTask(Runnable task, Priority priority) {
+        return createPendingTask(() -> {
+            task.run();
+            return null;
+        }, priority);
+    }
+
+    public <T> PendingTask<T> createPendingTask(Supplier<T> task) {
+        return createPendingTask(task, Priority.NORMAL);
+    }
+
+    public <T> PendingTask<T> createPendingTask(Supplier<T> task, Priority priority) {
+        return new PendingTask<>(task, priority);
+    }
+
+    public PendingTask<Void> submitTask(Runnable run) {
+        return submitTask(createPendingTask(run));
+    }
+
+    public PendingTask<Void> submitTask(Runnable run, Priority priority) {
+        return submitTask(createPendingTask(run, priority));
+    }
+
+    public <T> PendingTask<T> submitTask(Supplier<T> run) {
+        return submitTask(createPendingTask(run));
+    }
+
+    public <T> PendingTask<T> submitTask(Supplier<T> run, Priority priority) {
+        return submitTask(createPendingTask(run, priority));
+    }
+
+    public <T> PendingTask<T> submitTask(PendingTask<T> task) {
+        if (shuttingDown) {
+            handler.onRejection(task, this);
+            return task;
+        }
+        task.submit(this);
+        return task;
+    }
+
+    @Override
+    public void execute(@Nonnull Runnable command) {
+        submitTask(command);
+    }
+
+    private Runnable getTask() {
+        Runnable run = high.poll();
+        if (run != null) {
+            return run;
+        }
+        return normal.poll();
+    }
+
+    private void processQueues() {
+        Runnable run = null;
+        while (true) {
+            if (run != null) {
+                run.run();
+            }
+            if (shuttingDownNow) {
+                return;
+            }
+            if ((run = getTask()) != null) {
+                continue;
+            }
+            synchronized (PriorityQueuedExecutor.this) {
+                if ((run = getTask()) != null) {
+                    continue;
+                }
+
+                if (shuttingDown || shuttingDownNow) {
+                    return;
+                }
+                try {
+                    PriorityQueuedExecutor.this.wait();
+                } catch (InterruptedException ignored) {
+                }
+            }
+        }
+    }
+
+    public enum Priority {
+        NORMAL, HIGH
+    }
+
+    public class PendingTask <T> implements Runnable {
+
+        private final AtomicBoolean hasRan = new AtomicBoolean();
+        private final AtomicInteger submitted = new AtomicInteger(-1);
+        private final AtomicInteger priority;
+        private final Supplier<T> run;
+        private final CompletableFuture<T> future = new CompletableFuture<>();
+        private volatile PriorityQueuedExecutor executor;
+
+        public PendingTask(Supplier<T> run) {
+            this(run, Priority.NORMAL);
+        }
+
+        public PendingTask(Supplier<T> run, Priority priority) {
+            this.priority = new AtomicInteger(priority.ordinal());
+            this.run = run;
+        }
+
+        public boolean cancel() {
+            return hasRan.compareAndSet(false, true);
+        }
+
+        @Override
+        public void run() {
+            if (!hasRan.compareAndSet(false, true)) {
+                return;
+            }
+
+            try {
+                future.complete(run.get());
+            } catch (Throwable e) {
+                future.completeExceptionally(e);
+            }
+        }
+
+        public void bumpPriority() {
+            if (!priority.compareAndSet(Priority.NORMAL.ordinal(), Priority.HIGH.ordinal())) {
+                return;
+            }
+
+            if (this.executor == null) {
+                return;
+            }
+            // If we have already been submitted, resubmit with new priority
+            submit(this.executor);
+        }
+
+        public CompletableFuture<T> onDone() {
+            return future;
+        }
+
+        public void submit(PriorityQueuedExecutor executor) {
+            for (;;) {
+                final int submitted = this.submitted.get();
+                final int priority = this.priority.get();
+                if (submitted == priority) {
+                    return;
+                }
+                if (this.submitted.compareAndSet(submitted, priority)) {
+                    if (priority == Priority.HIGH.ordinal()) {
+                        high.add(this);
+                    } else {
+                        normal.add(this);
+                    }
+
+                    break;
+                }
+            }
+
+            //noinspection SynchronizationOnLocalVariableOrMethodParameter
+            synchronized (executor) {
+                // Wake up a thread to take this work
+                executor.notify();
+            }
+        }
+    }
+    public interface RejectionHandler {
+        void onRejection(Runnable run, PriorityQueuedExecutor executor);
+    }
+
+    public static final RejectionHandler ABORT_POLICY = (run, executor) -> {
+        throw new RejectedExecutionException("Executor has been shutdown");
+    };
+    public static final RejectionHandler CALLER_RUNS_POLICY = (run, executor) -> {
+        run.run();
+    };
+
+}
diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index edfcb107bd..cb99888707 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -184,6 +184,7 @@ public class Chunk implements IChunkAccess {
 
         for (k = 0; k < this.sections.length; ++k) {
             this.sections[k] = protochunk.getSections()[k];
+            if (this.sections[k] != null) this.sections[k].disableLocks(); // Paper - Async Chunks - disable locks used during world gen
         }
 
         Iterator iterator = protochunk.s().iterator();
diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
index 958a4084e6..56a76e17ef 100644
--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
+++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
@@ -38,9 +38,9 @@ public class ChunkProviderServer implements IChunkProvider {
     public final Long2ObjectMap<Chunk> chunks = Long2ObjectMaps.synchronize(new ChunkMap(8192));
     private Chunk lastChunk;
     private final ChunkTaskScheduler chunkScheduler;
-    private final SchedulerBatch<ChunkCoordIntPair, ChunkStatus, ProtoChunk> batchScheduler;
+    final SchedulerBatch<ChunkCoordIntPair, ChunkStatus, ProtoChunk> batchScheduler; // Paper
     public final WorldServer world;
-    private final IAsyncTaskHandler asyncTaskHandler;
+    final IAsyncTaskHandler asyncTaskHandler; // Paper
 
     public ChunkProviderServer(WorldServer worldserver, IChunkLoader ichunkloader, ChunkGenerator<?> chunkgenerator, IAsyncTaskHandler iasynctaskhandler) {
         this.world = worldserver;
@@ -77,10 +77,76 @@ public class ChunkProviderServer implements IChunkProvider {
         this.unloadQueue.remove(ChunkCoordIntPair.a(i, j));
     }
 
+    // Paper start - defaults if Async Chunks is not enabled
+    boolean chunkGoingToExists(int x, int z) {
+        final long k = ChunkCoordIntPair.asLong(x, z);
+        return chunkScheduler.progressCache.containsKey(k);
+    }
+    public void bumpPriority(ChunkCoordIntPair coords) {
+        // do nothing, override in async
+    }
+
+    public List<ChunkCoordIntPair> getSpiralOutChunks(BlockPosition blockposition, int radius) {
+        List<ChunkCoordIntPair> list = com.google.common.collect.Lists.newArrayList();
+
+        for (int r = 1; r <= radius; r++) {
+            int x = -r;
+            int z = r;
+            list.add(new ChunkCoordIntPair(blockposition.getX(), blockposition.getZ()));
+            // Iterates the edge of half of the box; then negates for other half.
+            while (x <= r && z > -r) {
+                list.add(new ChunkCoordIntPair(blockposition.getX() + x, blockposition.getZ() + z));
+                list.add(new ChunkCoordIntPair(blockposition.getX() - x, blockposition.getZ() - z));
+
+                if (x < r) {
+                    x++;
+                } else {
+                    z--;
+                }
+            }
+        }
+        return list;
+    }
+
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen, Consumer<Chunk> consumer) {
+        return getChunkAt(x, z, load, gen, false, consumer);
+    }
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        Chunk chunk = getChunkAt(x, z, load, gen);
+        if (consumer != null) {
+            consumer.accept(chunk);
+        }
+        return chunk;
+    }
+
+    PaperAsyncChunkProvider.CancellableChunkRequest requestChunk(int x, int z, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        Chunk chunk = getChunkAt(x, z, gen, priority, consumer);
+        return new PaperAsyncChunkProvider.CancellableChunkRequest() {
+            @Override
+            public void cancel() {
+
+            }
+
+            @Override
+            public Chunk getChunk() {
+                return chunk;
+            }
+        };
+    }
+    // Paper end
+
     @Nullable
     public Chunk getChunkAt(int i, int j, boolean flag, boolean flag1) {
         IChunkLoader ichunkloader = this.chunkLoader;
         Chunk chunk;
+        // Paper start - do already loaded checks before synchronize
+        long k = ChunkCoordIntPair.a(i, j);
+        chunk = (Chunk) this.chunks.get(k);
+        if (chunk != null) {
+            //this.lastChunk = chunk; // Paper remove vanilla lastChunk
+            return chunk;
+        }
+        // Paper end
 
         synchronized (this.chunkLoader) {
             // Paper start - remove vanilla lastChunk, we do it more accurately
@@ -88,13 +154,15 @@ public class ChunkProviderServer implements IChunkProvider {
                 return this.lastChunk;
             }*/ // Paper end
 
-            long k = ChunkCoordIntPair.a(i, j);
+            // Paper start - move up
+            //long k = ChunkCoordIntPair.a(i, j);
 
-            chunk = (Chunk) this.chunks.get(k);
+            /*chunk = (Chunk) this.chunks.get(k);
             if (chunk != null) {
                 //this.lastChunk = chunk; // Paper remove vanilla lastChunk
                 return chunk;
-            }
+            }*/
+            // Paper end
 
             if (flag) {
                 try (co.aikar.timings.Timing timing = world.timings.syncChunkLoadTimer.startTiming()) { // Paper
@@ -150,7 +218,8 @@ public class ChunkProviderServer implements IChunkProvider {
         return (IChunkAccess) (chunk != null ? chunk : (IChunkAccess) this.chunkScheduler.b(new ChunkCoordIntPair(i, j), flag));
     }
 
-    public CompletableFuture<ProtoChunk> a(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) {
+    public CompletableFuture<Void> loadAllChunks(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) { return a(iterable, consumer).thenCompose(protoChunk -> null); } // Paper - overriden in async chunk provider
+    private CompletableFuture<ProtoChunk> a(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) { // Paper - mark private, use above method
         this.batchScheduler.b();
         Iterator iterator = iterable.iterator();
 
