Load chunks asynchronously for players.

When a player triggers a chunk load via walking around or teleporting there
is no need to stop everything and get this chunk on the main thread. The
client is used to having to wait some time for this chunk and the server
doesn't immediately do anything with it except send it to the player. At
the same time chunk loading is the last major source of file IO that still
runs on the main thread.

These two facts make it possible to offload chunks loaded for this reason
to another thread. However, not all parts of chunk loading can happen off
the main thread. For this we use the new AsynchronousExecutor system to
split chunk loading in to three pieces. The first is loading data from
disk, decompressing it, and parsing it in to an NBT structure.  The second
piece is creating entities and tile entities in the chunk and adding them
to the world, this is still done on the main thread. The third piece is
informing everyone who requested a chunk load that the load is finished.
For this we register callbacks and then run them on the main thread once
the previous two stages are finished.

There are still cases where a chunk is needed immediately and these will
still trigger chunk loading entirely on the main thread. The most obvious
case is plugins using the API to request a chunk load. We also must load
the chunk immediately when something in the world tries to access it. In
these cases we ignore any possibly pending or in progress chunk loading
that is happening asynchronously as we will have the chunk loaded by the
time they are finished.

The hope is that overall this system will result in less CPU time and
pauses due to blocking file IO on the main thread thus giving more
consistent performance. Testing so far has shown that this also speeds up
chunk loading client side although some of this is likely to be because
we are sending less chunks at once for the client to process.

Thanks for @ammaraskar for help with the implementation of this feature.

