Improve Thread Pool usage to allow single threads for single cpu servers

Switch to a standard fixed size ThreadPoolExecutor as we don't use the
advanced capabilities of a ForkJoinPool.

ForkJoinPool does not allow single threads, and really rather not use
2 different executor types based on core count.

Also, change thread priorities so that main thread is prioritized by
the OS at a higher priority than the other threads. May not help too much
but it at least signals the OS the information to know main is more important.

Spigot-Server-Patches/Asynchronous-chunk-IO-and-loading.patch

@@ -3830,19 +3830,6 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      }
 +    // Paper end
  }
-diff --git a/src/main/java/net/minecraft/server/SystemUtils.java b/src/main/java/net/minecraft/server/SystemUtils.java
-index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
---- a/src/main/java/net/minecraft/server/SystemUtils.java
-+++ b/src/main/java/net/minecraft/server/SystemUtils.java
-@@ -0,0 +0,0 @@ public class SystemUtils {
-         i = Integer.getInteger("Paper.WorkerThreadCount", i); // Paper - allow overriding
-         Object object;
- 
--        if (i <= 0) {
-+        if (i <= 0 || (Runtime.getRuntime().availableProcessors() == 1 && !Boolean.getBoolean("Paper.allowAsyncChunksSingleCore"))) { // Paper - disable server worker queue if single core system
-             object = MoreExecutors.newDirectExecutorService();
-         } else {
-             object = new ForkJoinPool(i, (forkjoinpool) -> {
 diff --git a/src/main/java/net/minecraft/server/TicketType.java b/src/main/java/net/minecraft/server/TicketType.java
 index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
 --- a/src/main/java/net/minecraft/server/TicketType.java

Spigot-Server-Patches/Improve-Server-Thread-Pool-and-Thread-Priorities.patch

@@ -0,0 +1,90 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Tue, 23 Oct 2018 23:14:38 -0400
+Subject: [PATCH] Improve Server Thread Pool and Thread Priorities
+
+Use a simple executor since Fork join is a much more complex pool
+type and we are not using its capabilities.
+
+Set thread priorities so main thread has above normal priority over
+server threads
+
+Allow usage of a single thread executor by not using ForkJoin so single core CPU's.
+
+diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/net/minecraft/server/MinecraftServer.java
++++ b/src/main/java/net/minecraft/server/MinecraftServer.java
+@@ -0,0 +0,0 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
+                 dedicatedserver.setEraseCache(true);
+             }
+ 
++            dedicatedserver.serverThread.setPriority(Thread.NORM_PRIORITY+2); // Paper - boost priority
+             dedicatedserver.serverThread.start();
+             // CraftBukkit end
+         } catch (Exception exception) {
+diff --git a/src/main/java/net/minecraft/server/ServerWorkerThread.java b/src/main/java/net/minecraft/server/ServerWorkerThread.java
+new file mode 100644
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000
+--- /dev/null
++++ b/src/main/java/net/minecraft/server/ServerWorkerThread.java
+@@ -0,0 +0,0 @@
++package net.minecraft.server;
++
++import java.util.concurrent.CompletionException;
++import java.util.concurrent.atomic.AtomicInteger;
++
++public class ServerWorkerThread extends Thread {
++    private static final AtomicInteger threadId = new AtomicInteger(1);
++    public ServerWorkerThread(Runnable target) {
++        super(target, "Server-Worker-" + threadId.getAndIncrement());
++        setPriority(Thread.NORM_PRIORITY-1); // Deprioritize over main
++        this.setUncaughtExceptionHandler((thread, throwable) -> {
++            if (throwable instanceof CompletionException) {
++                throwable = throwable.getCause();
++            }
++
++            if (throwable instanceof ReportedException) {
++                DispenserRegistry.a(((ReportedException) throwable).a().e());
++                System.exit(-1);
++            }
++
++            MinecraftServer.LOGGER.error(String.format("Caught exception in thread %s", thread), throwable);
++        });
++    }
++}
+diff --git a/src/main/java/net/minecraft/server/SystemUtils.java b/src/main/java/net/minecraft/server/SystemUtils.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/net/minecraft/server/SystemUtils.java
++++ b/src/main/java/net/minecraft/server/SystemUtils.java
+@@ -0,0 +0,0 @@ public class SystemUtils {
+     }
+ 
+     private static ExecutorService k() {
+-        int i = MathHelper.clamp(Runtime.getRuntime().availableProcessors() - 1, 1, 7);
+-        Object object;
++        // Paper start - use simpler thread pool that allows 1 thread
++        int i = Math.min(8, Math.max(Runtime.getRuntime().availableProcessors() - 2, 1));
++        i = Integer.getInteger("Paper.WorkerThreadCount", i);
++        ExecutorService object;
+ 
+         if (i <= 0) {
+             object = MoreExecutors.newDirectExecutorService();
+         } else {
+-            object = new ForkJoinPool(i, (forkjoinpool) -> {
+-                ForkJoinWorkerThread forkjoinworkerthread = new ForkJoinWorkerThread(forkjoinpool) {
++            object = new java.util.concurrent.ThreadPoolExecutor(i, i,0L, TimeUnit.MILLISECONDS, new java.util.concurrent.LinkedBlockingQueue<Runnable>(), ServerWorkerThread::new);
++        }
++        /*
+                     protected void onTermination(Throwable throwable) {
+                         if (throwable != null) {
+                             SystemUtils.LOGGER.warn("{} died", this.getName(), throwable);
+@@ -0,0 +0,0 @@ public class SystemUtils {
+ 
+                 SystemUtils.LOGGER.error(String.format("Caught exception in thread %s", thread), throwable);
+             }, true);
+-        }
++        }*/ // Paper end
+ 
+         return (ExecutorService) object;
+     }

Spigot-Server-Patches/Use-more-reasonable-thread-count-default-for-bootstr.patch

@@ -1,20 +0,0 @@
-From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
-From: Aikar <aikar@aikar.co>
-Date: Tue, 23 Oct 2018 23:14:38 -0400
-Subject: [PATCH] Use more reasonable thread count default for bootstrap
-
-
-diff --git a/src/main/java/net/minecraft/server/SystemUtils.java b/src/main/java/net/minecraft/server/SystemUtils.java
-index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
---- a/src/main/java/net/minecraft/server/SystemUtils.java
-+++ b/src/main/java/net/minecraft/server/SystemUtils.java
-@@ -0,0 +0,0 @@ public class SystemUtils {
-     }
- 
-     private static ExecutorService k() {
--        int i = MathHelper.clamp(Runtime.getRuntime().availableProcessors() - 1, 1, 7);
-+        int i = Math.min(8, Math.max(Runtime.getRuntime().availableProcessors() - 2, 2)); // Paper - use more reasonable default - 2 is hard minimum to avoid using unlimited threads
-+        i = Integer.getInteger("Paper.WorkerThreadCount", i); // Paper - allow overriding
-         Object object;
- 
-         if (i <= 0) {