@@ -168,6 +237,7 @@ public class ChunkProviderServer implements IChunkProvider {
         return this.batchScheduler.c();
     }
 
+    ReportedException generateChunkError(int i, int j, Throwable throwable) { return a(i, j, throwable); } // Paper - OBFHELPER
     private ReportedException a(int i, int j, Throwable throwable) {
         CrashReport crashreport = CrashReport.a(throwable, "Exception generating new chunk");
         CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Chunk to be generated");
@@ -287,11 +357,13 @@ public class ChunkProviderServer implements IChunkProvider {
     }
 
     public void close() {
-        try {
+        // Paper start - we do not need to wait for chunk generations to finish on close
+        /*try {
             this.batchScheduler.a();
         } catch (InterruptedException interruptedexception) {
             ChunkProviderServer.a.error("Couldn\'t stop taskManager", interruptedexception);
-        }
+        }*/
+        // Paper end
 
     }
 
diff --git a/src/main/java/net/minecraft/server/ChunkRegionLoader.java b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
index c233b7e903..edd0742527 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -120,7 +120,7 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
     // CraftBukkit start
     private boolean check(ChunkProviderServer cps, int x, int z) throws IOException {
         if (cps != null) {
-            com.google.common.base.Preconditions.checkState(org.bukkit.Bukkit.isPrimaryThread(), "primary thread");
+            //com.google.common.base.Preconditions.checkState(org.bukkit.Bukkit.isPrimaryThread(), "primary thread"); // Paper - this is safe
             if (cps.isLoaded(x, z)) {
                 return true;
             }
@@ -380,11 +380,12 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
                     }
                 };
             } else {
+                /* // Paper start - we will never invoke this in an unsafe way
                 NBTTagCompound nbttagcompound2 = this.a(world, chunkcoordintpair.x, chunkcoordintpair.z);
 
                 if (nbttagcompound2 != null && this.a(nbttagcompound2) == ChunkStatus.Type.LEVELCHUNK) {
                     return;
-                }
+                }*/ // Paper end
 
                 completion = new Supplier<NBTTagCompound>() {
                     public NBTTagCompound get() {
diff --git a/src/main/java/net/minecraft/server/ChunkSection.java b/src/main/java/net/minecraft/server/ChunkSection.java
index bdfc7d81ff..a5c4564d60 100644
--- a/src/main/java/net/minecraft/server/ChunkSection.java
+++ b/src/main/java/net/minecraft/server/ChunkSection.java
@@ -24,7 +24,17 @@ public class ChunkSection {
             this.skyLight = new NibbleArray();
         }
 
+        // Paper start - Async Chunks - Lock during world gen
+        if (chunk instanceof ProtoChunk) {
+            this.blockIds.enableLocks();
+        } else {
+            this.blockIds.disableLocks();
+        }
+    }
+    void disableLocks() {
+        this.blockIds.disableLocks();
     }
+    // Paper end
 
     public IBlockData getType(int i, int j, int k) {
         return this.blockIds.a(i, j, k);
diff --git a/src/main/java/net/minecraft/server/ChunkTaskScheduler.java b/src/main/java/net/minecraft/server/ChunkTaskScheduler.java
index 34019bd1b3..fc9091c801 100644
--- a/src/main/java/net/minecraft/server/ChunkTaskScheduler.java
+++ b/src/main/java/net/minecraft/server/ChunkTaskScheduler.java
@@ -20,13 +20,14 @@ public class ChunkTaskScheduler extends Scheduler<ChunkCoordIntPair, ChunkStatus
     private final ChunkGenerator<?> d;
     private final IChunkLoader e;
     private final IAsyncTaskHandler f;
-    private final Long2ObjectMap<Scheduler.a> progressCache = new ExpiringMap<Scheduler.a>(8192, 5000) { // CraftBukkit - decompile error
+    final Long2ObjectMap<Scheduler.a> progressCache = new ExpiringMap<Scheduler.a>(8192, 5000) { // CraftBukkit - decompile error // Paper - protected
         protected boolean a(Scheduler.a scheduler_a) {
             ProtoChunk protochunk = (ProtoChunk) scheduler_a.a();
 
             return !protochunk.ab_() /*&& !protochunk.h()*/; // Paper
         }
     };
+    private final Long2ObjectMap<java.util.concurrent.CompletableFuture<Scheduler.a>> pendingSchedulers = new it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap<>(); // Paper
 
     public ChunkTaskScheduler(int i, World world, ChunkGenerator<?> chunkgenerator, IChunkLoader ichunkloader, IAsyncTaskHandler iasynctaskhandler) {
         super("WorldGen", i, ChunkStatus.FINALIZED, () -> {
@@ -50,8 +51,28 @@ public class ChunkTaskScheduler extends Scheduler<ChunkCoordIntPair, ChunkStatus
     protected Scheduler.a a(ChunkCoordIntPair chunkcoordintpair, boolean flag) {
         IChunkLoader ichunkloader = this.e;
 
-        synchronized (this.e) {
-            return flag ? (Scheduler.a) this.progressCache.computeIfAbsent(chunkcoordintpair.a(), (i) -> {
+        // Paper start - refactor a lot of this - avoid generating a chunk while holding lock on expiring map
+        java.util.concurrent.CompletableFuture<Scheduler.a> pending = null;
+        boolean created = false;
+        long key = chunkcoordintpair.a();
+        synchronized (pendingSchedulers) {
+            Scheduler.a existing = this.progressCache.get(key);
+            if (existing != null) {
+                return existing;
+            }
+            pending = this.pendingSchedulers.get(key);
+            if (pending == null) {
+                if (!flag) {
+                    return null;
+                }
+                created = true;
+                pending = new java.util.concurrent.CompletableFuture<>();
+                pendingSchedulers.put(key, pending);
+            }
+        }
+        if (created) {
+            java.util.function.Function<Long, Scheduler.a> get = (i) -> {
+                // Paper end
                 ProtoChunk protochunk;
 
                 try {
@@ -70,8 +91,18 @@ public class ChunkTaskScheduler extends Scheduler<ChunkCoordIntPair, ChunkStatus
                 } else {
                     return new Scheduler.a(chunkcoordintpair, new ProtoChunk(chunkcoordintpair, ChunkConverter.a, this.getWorld()), ChunkStatus.EMPTY); // Paper - Anti-Xray
                 }
-            }) : (Scheduler.a) this.progressCache.get(chunkcoordintpair.a());
+                // Paper start
+            };
+            Scheduler.a scheduler = get.apply(key);
+            progressCache.put(key, scheduler);
+            pending.complete(scheduler);
+            synchronized (pendingSchedulers) {
+                pendingSchedulers.remove(key);
+            }
+            return scheduler;
         }
+        return pending.join();
+        // Paper end
     }
 
     protected ProtoChunk a(ChunkCoordIntPair chunkcoordintpair, ChunkStatus chunkstatus, Map<ChunkCoordIntPair, ProtoChunk> map) {
diff --git a/src/main/java/net/minecraft/server/DataPaletteBlock.java b/src/main/java/net/minecraft/server/DataPaletteBlock.java
index 71a3636be6..ff0fe25417 100644
--- a/src/main/java/net/minecraft/server/DataPaletteBlock.java
+++ b/src/main/java/net/minecraft/server/DataPaletteBlock.java
@@ -3,7 +3,7 @@ package net.minecraft.server;
 import com.destroystokyo.paper.antixray.ChunkPacketInfo; // Paper - Anti-Xray
 import java.util.Arrays;
 import java.util.Objects;
-import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
@@ -20,25 +20,42 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     protected DataBits a; protected DataBits getDataBits() { return this.a; } // Paper - OBFHELPER
     private DataPalette<T> h; private DataPalette<T> getDataPalette() { return this.h; } // Paper - OBFHELPER
     private int i; private int getBitsPerObject() { return this.i; } // Paper - OBFHELPER
-    private final ReentrantLock j = new ReentrantLock();
+    // Paper start - use read write locks only during generation, disable once back on main thread
+    private static final NoopLock NOOP_LOCK = new NoopLock();
+    private java.util.concurrent.locks.Lock readLock = NOOP_LOCK;
+    private java.util.concurrent.locks.Lock writeLock = NOOP_LOCK;
+
+    private static class NoopLock extends ReentrantReadWriteLock.WriteLock {
+        private NoopLock() {
+            super(new ReentrantReadWriteLock());
+        }
+
+        @Override
+        public final void lock() {
+        }
+
+        @Override
+        public final void unlock() {
 
-    private void b() {
-        if (this.j.isLocked() && !this.j.isHeldByCurrentThread()) {
-            String s = (String)Thread.getAllStackTraces().keySet().stream().filter(Objects::nonNull).map((thread) -> {
-                return thread.getName() + ": \n\tat " + (String)Arrays.stream(thread.getStackTrace()).map(Object::toString).collect(Collectors.joining("\n\tat "));
-            }).collect(Collectors.joining("\n"));
-            CrashReport crashreport = new CrashReport("Writing into PalettedContainer from multiple threads", new IllegalStateException());
-            CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Thread dumps");
-            crashreportsystemdetails.a("Thread dumps", s);
-            throw new ReportedException(crashreport);
-        } else {
-            this.j.lock();
         }
     }
 
+    synchronized void enableLocks() {
+        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+        readLock = lock.readLock();
+        writeLock = lock.writeLock();
+    }
+    synchronized void disableLocks() {
+        readLock = NOOP_LOCK;
+        writeLock = NOOP_LOCK;
+    }
+    private void b() {
+        writeLock.lock();
+    }
     private void c() {
-        this.j.unlock();
+        writeLock.unlock();
     }
+    // Paper end
 
     public DataPaletteBlock(DataPalette<T> datapalette, RegistryBlockID<T> registryblockid, Function<NBTTagCompound, T> function, Function<T, NBTTagCompound> function1, T object) {
         // Paper start - Anti-Xray - Support default constructor
@@ -147,8 +164,13 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     }
 
     protected T a(int ix) {
-        T object = this.h.a(this.a.a(ix)); // Paper - decompile fix
-        return (T)(object == null ? this.g : object);
+        try { // Paper start - read lock
+            readLock.lock();
+            T object = this.h.a(this.a.a(ix)); // Paper - decompile fix
+            return (T)(object == null ? this.g : object);
+        } finally {
+            readLock.unlock();
+        } // Paper end
     }
 