3 files changed, 141 insertions, 0 deletions

diff --git a/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOExecutor.java b/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOExecutor.java
new file mode 100644
index 00000000..92fbc4f9
--- /dev/null
+++ b/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOExecutor.java
@@ -0,0 +1,32 @@
+package org.bukkit.craftbukkit.chunkio;
+
+import net.minecraft.server.Chunk;
+import net.minecraft.server.ChunkProviderServer;
+import net.minecraft.server.ChunkRegionLoader;
+import net.minecraft.server.World;
+import org.bukkit.craftbukkit.util.AsynchronousExecutor;
+import org.bukkit.craftbukkit.util.LongHash;
+
+public class ChunkIOExecutor {
+    static final int BASE_THREADS = 1;
+    static final int PLAYERS_PER_THREAD = 50;
+
+    private static final AsynchronousExecutor<QueuedChunk, Chunk, Runnable, RuntimeException> instance = new AsynchronousExecutor<QueuedChunk, Chunk, Runnable, RuntimeException>(new ChunkIOProvider(), BASE_THREADS);
+
+    public static void waitForChunkLoad(World world, int x, int z) {
+        instance.get(new QueuedChunk(LongHash.toLong(x, z), null, world, null));
+    }
+
+    public static void queueChunkLoad(World world, ChunkRegionLoader loader, ChunkProviderServer provider, int x, int z, Runnable runnable) {
+        instance.add(new QueuedChunk(LongHash.toLong(x, z), loader, world, provider), runnable);
+    }
+
+    public static void adjustPoolSize(int players) {
+        int size = Math.max(BASE_THREADS, (int) Math.ceil(players / PLAYERS_PER_THREAD));
+        instance.setActiveThreads(size);
+    }
+
+    public static void tick() {
+        instance.finishActive();
+    }
+}
diff --git a/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java b/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java
new file mode 100644
index 00000000..48cf5bac
--- /dev/null
+++ b/src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java
@@ -0,0 +1,73 @@
+package org.bukkit.craftbukkit.chunkio;
+
+import net.minecraft.server.Chunk;
+import net.minecraft.server.ChunkRegionLoader;
+import net.minecraft.server.NBTTagCompound;
+
+import org.bukkit.Server;
+import org.bukkit.craftbukkit.util.AsynchronousExecutor;
+import org.bukkit.craftbukkit.util.LongHash;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+class ChunkIOProvider implements AsynchronousExecutor.CallBackProvider<QueuedChunk, Chunk, Runnable, RuntimeException> {
+    private final AtomicInteger threadNumber = new AtomicInteger(1);
+
+    // async stuff
+    public Chunk callStage1(QueuedChunk queuedChunk) throws RuntimeException {
+        ChunkRegionLoader loader = queuedChunk.loader;
+        Object[] data = loader.loadChunk(queuedChunk.world, LongHash.msw(queuedChunk.coords), LongHash.lsw(queuedChunk.coords));
+
+        if (data != null) {
+            queuedChunk.compound = (NBTTagCompound) data[1];
+            return (Chunk) data[0];
+        }
+
+        return null;
+    }
+
+    // sync stuff
+    public void callStage2(QueuedChunk queuedChunk, Chunk chunk) throws RuntimeException {
+        if(chunk == null) {
+            // If the chunk loading failed just do it synchronously (may generate)
+            queuedChunk.provider.getChunkAt(LongHash.msw(queuedChunk.coords), LongHash.lsw(queuedChunk.coords));
+            return;
+        }
+
+        int x = LongHash.msw(queuedChunk.coords);
+        int z = LongHash.lsw(queuedChunk.coords);
+
+        // See if someone already loaded this chunk while we were working on it (API, etc)
+        if (queuedChunk.provider.chunks.containsKey(queuedChunk.coords)) {
+            // Make sure it isn't queued for unload, we need it
+            queuedChunk.provider.unloadQueue.remove(queuedChunk.coords);
+            return;
+        }
+
+        queuedChunk.loader.loadEntities(chunk, queuedChunk.compound.getCompound("Level"), queuedChunk.world);
+        chunk.n = queuedChunk.provider.world.getTime();
+        queuedChunk.provider.chunks.put(queuedChunk.coords, chunk);
+        chunk.addEntities();
+
+        if (queuedChunk.provider.chunkProvider != null) {
+            queuedChunk.provider.chunkProvider.recreateStructures(x, z);
+        }
+
+        Server server = queuedChunk.provider.world.getServer();
+        if (server != null) {
+            server.getPluginManager().callEvent(new org.bukkit.event.world.ChunkLoadEvent(chunk.bukkitChunk, false));
+        }
+        
+        chunk.a(queuedChunk.provider, queuedChunk.provider, x, z);
+    }
+
+    public void callStage3(QueuedChunk queuedChunk, Chunk chunk, Runnable runnable) throws RuntimeException {
+        runnable.run();
+    }
+
+    public Thread newThread(Runnable runnable) {
+        Thread thread = new Thread(runnable, "Chunk I/O Executor Thread-" + threadNumber.getAndIncrement());
+        thread.setDaemon(true);
+        return thread;
+    }
+}
diff --git a/src/main/java/org/bukkit/craftbukkit/chunkio/QueuedChunk.java b/src/main/java/org/bukkit/craftbukkit/chunkio/QueuedChunk.java
new file mode 100644
index 00000000..299b1d8a
--- /dev/null
+++ b/src/main/java/org/bukkit/craftbukkit/chunkio/QueuedChunk.java
@@ -0,0 +1,36 @@
+package org.bukkit.craftbukkit.chunkio;
+
+import net.minecraft.server.ChunkProviderServer;
+import net.minecraft.server.ChunkRegionLoader;
+import net.minecraft.server.NBTTagCompound;
+import net.minecraft.server.World;
+
+class QueuedChunk {
+    long coords;
+    ChunkRegionLoader loader;
+    World world;
+    ChunkProviderServer provider;
+    NBTTagCompound compound;
+
+    public QueuedChunk(long coords, ChunkRegionLoader loader, World world, ChunkProviderServer provider) {
+        this.coords = coords;
+        this.loader = loader;
+        this.world = world;
+        this.provider = provider;
+    }
+
+    @Override
+    public int hashCode() {
+        return (int) coords ^ world.hashCode();
+    }
+
+    @Override
+    public boolean equals(Object object) {
+        if (object instanceof QueuedChunk) {
+            QueuedChunk other = (QueuedChunk) object;
+            return coords == other.coords && world == other.world;
+        }
+
+        return false;
+    }
+}