Optimize Chunk Unload Queue

Removing chunks from the unload queue when performing chunk lookups is a costly activity.

It drastically slows down server performance as many methods call getChunkAt, resulting in a bandaid
to skip removing chunks from the unload queue.

This patch optimizes the unload queue to instead use a boolean on the Chunk object itself to mark
if the chunk is active, and then insert into a LinkedList queue.

The benefits here is that all chunk unload queue actions are now O(1) constant time.

A LinkedList will never need to resize, and can be removed from in constant time when
used in a queue like method.

We mark the chunk as active in many places that notify it is still being used, so that
when the chunk unload queue reaches that chunk, and sees the chunk became active again,
it will skip it and move to next.

Spigot-Server-Patches/0091-Optimize-Chunk-Unload-Queue.patch

@@ -0,0 +1,177 @@
+From 5036fed452975d8857105532162be561fcd8effc Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Fri, 18 Mar 2016 17:57:25 -0400
+Subject: [PATCH] Optimize Chunk Unload Queue
+
+Removing chunks from the unload queue when performing chunk lookups is a costly activity.
+
+It drastically slows down server performance as many methods call getChunkAt, resulting in a bandaid
+to skip removing chunks from the unload queue.
+
+This patch optimizes the unload queue to instead use a boolean on the Chunk object itself to mark
+if the chunk is active, and then insert into a LinkedList queue.
+
+The benefits here is that all chunk unload queue actions are now O(1) constant time.
+
+A LinkedList will never need to resize, and can be removed from in constant time when
+used in a queue like method.
+
+We mark the chunk as active in many places that notify it is still being used, so that
+when the chunk unload queue reaches that chunk, and sees the chunk became active again,
+it will skip it and move to next.
+
+diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
+index b6d84d7..a4c0b4e 100644
+--- a/src/main/java/net/minecraft/server/Chunk.java
++++ b/src/main/java/net/minecraft/server/Chunk.java
+@@ -55,6 +55,8 @@ public class Chunk {
+     // Keep this synced with entitySlices.add() and entitySlices.remove()
+     private final int[] itemCounts = new int[16];
+     private final int[] inventoryEntityCounts = new int[16];
++    public boolean isChunkActive = true;
++    public boolean isInUnloadQueue = false;
+     // Paper end
+ 
+     // CraftBukkit start - Neighbor loaded cache for chunk lighting and entity ticking
+@@ -780,6 +782,7 @@ public class Chunk {
+     }
+ 
+     public void addEntities() {
++        isChunkActive = true; // Paper
+         this.i = true;
+         this.world.b(this.tileEntities.values());
+ 
+@@ -798,6 +801,7 @@ public class Chunk {
+     }
+ 
+     public void removeEntities() {
++        isChunkActive = false; // Paper
+         this.i = false;
+         Iterator iterator = this.tileEntities.values().iterator();
+ 
+diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+index 9ef6246..f62a2c7 100644
+--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
++++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+@@ -21,7 +21,7 @@ import org.bukkit.event.world.ChunkUnloadEvent;
+ public class ChunkProviderServer implements IChunkProvider {
+ 
+     private static final Logger a = LogManager.getLogger();
+-    public final LongHashSet unloadQueue = new LongHashSet(); // CraftBukkit - LongHashSet
++    public final ChunkUnloadQueue unloadQueue = new ChunkUnloadQueue(); // CraftBukkit - LongHashSet // Paper -> ChunkUnloadQueue
+     public final ChunkGenerator chunkGenerator; // CraftBukkit - public
+     private final IChunkLoader chunkLoader;
+     public LongObjectHashMap<Chunk> chunks = new LongObjectHashMap<Chunk>(); // CraftBukkit
+@@ -79,18 +79,18 @@ public class ChunkProviderServer implements IChunkProvider {
+ 
+     // CraftBukkit start - Add async variant, provide compatibility
+     public Chunk getOrCreateChunkFast(int x, int z) {
+-        Chunk chunk = chunks.get(LongHash.toLong(x, z));
+-        return (chunk == null) ? getChunkAt(x, z) : chunk;
++        return getChunkAt(x, z); // Paper
+     }
+ 