     // Paper start - Anti-Xray - Support default methods
diff --git a/src/main/java/net/minecraft/server/DefinedStructureManager.java b/src/main/java/net/minecraft/server/DefinedStructureManager.java
index 271dc41d45..bd15534c23 100644
--- a/src/main/java/net/minecraft/server/DefinedStructureManager.java
+++ b/src/main/java/net/minecraft/server/DefinedStructureManager.java
@@ -19,7 +19,7 @@ import org.apache.logging.log4j.Logger;
 
 public class DefinedStructureManager implements IResourcePackListener {
     private static final Logger a = LogManager.getLogger();
-    private final Map<MinecraftKey, DefinedStructure> b = Maps.newHashMap();
+    private final Map<MinecraftKey, DefinedStructure> b = Maps.newConcurrentMap(); // Paper
     private final DataFixer c;
     private final MinecraftServer d;
     private final java.nio.file.Path e;
diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index 0237049a40..cd601f29a3 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -209,7 +209,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
         this.random = SHARED_RANDOM; // Paper
         this.fireTicks = -this.getMaxFireTicks();
         this.justCreated = true;
-        this.uniqueID = MathHelper.a(this.random);
+        this.uniqueID = MathHelper.a(java.util.concurrent.ThreadLocalRandom.current()); // Paper
         this.au = this.uniqueID.toString();
         this.aJ = Sets.newHashSet();
         this.aL = new double[] { 0.0D, 0.0D, 0.0D};
diff --git a/src/main/java/net/minecraft/server/IChunkLoader.java b/src/main/java/net/minecraft/server/IChunkLoader.java
index 4698ee99f8..dfb45cc4ea 100644
--- a/src/main/java/net/minecraft/server/IChunkLoader.java
+++ b/src/main/java/net/minecraft/server/IChunkLoader.java
@@ -6,6 +6,8 @@ import javax.annotation.Nullable;
 
 public interface IChunkLoader {
 
+    void loadEntities(NBTTagCompound nbttagcompound, Chunk chunk); // Paper - Async Chunks
+    Object[] loadChunk(GeneratorAccess generatoraccess, int i, int j, Consumer<Chunk> consumer) throws IOException; // Paper - Async Chunks
     @Nullable
     Chunk a(GeneratorAccess generatoraccess, int i, int j, Consumer<Chunk> consumer) throws IOException;
 
diff --git a/src/main/java/net/minecraft/server/MathHelper.java b/src/main/java/net/minecraft/server/MathHelper.java
index 49fba0979e..9ad646f8d4 100644
--- a/src/main/java/net/minecraft/server/MathHelper.java
+++ b/src/main/java/net/minecraft/server/MathHelper.java
@@ -142,6 +142,7 @@ public class MathHelper {
         return Math.floorMod(i, j);
     }
 
+    public static float normalizeYaw(float fx) { return g(fx); } // Paper - OBFHELPER
     public static float g(float fx) {
         fx = fx % 360.0F;
         if (fx >= 180.0F) {
diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
index 98d182fdb8..487d98eb1b 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -503,6 +503,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
 
         // CraftBukkit start - fire WorldLoadEvent and handle whether or not to keep the spawn in memory
         Stopwatch stopwatch = Stopwatch.createStarted();
+        boolean waitForChunks = Boolean.getBoolean("paper.waitforchunks"); // Paper
         for (WorldServer worldserver : this.getWorlds()) {
             MinecraftServer.LOGGER.info("Preparing start region for level " + worldserver.dimension + " (Seed: " + worldserver.getSeed() + ")");
             if (!worldserver.getWorld().getKeepSpawnInMemory()) {
@@ -510,29 +511,25 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
             }
 
             BlockPosition blockposition = worldserver.getSpawn();
-            ArrayList arraylist = Lists.newArrayList();
+            List<ChunkCoordIntPair> arraylist = worldserver.getChunkProviderServer().getSpiralOutChunks(blockposition,  worldserver.paperConfig.keepLoadedRange >> 4); // Paper
             Set set = Sets.newConcurrentHashSet();
 
-            // Paper start
-            short radius = worldserver.paperConfig.keepLoadedRange;
-            for (int i = -radius; i <= radius && this.isRunning(); i += 16) {
-                for (int j = -radius; j <= radius && this.isRunning(); j += 16) {
-                    // Paper end
-                    arraylist.add(new ChunkCoordIntPair(blockposition.getX() + i >> 4, blockposition.getZ() + j >> 4));
-                }
-            } // Paper
+            // Paper - remove arraylist creation, call spiral above
             if (this.isRunning()) { // Paper
                 int expected = arraylist.size(); // Paper
 
 
-                CompletableFuture completablefuture = worldserver.getChunkProviderServer().a((Iterable) arraylist, (chunk) -> {
+                CompletableFuture completablefuture = worldserver.getChunkProviderServer().loadAllChunks(arraylist, (chunk) -> { // Paper
                     set.add(chunk.getPos());
-                    if (set.size() < expected && set.size() % 25 == 0) this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                    if (waitForChunks && (set.size() == expected || (set.size() < expected && set.size() % (set.size() / 10) == 0))) {
+                        this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                    }
                 });
 
-                while (!completablefuture.isDone()) {
+                while (waitForChunks && !completablefuture.isDone() && isRunning()) { // Paper
                     try {
-                        completablefuture.get(1L, TimeUnit.SECONDS);
+                        PaperAsyncChunkProvider.processChunkLoads(worldserver); // Paper
+                        completablefuture.get(50L, TimeUnit.MILLISECONDS); // Paper
                     } catch (InterruptedException interruptedexception) {
                         throw new RuntimeException(interruptedexception);
                     } catch (ExecutionException executionexception) {
@@ -542,11 +539,11 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
 
                         throw new RuntimeException(executionexception.getCause());
                     } catch (TimeoutException timeoutexception) {
-                        this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                        //this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
                     }
                 }
 
-                this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                if (waitForChunks) this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
             }
         }
 
@@ -650,6 +647,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
             if (hasStopped) return;
             hasStopped = true;
         }
+        PaperAsyncChunkProvider.stop(this); // Paper
         // CraftBukkit end
         MinecraftServer.LOGGER.info("Stopping server");
         MinecraftTimings.stopServer(); // Paper
@@ -1017,6 +1015,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
         while ((futuretask = (FutureTask) this.f.poll()) != null) {
             SystemUtils.a(futuretask, MinecraftServer.LOGGER);
         }
+        PaperAsyncChunkProvider.processChunkLoads(this); // Paper
         MinecraftTimings.minecraftSchedulerTimer.stopTiming(); // Paper
 
         this.methodProfiler.c("commandFunctions");
@@ -1053,6 +1052,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
         // CraftBukkit - dropTickTime
         for (Iterator iterator = this.getWorlds().iterator(); iterator.hasNext();) {
             WorldServer worldserver = (WorldServer) iterator.next();
+            PaperAsyncChunkProvider.processChunkLoads(worldserver); // Paper
             TileEntityHopper.skipHopperEvents = worldserver.paperConfig.disableHopperMoveEvents || org.bukkit.event.inventory.InventoryMoveItemEvent.getHandlerList().getRegisteredListeners().length == 0; // Paper
             i = SystemUtils.c();
             if (true || worldserver.worldProvider.getDimensionManager() == DimensionManager.OVERWORLD || this.getAllowNether()) { // CraftBukkit
@@ -1109,6 +1109,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
                 this.methodProfiler.e();
                 this.methodProfiler.e();
                 worldserver.explosionDensityCache.clear(); // Paper - Optimize explosions
+                PaperAsyncChunkProvider.processChunkLoads(worldserver); // Paper
             }
         }
 
