PaperMC/Spigot-Server-Patches/0303-Optimize-Region-File-Cache.patch

From 70b73f56aafb1a6e9209bb568fd46fdb7cbf64bf Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Mon, 23 Jul 2018 23:40:04 -0400
Subject: [PATCH] Optimize Region File Cache

CraftBukkit added synchronization to read and write methods. This adds
much more contention on this object for accessing region files, as
the entire read and write of NBT data is now a blocking operation.

This causes issues when something then simply needs to check if a chunk exists
on the main thread, causing a block...

However, this synchronization was unnecessary, because there is already
enough synchronization done to keep things safe

1) Obtaining a Region File: Those methods are still static synchronized.
   Meaning we can safely obtain a Region File concurrently.

2) RegionFile data access: Methods reading and manipulating data from
   a region file are also marked synchronized, ensuring that no 2 processes
   are reading or writing data at the same time.

3) Checking a region file for chunkExists: getOffset is also synchronized
   ensuring that even if a chunk is currently being written, it will be safe.

By removing these synchronizations, we reduce the locking to only
when data is being write or read.

GZIP compression and NBT Buffer creation will no longer be part of the
synchronized context, reducing lock times.

Ultimately: This brings us back to Vanilla, which has had no indication of region file loss.

diff --git a/src/main/java/net/minecraft/server/RegionFileCache.java b/src/main/java/net/minecraft/server/RegionFileCache.java
index 964996976..d07222239 100644
--- a/src/main/java/net/minecraft/server/RegionFileCache.java
+++ b/src/main/java/net/minecraft/server/RegionFileCache.java
@@ -99,7 +99,7 @@ public class RegionFileCache {
 
     @Nullable
     // CraftBukkit start - call sites hoisted for synchronization
-    public static synchronized NBTTagCompound read(File file, int i, int j) throws IOException {
+    public static NBTTagCompound read(File file, int i, int j) throws IOException { // Paper - remove synchronization
         RegionFile regionfile = a(file, i, j);
 
         DataInputStream datainputstream = regionfile.a(i & 31, j & 31);
@@ -112,7 +112,7 @@ public class RegionFileCache {
     }
 
     @Nullable
-    public static synchronized void write(File file, int i, int j, NBTTagCompound nbttagcompound) throws IOException {
+    public static void write(File file, int i, int j, NBTTagCompound nbttagcompound) throws IOException {
         int attempts = 0; Exception laste = null; while (attempts++ < 5) { try { // Paper
         RegionFile regionfile = a(file, i, j);
 
@@ -138,7 +138,7 @@ public class RegionFileCache {
         // Paper end
     }
 
-    public static synchronized boolean chunkExists(File file, int i, int j) {
+    public static boolean chunkExists(File file, int i, int j) { // Paper - remove synchronization
         RegionFile regionfile = b(file, i, j);
 
         return regionfile != null ? regionfile.d(i & 31, j & 31) : false;
-- 
2.20.1