+     public Chunk getChunkIfLoaded(int x, int z) {
+-        return chunks.get(LongHash.toLong(x, z));
++        return getLoadedChunkAt(x, z); // Paper - Bukkit has a duplicate method now.
+     }
+ 
+     public Chunk getLoadedChunkAt(int i, int j) {
+         Chunk chunk = chunks.get(LongHash.toLong(i, j)); // CraftBukkit
+ 
+         this.unloadQueue.remove(i, j); // CraftBukkit
++        if (chunk != null) { chunk.isChunkActive = true; }// Paper
+         return chunk;
+     }
+ 
+@@ -151,6 +151,7 @@ public class ChunkProviderServer implements IChunkProvider {
+             runnable.run();
+         }
+ 
++        chunk.isChunkActive = true; // Paper
+         return chunk;
+     }
+ 
+@@ -300,10 +301,17 @@ public class ChunkProviderServer implements IChunkProvider {
+         if (!this.world.savingDisabled) {
+             // CraftBukkit start
+             Server server = this.world.getServer();
+-            for (int i = 0; i < 100 && !this.unloadQueue.isEmpty(); ++i) {
+-                long chunkcoordinates = this.unloadQueue.popFirst();
+-                Chunk chunk = this.chunks.get(chunkcoordinates);
+-                if (chunk == null) continue;
++            // Paper start
++            final Iterator<Chunk> iterator = this.unloadQueue.iterator();
++            for (int i = 0; i < 100 && iterator.hasNext(); ++i) {
++                Chunk chunk = iterator.next();
++                iterator.remove();
++                long chunkcoordinates = LongHash.toLong(chunk.locX, chunk.locZ);
++                chunk.isInUnloadQueue = false;
++                if (chunk.isChunkActive) {
++                    continue;
++                }
++                // Paper end
+ 
+                 ChunkUnloadEvent event = new ChunkUnloadEvent(chunk.bukkitChunk);
+                 server.getPluginManager().callEvent(event);
+@@ -367,4 +375,22 @@ public class ChunkProviderServer implements IChunkProvider {
+     public boolean e(int i, int j) {
+         return this.chunks.containsKey(LongHash.toLong(i, j)); // CraftBukkit
+     }
++
++    // Paper start
++    public class ChunkUnloadQueue extends java.util.LinkedList<Chunk> {
++        public void remove(int x, int z) {
++            // nothing! Just to reduce diff
++        }
++        public void add(int x, int z) {
++            final Chunk chunk = chunks.get(LongHash.toLong(x, z));
++            if (chunk != null) {
++                chunk.isChunkActive = false;
++                if (!chunk.isInUnloadQueue) {
++                    chunk.isInUnloadQueue = true;
++                    add(chunk);
++                }
++            }
++        }
++    }
++    // Paper end
+ }
+diff --git a/src/main/java/net/minecraft/server/SpawnerCreature.java b/src/main/java/net/minecraft/server/SpawnerCreature.java
+index 63e118d..721bcae 100644
+--- a/src/main/java/net/minecraft/server/SpawnerCreature.java
++++ b/src/main/java/net/minecraft/server/SpawnerCreature.java
+@@ -28,7 +28,7 @@ public final class SpawnerCreature {
+             Long coord = it.next();
+             int x = LongHash.msw( coord );
+             int z = LongHash.lsw( coord );
+-            if ( !((ChunkProviderServer)server.chunkProvider).unloadQueue.contains( coord ) && server.isChunkLoaded( x, z, true ) )
++            if ( server.isChunkLoaded( x, z, true ) ) // Paper
+             {
+                 i += server.getChunkAt( x, z ).entityCount.get( oClass );
+             }
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+index 40338c0..bd28154 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+@@ -302,7 +302,7 @@ public class CraftWorld implements World {
+             world.timings.syncChunkLoadTimer.startTiming(); // Spigot
+             chunk = world.getChunkProviderServer().getOrLoadChunkAt(x, z);
+             world.timings.syncChunkLoadTimer.stopTiming(); // Spigot
+-        }
++        } else { chunk.isChunkActive = true; } // Paper
+         return chunk != null;
+     }
+ 
+@@ -1538,7 +1538,7 @@ public class CraftWorld implements World {
+             }
+ 
+             // Already unloading?
+-            if (cps.unloadQueue.contains(chunk.locX, chunk.locZ)) {
++            if (!chunk.isChunkActive) { // Paper
+                 continue;
+             }
+ 
+-- 
+2.7.4
+