diff --git a/src/main/java/net/minecraft/server/PaperAsyncChunkProvider.java b/src/main/java/net/minecraft/server/PaperAsyncChunkProvider.java
new file mode 100644
index 0000000000..5823917a65
--- /dev/null
+++ b/src/main/java/net/minecraft/server/PaperAsyncChunkProvider.java
@@ -0,0 +1,593 @@
+/*
+ * This file is licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2018 Daniel Ennis <http://aikar.co>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package net.minecraft.server;
+
+import com.destroystokyo.paper.PaperConfig;
+import com.destroystokyo.paper.util.PriorityQueuedExecutor;
+import com.destroystokyo.paper.util.PriorityQueuedExecutor.Priority;
+import it.unimi.dsi.fastutil.longs.Long2ObjectMap;
+import it.unimi.dsi.fastutil.longs.Long2ObjectMaps;
+import it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap;
+import org.bukkit.Bukkit;
+import org.bukkit.craftbukkit.generator.CustomChunkGenerator;
+import org.bukkit.craftbukkit.generator.InternalChunkGenerator;
+
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Consumer;
+
+@SuppressWarnings("unused")
+public class PaperAsyncChunkProvider extends ChunkProviderServer {
+
+    private static final PriorityQueuedExecutor EXECUTOR = new PriorityQueuedExecutor("PaperChunkLoader", PaperConfig.asyncChunks ? PaperConfig.asyncChunkLoadThreads : 0);
+    private static final PriorityQueuedExecutor SINGLE_GEN_EXECUTOR = new PriorityQueuedExecutor("PaperChunkGenerator", PaperConfig.asyncChunks && PaperConfig.asyncChunkGeneration && !PaperConfig.asyncChunkGenThreadPerWorld ? 1 : 0);
+    private static final ConcurrentLinkedQueue<Runnable> MAIN_THREAD_QUEUE = new ConcurrentLinkedQueue<>();
+    private static final ThreadLocal<Boolean> IS_CHUNK_THREAD = ThreadLocal.withInitial(() -> false);
+    private static final ThreadLocal<Boolean> IS_CHUNK_GEN_THREAD = ThreadLocal.withInitial(() -> false);
+
+    private final PriorityQueuedExecutor generationExecutor;
+    //private static final PriorityQueuedExecutor generationExecutor = new PriorityQueuedExecutor("PaperChunkGen", 1);
+    private final Long2ObjectMap<PendingChunk> pendingChunks = Long2ObjectMaps.synchronize(new Long2ObjectOpenHashMap<>());
+    private final IAsyncTaskHandler asyncHandler;
+
+    private final WorldServer world;
+    private final IChunkLoader chunkLoader;
+    private final MinecraftServer server;
+    private final boolean shouldGenSync;
+
+    public PaperAsyncChunkProvider(WorldServer world, IChunkLoader chunkLoader, InternalChunkGenerator generator, MinecraftServer server) {
+        super(world, chunkLoader, generator, server);
+
+        this.server = world.getMinecraftServer();
+        this.world = world;
+        this.asyncHandler = server;
+        this.chunkLoader = chunkLoader;
+        String worldName = this.world.getWorld().getName();
+        this.shouldGenSync = generator instanceof CustomChunkGenerator && !(((CustomChunkGenerator) generator).asyncSupported) || !PaperConfig.asyncChunkGeneration;
+        this.generationExecutor = PaperConfig.asyncChunkGenThreadPerWorld ? new PriorityQueuedExecutor("PaperChunkGen-" + worldName, shouldGenSync ? 0 : 1) : SINGLE_GEN_EXECUTOR;
+    }
+
+    static void processChunkLoads(MinecraftServer server) {
+        for (WorldServer world : server.getWorlds()) {
+            processChunkLoads(world);
+        }
+    }
+
+    static void processChunkLoads(World world) {
+        IChunkProvider chunkProvider = world.getChunkProvider();
+        if (chunkProvider instanceof PaperAsyncChunkProvider) {
+            ((PaperAsyncChunkProvider) chunkProvider).processChunkLoads();
+        }
+    }
+
+    static void stop(MinecraftServer server) {
+        for (WorldServer world : server.getWorlds()) {
+            world.getPlayerChunkMap().shutdown();
+        }
+    }
+
+    private boolean processChunkLoads() {
+        Runnable run;
+        boolean hadLoad = false;
+        while ((run = MAIN_THREAD_QUEUE.poll()) != null) {
+            run.run();
+            hadLoad = true;
+        }
+        return hadLoad;
+    }
+
+    @Override
+    public void bumpPriority(ChunkCoordIntPair coords) {
+        PendingChunk pending = pendingChunks.get(coords.asLong());
+        if (pending != null) {
+            pending.bumpPriority();
+        }
+    }
+
+    @Nullable
+    @Override
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen) {
+        return getChunkAt(x, z, load, gen, null);
+    }
+
+    @Nullable
+    @Override
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        long key = ChunkCoordIntPair.asLong(x, z);
+        Chunk chunk = this.chunks.get(key);
+        if (chunk != null || !load) { // return null if we aren't loading
+            if (consumer != null) {
+                consumer.accept(chunk);
+            }
+            return chunk;
+        }
+        return loadOrGenerateChunk(x, z, gen, priority, consumer); // Async overrides this method
+    }
+
+    private Chunk loadOrGenerateChunk(int x, int z, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        return requestChunk(x, z, gen, priority, consumer).getChunk();
+    }
+
+    PendingChunkRequest requestChunk(int x, int z, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        final long key = ChunkCoordIntPair.asLong(x, z);
+
+        // Obtain a PendingChunk
+        final PendingChunk pending;
+        final boolean isBlockingMain = consumer == null && server.isMainThread();
+        synchronized (pendingChunks) {
+            PendingChunk pendingChunk = pendingChunks.get(key);
+            if (pendingChunk == null) {
+                pending = new PendingChunk(x, z, key, gen, priority || isBlockingMain);
+                pendingChunks.put(key, pending);
+            } else if (pendingChunk.hasFinished && gen && !pendingChunk.canGenerate && pendingChunk.chunk == null) {
+                // need to overwrite the old
+                pending = new PendingChunk(x, z, key, true, priority || isBlockingMain);
+                pendingChunks.put(key, pending);
+            } else {
+                pending = pendingChunk;
+                if (priority || isBlockingMain) {
+                    pending.bumpPriority();
+                }
+            }
+        }
+        // Listen for when result is ready
+        final CompletableFuture<Chunk> future = new CompletableFuture<>();
+        PendingChunkRequest request = pending.addListener(future, gen);
+        if (IS_CHUNK_THREAD.get()) {
+            pending.loadTask.run();
+        }
+
+        if (isBlockingMain && pending.hasFinished) {
+            processChunkLoads();
+            request.initialReturnChunk = pending.postChunk();
+            return request;
+        }
+
+        if (isBlockingMain) {
+            try (co.aikar.timings.Timing timing = world.timings.syncChunkLoadTimer.startTiming()) {
+                while (!future.isDone()) {
+                    // We aren't done, obtain lock on queue
+                    synchronized (MAIN_THREAD_QUEUE) {
+                        // We may of received our request now, check it
+                        if (processChunkLoads()) {
+                            // If we processed SOMETHING, don't wait
+                            continue;
+                        }
+                        try {
+                            // We got nothing from the queue, wait until something has been added
+                            MAIN_THREAD_QUEUE.wait(1);
+                        } catch (InterruptedException ignored) {
+                        }
+                    }
+                    // Queue has been notified or timed out, process it
+                    processChunkLoads();
+                }
+                // We should be done AND posted into chunk map now, return it
+                request.initialReturnChunk = future.join();
+            }
+        } else if (consumer == null) {
+            // This is on another thread
+            request.initialReturnChunk = future.join();
+        } else {
+            future.thenAccept((c) -> this.asyncHandler.postToMainThread(() -> consumer.accept(c)));
+        }
+
+        return request;
+    }
+
+    @Override
+    public CompletableFuture<Void> loadAllChunks(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) {
+        Iterator<ChunkCoordIntPair> iterator = iterable.iterator();
+
+        List<CompletableFuture<Chunk>> all = new ArrayList<>();
+        while (iterator.hasNext()) {
+            ChunkCoordIntPair chunkcoordintpair = iterator.next();
+            CompletableFuture<Chunk> future = new CompletableFuture<>();
+            all.add(future);
+            this.getChunkAt(chunkcoordintpair.x, chunkcoordintpair.z, true, true, chunk -> {
+                future.complete(chunk);
+                if (consumer != null) {
+                    consumer.accept(chunk);
+                }
+            });
+        }
+        return CompletableFuture.allOf(all.toArray(new CompletableFuture[0]));
+    }
+
+    boolean chunkGoingToExists(int x, int z) {
+        synchronized (pendingChunks) {
+            long key = ChunkCoordIntPair.asLong(x, z);
+            PendingChunk pendingChunk = pendingChunks.get(key);
+            return pendingChunk != null && pendingChunk.canGenerate;
+        }
+    }
+
+    private enum PendingStatus {
+        /**
+         * Request has just started
+         */
+        STARTED,
+        /**
+         * Chunk is attempting to be loaded from disk
+         */
+        LOADING,
+        /**
+         * Chunk must generate on main and is pending main
+         */
+        GENERATION_PENDING,
+        /**
+         * Chunk is generating
+         */
+        GENERATING,
+        /**
+         * Chunk is ready and is pending post to main
+         */
+        PENDING_MAIN,
+        /**
+         * Could not load chunk, and did not need to generat
+         */
+        FAIL,
+        /**
+         * Fully done with this request (may or may not of loaded)
+         */
+        DONE,
+        /**
+         * Chunk load was cancelled (no longer needed)
+         */
+        CANCELLED
+    }
+
+    public interface CancellableChunkRequest {
+        void cancel();
+        Chunk getChunk();
+    }
+
+    public static class PendingChunkRequest implements CancellableChunkRequest {
+        private final PendingChunk pending;
+        private final AtomicBoolean cancelled = new AtomicBoolean(false);
+        private volatile boolean generating;
+        private volatile Chunk initialReturnChunk;
+
+        private PendingChunkRequest(PendingChunk pending) {
+            this.pending = pending;
+            this.cancelled.set(true);
+        }
+
+        private PendingChunkRequest(PendingChunk pending, boolean gen) {
+            this.pending = pending;
+            this.generating = gen;
+        }
+
+        public void cancel() {
+            this.pending.cancel(this);
+        }
+
+        /**
+         * Will be null on asynchronous loads
+         */
+        @Override @Nullable
+        public Chunk getChunk() {
+            return initialReturnChunk;
+        }
+    }
+
+    private class PendingChunk implements Runnable {
+        private final int x;
+        private final int z;
+        private final long key;
+        private final CompletableFuture<Chunk> loadOnly = new CompletableFuture<>();
+        private final CompletableFuture<Chunk> generate = new CompletableFuture<>();
+        private final AtomicInteger requests = new AtomicInteger(0);
+
+        private volatile PendingStatus status = PendingStatus.STARTED;
+        private volatile PriorityQueuedExecutor.PendingTask<Void> loadTask;
+        private volatile PriorityQueuedExecutor.PendingTask<Chunk> genTask;
+        private volatile Priority taskPriority;
+        private volatile boolean generating;
+        private volatile boolean canGenerate;
+        private volatile boolean isHighPriority;
+        private volatile boolean hasPosted;
+        private volatile boolean hasFinished;
+        private volatile Chunk chunk;
+        private volatile NBTTagCompound pendingLevel;
+
+        PendingChunk(int x, int z, long key, boolean canGenerate, boolean priority) {
+            this.x = x;
+            this.z = z;
+            this.key = key;
+            this.canGenerate = canGenerate;
+            taskPriority = priority ? Priority.HIGH : Priority.NORMAL;
+        }
+
+        private synchronized void setStatus(PendingStatus status) {
+            this.status = status;
+        }
+
+        private Chunk loadChunk(int x, int z) throws IOException {
+            setStatus(PendingStatus.LOADING);
+            Object[] data = chunkLoader.loadChunk(world, x, z, null);
+            if (data != null) {
+                // Level must be loaded on main
