Cache palette array (#6767)

Instead of allocating the 4KB for every chunk section, cache it locally and
reuse it for other chunk sections to save on allocations. These allocations add
up very quickly when saving chunks frequently.

For example, with 30,000 chunks and 16 chunk sections per chunk (which will
actually get larger in 1.18) it allocates nearly 2GB, which this patch
eliminates entirely.

patches/server/0836-Cache-palette-array.patch

@@ -0,0 +1,61 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Paul Sauve <paul@technove.co>
+Date: Thu, 4 Feb 2021 23:28:46 -0600
+Subject: [PATCH] Cache palette array
+
+Instead of allocating the 4KB for every chunk section, cache it locally and
+reuse it for other chunk sections to save on allocations. These allocations add
+up very quickly when saving chunks frequently.
+
+diff --git a/src/main/java/net/minecraft/world/level/chunk/PalettedContainer.java b/src/main/java/net/minecraft/world/level/chunk/PalettedContainer.java
+index c9e942669458668a184aaec3bc0a5509dd6ab5f0..ac84d49d7819666a89cacbe9ef1199cf22ac2ac3 100644
+--- a/src/main/java/net/minecraft/world/level/chunk/PalettedContainer.java
++++ b/src/main/java/net/minecraft/world/level/chunk/PalettedContainer.java
+@@ -263,13 +263,17 @@ public class PalettedContainer<T> implements PaletteResize<T> {
+ 
+     }
+ 
++    // Paper start - allow reusing int array
+     public synchronized void write(CompoundTag nbt, String paletteKey, String dataKey) { // Paper - synchronize
++        this.write(nbt, paletteKey, dataKey, new int[4096]);
++    }
++    public synchronized void write(CompoundTag nbt, String paletteKey, String dataKey, int[] is) { // Paper - synchronize // Paper end
+         try {
+             this.acquire();
+             HashMapPalette<T> hashMapPalette = new HashMapPalette<>(this.registry, this.bits, this.dummyPaletteResize, this.reader, this.writer);
+             T object = this.defaultValue;
+             int i = hashMapPalette.idFor(this.defaultValue);
+-            int[] is = new int[4096];
++            //int[] is = new int[4096]; // Paper - will now be pulled from ThreadLocal
+ 
+             for(int j = 0; j < 4096; ++j) {
+                 T object2 = this.get(j);
+diff --git a/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java b/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java
+index 7921ee2786d0d6a60d43786b20efc03a0f9178e3..10f15b8c7763a980aec47a4b4dda5686e60642ce 100644
+--- a/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java
++++ b/src/main/java/net/minecraft/world/level/chunk/storage/ChunkSerializer.java
+@@ -500,6 +500,7 @@ public class ChunkSerializer {
+         return new AsyncSaveData(blockLight, skyLight, blockTickListSerialized, fluidTickListSerialized, blockEntitiesSerialized, world.getGameTime());
+     }
+ 
++    private static final ThreadLocal<int[]> PALETTE_ARRAY = ThreadLocal.withInitial(() -> new int[4096]); // Paper - maintain a per-thread cache for saving palettes
+     public static CompoundTag write(ServerLevel world, ChunkAccess chunk) {
+         return saveChunk(world, chunk, null);
+     }
+@@ -533,6 +534,7 @@ public class ChunkSerializer {
+         ThreadedLevelLightEngine lightenginethreaded = world.getChunkSource().getLightEngine();
+         boolean flag = chunk.isLightCorrect();
+ 
++        int[] is = PALETTE_ARRAY.get(); // Paper - use cached
+         for (int i = lightenginethreaded.getMinLightSection(); i < lightenginethreaded.getMaxLightSection(); ++i) {
+             int finalI = i; // CraftBukkit - decompile errors
+             LevelChunkSection chunksection = (LevelChunkSection) Arrays.stream(achunksection).filter((chunksection1) -> {
+@@ -547,7 +549,7 @@ public class ChunkSerializer {
+ 
+                 nbttagcompound2.putByte("Y", (byte) (i & 255));
+                 if (chunksection != LevelChunk.EMPTY_SECTION) {
+-                    chunksection.getStates().write(nbttagcompound2, "Palette", "BlockStates");
++                    chunksection.getStates().write(nbttagcompound2, "Palette", "BlockStates", is); // Paper - reuse array
+                 }
+ 
+                 // Paper start - replace light engine