+                this.pendingLevel = ((NBTTagCompound) data[1]).getCompound("Level");
+                return (Chunk) data[0];
+            } else {
+                return null;
+            }
+        }
+
+        private Chunk generateChunkExecutor() {
+            IS_CHUNK_THREAD.set(true);
+            IS_CHUNK_GEN_THREAD.set(true);
+            return generateChunk();
+        }
+        private Chunk generateChunk() {
+            synchronized (this) {
+                if (requests.get() <= 0) {
+                    return null;
+                }
+            }
+            CompletableFuture<Chunk> pending = new CompletableFuture<>();
+            batchScheduler.startBatch();
+            batchScheduler.add(new ChunkCoordIntPair(x, z));
+            try {
+                ProtoChunk protoChunk = batchScheduler.executeBatch().join();
+                boolean saved = false;
+                if (!Bukkit.isPrimaryThread()) {
+                    // If we are async, dispatch later
+                    try {
+                        chunkLoader.saveChunk(world, protoChunk, true);
+                        saved = true;
+                    } catch (IOException | ExceptionWorldConflict e) {
+                        e.printStackTrace();
+                    }
+                }
+                Chunk chunk = new Chunk(world, protoChunk, x, z);
+                if (saved) {
+                    chunk.setLastSaved(world.getTime());
+                }
+                generateFinished(chunk);
+
+                return chunk;
+            } catch (Exception e) {
+                MinecraftServer.LOGGER.error("Couldn't generate chunk (" +world.getWorld().getName() + ":" + x + "," + z + ")", e);
+                generateFinished(null);
+                return null;
+            }
+        }
+
+        boolean loadFinished(Chunk chunk) {
+            if (chunk != null) {
+                postChunkToMain(chunk);
+                return false;
+            }
+            loadOnly.complete(null);
+
+            synchronized (this) {
+                boolean cancelled = requests.get() <= 0;
+                if (!canGenerate || cancelled) {
+                    if (!cancelled) {
+                        setStatus(PendingStatus.FAIL);
+                    }
+                    this.chunk = null;
+                    this.hasFinished = true;
+                    pendingChunks.remove(key);
+                    return false;
+                } else {
+                    setStatus(PendingStatus.GENERATING);
+                    generating = true;
+                    return true;
+                }
+            }
+        }
+
+        void generateFinished(Chunk chunk) {
+            synchronized (this) {
+                this.chunk = chunk;
+                this.hasFinished = true;
+            }
+            if (chunk != null) {
+                postChunkToMain(chunk);
+            } else {
+                synchronized (this) {
+                    pendingChunks.remove(key);
+                    completeFutures(null);
+                }
+            }
+        }
+
+        synchronized private void completeFutures(Chunk chunk) {
+            loadOnly.complete(chunk);
+            generate.complete(chunk);
+        }
+
+        private void postChunkToMain(Chunk chunk) {
+            synchronized (this) {
+                setStatus(PendingStatus.PENDING_MAIN);
+                this.chunk = chunk;
+                this.hasFinished = true;
+            }
+
+            // Don't post here, even if on main, it must enter the queue so we can exit any open batch
+            // schedulers, as post stage may trigger a new generation and cause errors
+            synchronized (MAIN_THREAD_QUEUE) {
+                MAIN_THREAD_QUEUE.add(this::postChunk);
+                MAIN_THREAD_QUEUE.notify();
+            }
+        }
+
+        Chunk postChunk() {
+            if (!server.isMainThread()) {
+                throw new IllegalStateException("Must post from main");
+            }
+            synchronized (this) {
+                if (hasPosted || requests.get() <= 0) { // if pending is 0, all were cancelled
+                    return chunk;
+                }
+                hasPosted = true;
+            }
+            try {
+                if (chunk == null) {
+                    chunk = chunks.get(key);
+                    completeFutures(chunk);
+                    return chunk;
+                }
+                if (pendingLevel != null) {
+                    chunkLoader.loadEntities(pendingLevel, chunk);
+                    pendingLevel = null;
+                }
+                synchronized (chunks) {
+                    final Chunk other = chunks.get(key);
+                    if (other != null) {
+                        this.chunk = other;
+                        completeFutures(other);
+                        return other;
+                    }
+                    if (chunk != null) {
+                        chunks.put(key, chunk);
+                    }
+                }
+
+                chunk.addEntities();
+
+                completeFutures(chunk);
+                return chunk;
+            } finally {
+                pendingChunks.remove(key);
+                setStatus(PendingStatus.DONE);
+            }
+        }
+
+        synchronized PendingChunkRequest addListener(CompletableFuture<Chunk> future, boolean gen) {
+            if (hasFinished) {
+                future.complete(chunk);
+                return new PendingChunkRequest(this);
+            } else if (gen) {
+                canGenerate = true;
+                generate.thenAccept(future::complete);
+            } else {
+                if (generating) {
+                    future.complete(null);
+                    return new PendingChunkRequest(this);
+                } else {
+                    loadOnly.thenAccept(future::complete);
+                }
+            }
+
+            requests.incrementAndGet();
+            if (loadTask == null) {
+                // Take care of a race condition in that a request could be cancelled after the synchronize
+                // on pendingChunks, but before a listener is added, which would erase these pending tasks.
+                if (shouldGenSync) {
+                    genTask = generationExecutor.createPendingTask(this::generateChunk, taskPriority);
+                } else {
+                    genTask = generationExecutor.createPendingTask(this::generateChunkExecutor, taskPriority);
+                }
+                loadTask = EXECUTOR.createPendingTask(this, taskPriority);
+                if (!IS_CHUNK_THREAD.get()) {
+                    // We will execute it outside of the synchronized context immediately after
+                    EXECUTOR.submitTask(loadTask);
+                }
+            }
+            return new PendingChunkRequest(this, gen);
+        }
+
+
+        @Override
+        public void run() {
+            IS_CHUNK_THREAD.set(true);
+            try {
+                if (!loadFinished(loadChunk(x, z))) {
+                    return;
+                }
+            } catch (Exception ex) {
+                MinecraftServer.LOGGER.error("Couldn't load chunk (" +world.getWorld().getName() + ":" + x + "," + z + ")", ex);
+                if (ex instanceof IOException) {
+                    generateFinished(null);
+                    return;
+                }
+            }
+
+            if (shouldGenSync) {
+                synchronized (this) {
+                    setStatus(PendingStatus.GENERATION_PENDING);
+                    MAIN_THREAD_QUEUE.add(() -> generateFinished(this.generateChunk()));
+                }
+                synchronized (MAIN_THREAD_QUEUE) {
+                    MAIN_THREAD_QUEUE.notify();
+                }
+            } else {
+                if (IS_CHUNK_GEN_THREAD.get()) {
+                    // ideally we should never run into 1 chunk generating another chunk...
+                    // but if we do, let's apply same solution
+                    genTask.run();
+                } else {
+                    generationExecutor.submitTask(genTask);
+                }
+            }
+        }
+
+        void bumpPriority() {
+            this.taskPriority = Priority.HIGH;
+            PriorityQueuedExecutor.PendingTask<Void> loadTask = this.loadTask;
+            PriorityQueuedExecutor.PendingTask<Chunk> genTask = this.genTask;
+            if (loadTask != null) {
+                loadTask.bumpPriority();
+            }
+            if (genTask != null) {
+                genTask.bumpPriority();
+            }
+        }
+
+        public synchronized boolean isCancelled() {
+            return requests.get() <= 0;
+        }
+
+        public synchronized void cancel(PendingChunkRequest request) {
+            synchronized (pendingChunks) {
+                if (!request.cancelled.compareAndSet(false, true)) {
+                    return;
+                }
+
+                if (requests.decrementAndGet() > 0) {
+                    return;
+                }
+
+                boolean c1 = genTask.cancel();
+                boolean c2 = loadTask.cancel();
+                loadTask = null;
+                genTask = null;
+                pendingChunks.remove(key);
+                setStatus(PendingStatus.CANCELLED);
+            }
+        }
+    }
+
+}
diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
index 2c7c8adf7c..aabd107fe1 100644
--- a/src/main/java/net/minecraft/server/PlayerChunk.java
+++ b/src/main/java/net/minecraft/server/PlayerChunk.java
@@ -29,16 +29,62 @@ public class PlayerChunk {
     // You know the drill, https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse
     // All may seem good at first, but there's deeper issues if you play for a bit
     boolean chunkExists; // Paper
-    private boolean loadInProgress = false;
-    private Runnable loadedRunnable = new Runnable() {
-        public void run() {
-            loadInProgress = false;
-            PlayerChunk.this.chunk = PlayerChunk.this.playerChunkMap.getWorld().getChunkProviderServer().getChunkAt(location.x, location.z, true, true);
-            markChunkUsed(); // Paper - delay chunk unloads
+    PaperAsyncChunkProvider.CancellableChunkRequest chunkRequest;
+    // Paper start
+    private java.util.function.Consumer<Chunk> chunkLoadedConsumer = chunk -> {
+        chunkRequest = null;
+        PlayerChunk pChunk = PlayerChunk.this;
+        if (chunk == null) {
+            new Throwable().printStackTrace();
         }
+        pChunk.chunk = chunk;
+        markChunkUsed(); // Paper - delay chunk unloads
     };
+    private boolean markedHigh = false;
+    void checkHighPriority(EntityPlayer player) {
+        if (done || markedHigh || chunk != null) {
+            return;
+        }
+        final double dist = getDistance(player.locX, player.locZ);
+        if (dist > 8) {
+            return;
+        }
+        if (dist >= 3 && getDistance(player, 5) > 3.5) {
+            return;
+        }
+
+        markedHigh = true;
+        playerChunkMap.getWorld().getChunkProviderServer().bumpPriority(location);
+        if (chunkRequest == null) {
+            requestChunkIfNeeded(PlayerChunkMap.CAN_GEN_CHUNKS.test(player));
+        }
+    }
+    private void requestChunkIfNeeded(boolean flag) {
+        if (chunkRequest == null) {
+            chunkRequest = this.playerChunkMap.getWorld().getChunkProviderServer().requestChunk(this.location.x, this.location.z, flag, markedHigh, chunkLoadedConsumer);
+            this.chunk = chunkRequest.getChunk(); // Paper)
+            markChunkUsed(); // Paper - delay chunk unloads
+        }
+    }
+    private double getDistance(EntityPlayer player, int inFront) {
+        final float yaw = MathHelper.normalizeYaw(player.yaw);
+        final double x = player.locX + (inFront * Math.cos(Math.toRadians(yaw)));
+        final double z = player.locZ + (inFront * Math.sin(Math.toRadians(yaw)));
+        return getDistance(x, z);
+    }
+
+    private double getDistance(double blockX, double blockZ) {
+        final double x = location.x - ((int)Math.floor(blockX) >> 4);
+        final double z = location.z - ((int)Math.floor(blockZ) >> 4);
+        return Math.sqrt((x * x) + (z * z));
+    }
+    // Paper end
     // Paper start - delay chunk unloads
     private void markChunkUsed() {
+        if (!chunkHasPlayers && chunkRequest != null) {
+            chunkRequest.cancel();
+            chunkRequest = null;
+        }
         if (chunk == null) {
             return;
         }
@@ -58,8 +104,8 @@ public class PlayerChunk {
         ChunkProviderServer chunkproviderserver = playerchunkmap.getWorld().getChunkProviderServer();
 
         chunkproviderserver.a(i, j);
-        this.chunk = chunkproviderserver.getChunkAt(i, j, true, false);
-        this.chunkExists = this.chunk != null || ChunkIOExecutor.hasQueuedChunkLoad(playerChunkMap.getWorld(), i, j); // Paper
+        this.chunk = chunkproviderserver.getChunkAt(i, j, false, false); // Paper
+        this.chunkExists = this.chunk != null || chunkproviderserver.chunkGoingToExists(i, j); // Paper
         markChunkUsed(); // Paper - delay chunk unloads
     }
 
@@ -80,7 +126,7 @@ public class PlayerChunk {
             this.c.add(entityplayer);
             if (this.done) {
                 this.sendChunk(entityplayer);
-            }
+            } else checkHighPriority(entityplayer); // Paper
 
         }
     }
@@ -105,8 +151,9 @@ public class PlayerChunk {
         if (this.chunk != null) {
             return true;
         } else {
-            this.chunk = this.playerChunkMap.getWorld().getChunkProviderServer().getChunkAt(this.location.x, this.location.z, true, flag);
-            markChunkUsed(); // Paper - delay chunk unloads
+            // Paper start - async chunks
+            requestChunkIfNeeded(flag);
+            // Paper end
             return this.chunk != null;
         }
     }
diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
index d1a443ca8d..1504bd113b 100644
--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
+++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
@@ -27,10 +27,10 @@ public class PlayerChunkMap {
     };
     private static final Predicate<EntityPlayer> b = (entityplayer) -> {
         return entityplayer != null && (!entityplayer.isSpectator() || entityplayer.getWorldServer().getGameRules().getBoolean("spectatorsGenerateChunks"));
-    };
+    }; static final Predicate<EntityPlayer> CAN_GEN_CHUNKS = b; // Paper - OBFHELPER
     private final WorldServer world;
     private final List<EntityPlayer> managedPlayers = Lists.newArrayList();
-    private final Long2ObjectMap<PlayerChunk> e = new Long2ObjectOpenHashMap(4096);
+    private final Long2ObjectMap<PlayerChunk> e = new Long2ObjectOpenHashMap(4096); Long2ObjectMap<PlayerChunk> getChunks() { return e; } // Paper - OBFHELPER
     private final Set<PlayerChunk> f = Sets.newHashSet();
     private final List<PlayerChunk> g = Lists.newLinkedList();
     private final List<PlayerChunk> h = Lists.newLinkedList();
@@ -349,7 +349,13 @@ public class PlayerChunkMap {
                             if (playerchunk != null) {
                                 playerchunk.b(entityplayer);
                             }
+                        } else { // Paper start
+                            PlayerChunk playerchunk = this.getChunk(l1 - j1, i2 - k1);
+                            if (playerchunk != null) {
+                                playerchunk.checkHighPriority(entityplayer); // Paper
+                            }
                         }
+                        // Paper end
                     }
                 }
 
@@ -360,7 +366,11 @@ public class PlayerChunkMap {
                 // CraftBukkit start - send nearest chunks first
                 Collections.sort(chunksToLoad, new ChunkCoordComparator(entityplayer));
                 for (ChunkCoordIntPair pair : chunksToLoad) {
-                    this.c(pair.x, pair.z).a(entityplayer);
+                    // Paper start
+                    PlayerChunk c = this.c(pair.x, pair.z);
+                    c.checkHighPriority(entityplayer);
+                    c.a(entityplayer);
+                    // Paper end
                 }
                 // CraftBukkit end
             }
@@ -432,6 +442,15 @@ public class PlayerChunkMap {
             }
         }
     }
+
+    void shutdown() {
+        getChunks().values().forEach(pchunk -> {
+                PaperAsyncChunkProvider.CancellableChunkRequest chunkRequest = pchunk.chunkRequest;
+                if (chunkRequest != null) {
+                    chunkRequest.cancel();
+                }
+        });
+    }
     // Paper end
 
     private void e() {
diff --git a/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java b/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java
index 3c35c0f481..187ca2813a 100644
--- a/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java
+++ b/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java
@@ -35,7 +35,7 @@ public class RegionLimitedWorldAccess implements GeneratorAccess {
         this.d = l;
         this.e = i;
         this.f = j;
-        this.g = world;
+        this.g = world.regionLimited(this); // Paper
         this.h = world.getSeed();
         this.m = world.getChunkProvider().getChunkGenerator().getSettings();
         this.i = world.getSeaLevel();
diff --git a/src/main/java/net/minecraft/server/SchedulerBatch.java b/src/main/java/net/minecraft/server/SchedulerBatch.java
index d868149d1a..0d45d933ee 100644
--- a/src/main/java/net/minecraft/server/SchedulerBatch.java
+++ b/src/main/java/net/minecraft/server/SchedulerBatch.java
@@ -9,6 +9,7 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
     private final Scheduler<K, T, R> b;
     private boolean c;
     private int d = 1000;
+    private final java.util.concurrent.locks.ReentrantLock lock = new java.util.concurrent.locks.ReentrantLock(true); // Paper
 
     public SchedulerBatch(Scheduler<K, T, R> scheduler) {
         this.b = scheduler;
@@ -18,8 +19,10 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
         this.b.b();
     }
 
+    public void startBatch() { b(); } // Paper - OBFHELPER
     public void b() {
-        if (this.c) {
+        lock.lock(); // Paper
+        if (false && this.c) { // Paper
             throw new RuntimeException("Batch already started.");
         } else {
             this.d = 1000;
@@ -27,6 +30,7 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
         }
     }
 
+    public CompletableFuture<R> add(K k0) { return a(k0); } // Paper - OBFHELPER
     public CompletableFuture<R> a(K object) {
         if (!this.c) {
             throw new RuntimeException("Batch not properly started. Please use startBatch to create a new batch.");
@@ -42,8 +46,14 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
         }
     }
 
+    public CompletableFuture<R> executeBatch() { return c(); } // Paper - OBFHELPER
     public CompletableFuture<R> c() {
-        if (!this.c) {
+        // Paper start
+        if (!lock.isHeldByCurrentThread()) {
+            throw new IllegalStateException("Current thread does not hold the write lock");
+        }
+        try {// Paper end
+        if (false && !this.c) { // Paper
             throw new RuntimeException("Batch not properly started. Please use startBatch to create a new batch.");
         } else {
             if (this.d != 1000) {
@@ -53,5 +63,6 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
             this.c = false;
             return this.b.c();
         }
+        } finally { lock.unlock(); } // Paper
     }
 }
diff --git a/src/main/java/net/minecraft/server/StructurePiece.java b/src/main/java/net/minecraft/server/StructurePiece.java
index a5cf017da1..def8730b86 100644
--- a/src/main/java/net/minecraft/server/StructurePiece.java
+++ b/src/main/java/net/minecraft/server/StructurePiece.java
@@ -14,7 +14,7 @@ public abstract class StructurePiece {
     private EnumBlockMirror b;
     private EnumBlockRotation c;
     protected int o;
-    private static final Set<Block> d = ImmutableSet.builder().add(Blocks.NETHER_BRICK_FENCE).add(Blocks.TORCH).add(Blocks.WALL_TORCH).add(Blocks.OAK_FENCE).add(Blocks.SPRUCE_FENCE).add(Blocks.DARK_OAK_FENCE).add(Blocks.ACACIA_FENCE).add(Blocks.BIRCH_FENCE).add(Blocks.JUNGLE_FENCE).add(Blocks.LADDER).add(Blocks.IRON_BARS).build();
+    private static final Set<Block> d = ImmutableSet.<Block>builder().add(Blocks.NETHER_BRICK_FENCE).add(Blocks.TORCH).add(Blocks.WALL_TORCH).add(Blocks.OAK_FENCE).add(Blocks.SPRUCE_FENCE).add(Blocks.DARK_OAK_FENCE).add(Blocks.ACACIA_FENCE).add(Blocks.BIRCH_FENCE).add(Blocks.JUNGLE_FENCE).add(Blocks.LADDER).add(Blocks.IRON_BARS).build(); // Paper - decompile error
 
     public StructurePiece() {
     }
@@ -63,11 +63,11 @@ public abstract class StructurePiece {
     }
 
     public static StructurePiece a(List<StructurePiece> list, StructureBoundingBox structureboundingbox) {
-        for(StructurePiece structurepiece : list) {
+        synchronized (list) { for(StructurePiece structurepiece : list) { // Paper - synchronize main structure list
             if (structurepiece.d() != null && structurepiece.d().a(structureboundingbox)) {
                 return structurepiece;
             }
-        }
+        }} // Paper
 
         return null;
     }
diff --git a/src/main/java/net/minecraft/server/StructureStart.java b/src/main/java/net/minecraft/server/StructureStart.java
index f87182b5c4..574930f5fe 100644
--- a/src/main/java/net/minecraft/server/StructureStart.java
+++ b/src/main/java/net/minecraft/server/StructureStart.java
@@ -6,7 +6,7 @@ import java.util.List;
 import java.util.Random;
 
 public abstract class StructureStart {
-    protected final List<StructurePiece> a = Lists.newArrayList();
+    protected final List<StructurePiece> a = java.util.Collections.synchronizedList(Lists.newArrayList()); // Paper
     protected StructureBoundingBox b;
     protected int c;
     protected int d;
@@ -49,9 +49,9 @@ public abstract class StructureStart {
     protected void a(IBlockAccess var1) {
         this.b = StructureBoundingBox.a();
 
-        for(StructurePiece structurepiece : this.a) {
+        synchronized (this.a) {for(StructurePiece structurepiece : this.a) { // Paper - synchronize
             this.b.b(structurepiece.d());
-        }
+        }} // Paper
 
     }
 
@@ -114,9 +114,9 @@ public abstract class StructureStart {
         int l = k - this.b.e;
         this.b.a(0, l, 0);
 
-        for(StructurePiece structurepiece : this.a) {
+        synchronized (this.a) {for(StructurePiece structurepiece : this.a) { // Paper - synchronize
             structurepiece.a(0, l, 0);
-        }
+        }} // Paper
 
     }
 
@@ -132,9 +132,9 @@ public abstract class StructureStart {
         int i1 = l - this.b.b;
         this.b.a(0, i1, 0);
 
-        for(StructurePiece structurepiece : this.a) {
+        synchronized (this.a) {for(StructurePiece structurepiece : this.a) { // Paper - synchronize
             structurepiece.a(0, i1, 0);
-        }
+        }} // Paper
 
     }
 
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index a2559f0c19..bbcedb8fc7 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -46,7 +46,7 @@ import org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason;
 import org.bukkit.generator.ChunkGenerator;
 // CraftBukkit end
 
-public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAccess, AutoCloseable {
+public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAccess, AutoCloseable, Cloneable { // Paper
 
     protected static final Logger e = LogManager.getLogger();
     private static final EnumDirection[] a = EnumDirection.values();
@@ -109,6 +109,24 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
     protected PersistentVillage villages;
     public final MethodProfiler methodProfiler;
     public final boolean isClientSide;
+    // Paper start - yes this is hacky as shit
+    RegionLimitedWorldAccess regionLimited;
+    World originalWorld;
+    public World regionLimited(RegionLimitedWorldAccess limitedWorldAccess) {
+        try {
+            World clone = (World) super.clone();
+            clone.regionLimited = limitedWorldAccess;
+            clone.originalWorld = this;
+            return clone;
+        } catch (CloneNotSupportedException e1) {
+        }
+        return null;
+    }
+    ChunkCoordIntPair[] strongholdCoords;
+    final java.util.concurrent.atomic.AtomicBoolean
+        strongholdInit = new java.util.concurrent.atomic.AtomicBoolean
+        (false);
+    // Paper end
     public boolean allowMonsters;
     public boolean allowAnimals;
     private boolean J;
@@ -742,17 +760,42 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
 
     }
 
-    public IBlockData getType(BlockPosition blockposition) {
-        // CraftBukkit start - tree generation
+    // Paper - async variant
+    public java.util.concurrent.CompletableFuture<IBlockData> getTypeAsync(BlockPosition blockposition) {
+        int x = blockposition.getX();
+        int z = blockposition.getZ();
         if (captureTreeGeneration) {
             Iterator<CraftBlockState> it = capturedBlockStates.iterator();
             while (it.hasNext()) {
                 CraftBlockState previous = it.next();
-                if (previous.getX() == blockposition.getX() && previous.getY() == blockposition.getY() && previous.getZ() == blockposition.getZ()) {
-                    return previous.getHandle();
+                if (previous.getX() == x && previous.getY() == blockposition.getY() && previous.getZ() == z) {
+                    return java.util.concurrent.CompletableFuture.completedFuture(previous.getHandle());
                 }
             }
         }
+        if (blockposition.isInvalidYLocation()) {
+            return java.util.concurrent.CompletableFuture.completedFuture(Blocks.VOID_AIR.getBlockData());
+        } else {
+            java.util.concurrent.CompletableFuture<IBlockData> future = new java.util.concurrent.CompletableFuture<>();
+            ((ChunkProviderServer) chunkProvider).getChunkAt(x << 4, z << 4, true, true, (chunk) -> {
+                future.complete(chunk.getType(blockposition));
+            });
+            return future;
+        }
+    }
+    // Paper end
+
+    public IBlockData getType(BlockPosition blockposition) {
+        // CraftBukkit start - tree generation
+        if (captureTreeGeneration) { // If any of this logic updates, update async variant above
+            Iterator<CraftBlockState> it = capturedBlockStates.iterator();
+            while (it.hasNext()) { // If any of this logic updates, update async variant above
+                CraftBlockState previous = it.next();
+                if (previous.getX() == blockposition.getX() && previous.getY() == blockposition.getY() && previous.getZ() == blockposition.getZ()) {
+                    return previous.getHandle(); // If any of this logic updates, update async variant above
+                }
+            } // If any of this logic updates, update async variant above
+        }
         // CraftBukkit end
         if (blockposition.isInvalidYLocation()) { // Paper
             return Blocks.VOID_AIR.getBlockData();
@@ -1023,6 +1066,11 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
     }
 
     public boolean addEntity(Entity entity, SpawnReason spawnReason) { // Changed signature, added SpawnReason
+        // Paper start
+        if (regionLimited != null) {
+            return regionLimited.addEntity(entity, spawnReason);
+        }
+        // Paper end
         org.spigotmc.AsyncCatcher.catchOp( "entity add"); // Spigot
         if (entity == null) return false;
         if (entity.valid) { MinecraftServer.LOGGER.error("Attempted Double World add on " + entity, new Throwable()); return true; } // Paper
diff --git a/src/main/java/net/minecraft/server/WorldGenStronghold.java b/src/main/java/net/minecraft/server/WorldGenStronghold.java
index fa99fe0146..4f49786aa3 100644
--- a/src/main/java/net/minecraft/server/WorldGenStronghold.java
+++ b/src/main/java/net/minecraft/server/WorldGenStronghold.java
@@ -9,24 +9,29 @@ import java.util.Random;
 import javax.annotation.Nullable;
 
 public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrongholdConfiguration> {
-    private boolean b;
-    private ChunkCoordIntPair[] c;
-    private long d;
+    // Paper start - no shared state
+    //private boolean b;
+    //private ChunkCoordIntPair[] c;
+    //private long d;
+    // Paper end
 
     public WorldGenStronghold() {
     }
 
     protected boolean a(ChunkGenerator<?> chunkgenerator, Random var2, int i, int j) {
-        if (this.d != chunkgenerator.getSeed()) {
+        // Paper start
+        /*if (this.d != chunkgenerator.getSeed()) {
             this.c();
-        }
+        }*/
 
-        if (!this.b) {
+        synchronized (chunkgenerator.getWorld().strongholdInit) {
+        if (chunkgenerator.getWorld().strongholdInit.compareAndSet(false, true)) { // Paper
             this.a(chunkgenerator);
-            this.b = true;
-        }
+            //this.b = true;
+        }} // Paper
+        // Paper end
 
-        for(ChunkCoordIntPair chunkcoordintpair : this.c) {
+        for(ChunkCoordIntPair chunkcoordintpair : chunkgenerator.getWorld().strongholdCoords) { // Paper
             if (i == chunkcoordintpair.x && j == chunkcoordintpair.z) {
                 return true;
             }
@@ -36,8 +41,8 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
     }
 
     private void c() {
-        this.b = false;
-        this.c = null;
+        //this.b = false; // Paper
+        //this.c = null; // Paper
     }
 
     protected boolean a(GeneratorAccess generatoraccess) {
@@ -69,20 +74,23 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
         if (!chunkgenerator.getWorldChunkManager().a(this)) {
             return null;
         } else {
-            if (this.d != world.getSeed()) {
-                this.c();
-            }
+            // Paper start
+        /*if (this.d != chunkgenerator.getSeed()) {
+            this.c();
+        }*/
 
-            if (!this.b) {
-                this.a(chunkgenerator);
-                this.b = true;
-            }
+            synchronized (chunkgenerator.getWorld().strongholdInit) {
+                if (chunkgenerator.getWorld().strongholdInit.compareAndSet(false, true)) { // Paper
+                    this.a(chunkgenerator);
+                    //this.b = true;
+                }} // Paper
+            // Paper end
 
             BlockPosition blockposition1 = null;
             BlockPosition.MutableBlockPosition blockposition$mutableblockposition = new BlockPosition.MutableBlockPosition(0, 0, 0);
             double d0 = Double.MAX_VALUE;
 
-            for(ChunkCoordIntPair chunkcoordintpair : this.c) {
+            for(ChunkCoordIntPair chunkcoordintpair : chunkgenerator.getWorld().strongholdCoords) { // Paper
                 blockposition$mutableblockposition.c((chunkcoordintpair.x << 4) + 8, 32, (chunkcoordintpair.z << 4) + 8);
                 double d1 = blockposition$mutableblockposition.n(blockposition);
                 if (blockposition1 == null) {
@@ -99,7 +107,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
     }
 
     private void a(ChunkGenerator<?> chunkgenerator) {
-        this.d = chunkgenerator.getSeed();
+        //this.d = chunkgenerator.getSeed(); // Paper
         ArrayList arraylist = Lists.newArrayList();
 
         for(BiomeBase biomebase : IRegistry.BIOME) {
@@ -111,7 +119,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
         int i2 = chunkgenerator.getSettings().e();
         int j2 = chunkgenerator.getSettings().f();
         int i = chunkgenerator.getSettings().g();
-        this.c = new ChunkCoordIntPair[j2];
+        ChunkCoordIntPair[] strongholdCoords = chunkgenerator.getWorld().strongholdCoords = new ChunkCoordIntPair[j2]; // Paper
         int j = 0;
         Long2ObjectMap long2objectmap = chunkgenerator.getStructureStartCache(this);
         synchronized(long2objectmap) {
@@ -119,8 +127,8 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
 
             while(objectiterator.hasNext()) {
                 StructureStart structurestart = (StructureStart)objectiterator.next();
-                if (j < this.c.length) {
-                    this.c[j++] = new ChunkCoordIntPair(structurestart.e(), structurestart.f());
+                if (j < strongholdCoords.length) { // Paper
+                    strongholdCoords[j++] = new ChunkCoordIntPair(structurestart.e(), structurestart.f()); // Paper
                 }
             }
         }
@@ -129,11 +137,11 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
         random.setSeed(chunkgenerator.getSeed());
         double d1 = random.nextDouble() * Math.PI * 2.0D;
         int k = long2objectmap.size();
-        if (k < this.c.length) {
+        if (k < strongholdCoords.length) { // Paper
             int l = 0;
             int i1 = 0;
 
-            for(int j1 = 0; j1 < this.c.length; ++j1) {
+            for(int j1 = 0; j1 < strongholdCoords.length; ++j1) { // Paper
                 double d0 = (double)(4 * i2 + i2 * i1 * 6) + (random.nextDouble() - 0.5D) * (double)i2 * 2.5D;
                 int k1 = (int)Math.round(Math.cos(d1) * d0);
                 int l1 = (int)Math.round(Math.sin(d1) * d0);
@@ -144,7 +152,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
                 }
 
                 if (j1 >= k) {
-                    this.c[j1] = new ChunkCoordIntPair(k1, l1);
+                    strongholdCoords[j1] = new ChunkCoordIntPair(k1, l1); // Paper
                 }
 
                 d1 += (Math.PI * 2D) / (double)i;
@@ -153,7 +161,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
                     ++i1;
                     l = 0;
                     i = i + 2 * i / (i1 + 1);
-                    i = Math.min(i, this.c.length - j1);
+                    i = Math.min(i, strongholdCoords.length - j1); // Paper
                     d1 += random.nextDouble() * Math.PI * 2.0D;
                 }
             }
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index ad3fea9c97..0a76482638 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -731,7 +731,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
             gen = new org.bukkit.craftbukkit.generator.NormalChunkGenerator(this, this.getSeed());
         }
 
-        return new ChunkProviderServer(this, ichunkloader, gen, this.server);
+        return com.destroystokyo.paper.PaperConfig.asyncChunks ? new PaperAsyncChunkProvider(this, ichunkloader, gen, this.server) : new ChunkProviderServer(this, ichunkloader, gen, this.server); // Paper - async chunks
         // CraftBukkit end
     }
 
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
index cb2255a5d0..19f2f2bbd8 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
@@ -1014,8 +1014,12 @@ public final class CraftServer implements Server {
         if (internal.getWorld().getKeepSpawnInMemory()) {
             short short1 = internal.paperConfig.keepLoadedRange; // Paper
             long i = System.currentTimeMillis();
-            for (int j = -short1; j <= short1; j += 16) {
-                for (int k = -short1; k <= short1; k += 16) {
+            // Paper start
+            for (ChunkCoordIntPair coords : internal.getChunkProviderServer().getSpiralOutChunks(internal.getSpawn(), short1 >> 4)) {{
+                    int j = coords.x;
+                    int k = coords.z;
+                // Paper end
+
                     long l = System.currentTimeMillis();
 
                     if (l < i) {
@@ -1031,7 +1035,7 @@ public final class CraftServer implements Server {
                     }
 
                     BlockPosition chunkcoordinates = internal.getSpawn();
-                    internal.getChunkProviderServer().getChunkAt(chunkcoordinates.getX() + j >> 4, chunkcoordinates.getZ() + k >> 4, true, true);
+                    internal.getChunkProviderServer().getChunkAt(chunkcoordinates.getX() + j >> 4, chunkcoordinates.getZ() + k >> 4, true, true, c -> {}); // Paper
                 }
             }
         }
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
index d0110070a9..02b6bf2990 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
@@ -157,6 +157,16 @@ public class CraftWorld implements World {
         }
     }
 
+    // Paper start - Async chunk load API
+    @Override
+    public java.util.concurrent.CompletableFuture<Chunk> getChunkAtAsync(final int x, final int z, final boolean gen) {
+        final ChunkProviderServer cps = this.world.getChunkProviderServer();
+        java.util.concurrent.CompletableFuture<Chunk> future = new java.util.concurrent.CompletableFuture<>();
+        cps.getChunkAt(x, z, true, gen, chunk -> future.complete(chunk != null ? chunk.bukkitChunk : null));
+        return future;
+    }
+    // Paper end
+
     public Chunk getChunkAt(int x, int z) {
         return this.world.getChunkProviderServer().getChunkAt(x, z, true, true).bukkitChunk;
     }
@@ -1333,10 +1343,13 @@ public class CraftWorld implements World {
         int chunkCoordZ = chunkcoordinates.getZ() >> 4;
         // Cycle through the 25x25 Chunks around it to load/unload the chunks.
         int radius = world.paperConfig.keepLoadedRange / 16; // Paper
-        for (int x = -radius; x <= radius; x++) { // Paper
-            for (int z = -radius; z <= radius; z++) { // Paper
+        // Paper start
+        for (ChunkCoordIntPair coords : world.getChunkProviderServer().getSpiralOutChunks(world.getSpawn(), radius)) {{
+            int x = coords.x;
+            int z = coords.z;
+            // Paper end
                 if (keepLoaded) {
-                    loadChunk(chunkCoordX + x, chunkCoordZ + z);
+                    getChunkAtAsync(chunkCoordX + x, chunkCoordZ + z, chunk -> {}); // Paper - Async Chunks
                 } else {
                     if (isChunkLoaded(chunkCoordX + x, chunkCoordZ + z)) {
                         unloadChunk(chunkCoordX + x, chunkCoordZ + z);
diff --git a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
index 9e903159d9..4ead18b66c 100644
--- a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+++ b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
@@ -78,6 +78,7 @@ public class CraftEventFactory {
     public static final DamageSource POISON = CraftDamageSource.copyOf(DamageSource.MAGIC);
     public static org.bukkit.block.Block blockDamage; // For use in EntityDamageByBlockEvent
     public static Entity entityDamage; // For use in EntityDamageByEntityEvent
+    public static boolean isWorldGen(GeneratorAccess world) { return world instanceof net.minecraft.server.RegionLimitedWorldAccess; } // Paper
 
     // helper methods
     private static boolean canBuild(CraftWorld world, Player player, int x, int z) {
@@ -301,6 +302,7 @@ public class CraftEventFactory {
         CraftServer craftServer = (CraftServer) entity.getServer();
 
         CreatureSpawnEvent event = new CreatureSpawnEvent(entity, spawnReason);
+        if (isWorldGen(entityliving.world)) return event; // Paper - do not call during world gen
         craftServer.getPluginManager().callEvent(event);
         return event;
     }
@@ -948,6 +950,7 @@ public class CraftEventFactory {
         }
 
         BlockIgniteEvent event = new BlockIgniteEvent(bukkitWorld.getBlockAt(block.getX(), block.getY(), block.getZ()), cause, igniter);
+        if (isWorldGen(world)) return event; // Paper - do not call during world gen
         world.getServer().getPluginManager().callEvent(event);
         return event;
     }
@@ -972,6 +975,7 @@ public class CraftEventFactory {
         }
 
         BlockIgniteEvent event = new BlockIgniteEvent(bukkitWorld.getBlockAt(pos.getX(), pos.getY(), pos.getZ()), cause, bukkitIgniter);
+        if (isWorldGen(world)) return event; // Paper - do not call during world gen
         world.getServer().getPluginManager().callEvent(event);
         return event;
     }
@@ -1179,7 +1183,8 @@ public class CraftEventFactory {
     public static BlockPhysicsEvent callBlockPhysicsEvent(GeneratorAccess world, BlockPosition blockposition) {
         org.bukkit.block.Block block = CraftBlock.at(world, blockposition);
         BlockPhysicsEvent event = new BlockPhysicsEvent(block, block.getBlockData());
-        world.getMinecraftWorld().getMinecraftServer().server.getPluginManager().callEvent(event);
+        if (isWorldGen(world)) return event; // Paper - do not call during world gen
+        Bukkit.getPluginManager().callEvent(event); // Paper
         return event;
     }
 
@@ -1215,6 +1220,7 @@ public class CraftEventFactory {
         }
 
         EntityPotionEffectEvent event = new EntityPotionEffectEvent((LivingEntity) entity.getBukkitEntity(), bukkitOldEffect, bukkitNewEffect, cause, action, willOverride);
+        if (isWorldGen(entity.world)) return event; // Paper - do not call during world gen
         Bukkit.getPluginManager().callEvent(event);
 
         return event;
@@ -1233,6 +1239,7 @@ public class CraftEventFactory {
         blockState.setData(block);
 
         BlockFormEvent event = (entity == null) ? new BlockFormEvent(blockState.getBlock(), blockState) : new EntityBlockFormEvent(entity.getBukkitEntity(), blockState.getBlock(), blockState);
+        if (isWorldGen(world)) return true; // Paper - do not call during world gen
         world.getServer().getPluginManager().callEvent(event);
 
         if (!event.isCancelled()) {
diff --git a/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java b/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java
index 9c2adb2351..62c197b80d 100644
--- a/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java
+++ b/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java
@@ -21,6 +21,7 @@ public class CustomChunkGenerator extends InternalChunkGenerator<GeneratorSettin
     private final WorldServer world;
     private final long seed;
     private final Random random;
+    public final boolean asyncSupported; // Paper
     private final WorldChunkManager chunkManager;
     private final WorldGenStronghold strongholdGen = new WorldGenStronghold();
     private final GeneratorSettingsDefault settings = new GeneratorSettingsDefault();
@@ -43,6 +44,15 @@ public class CustomChunkGenerator extends InternalChunkGenerator<GeneratorSettin
         this.world = (WorldServer) world;
         this.generator = generator;
         this.seed = seed;
+        // Paper start
+        boolean asyncSupported = false;
+        try {
+            java.lang.reflect.Field asyncSafe = generator.getClass().getDeclaredField("PAPER_ASYNC_SAFE");
+            asyncSafe.setAccessible(true);
+            asyncSupported = asyncSafe.getBoolean(generator);
+        } catch (NoSuchFieldException | IllegalAccessException ignored) {}
+        this.asyncSupported = asyncSupported;
+        // Paper end
 
         this.random = new Random(seed);
         this.chunkManager = world.worldProvider.getChunkGenerator().getWorldChunkManager();
-- 
2.19.1