From 010dbe3c734aff7b400c7be8666e5f0c055fca62 Mon Sep 17 00:00:00 2001
From: Robin Wang <micrari@gmail.com>
Date: Tue, 24 Apr 2018 15:19:01 +0800
Subject: [PATCH] Optimize space usage of FastThreadLocal (#7861)

Motivation:
A FastThreadLocal instance currently occupies 2 slots of InternalThreadLocalMap of every thread. Actually for a FastThreadLocalThread, it does not need to store cleaner flags of FastThreadLocal instances. Besides, using BitSet to store cleaner flags is also better for space usage.

Modification:
Use BitSet to optimize space usage of FastThreadLocal.

Result:
Avoid unnecessary slot occupancy. Cleaner flags are now stored into a BitSet.
---
 .../io/netty/util/concurrent/FastThreadLocal.java  | 11 +++--------
 .../util/internal/InternalThreadLocalMap.java      | 14 ++++++++++++++
 .../netty/util/concurrent/FastThreadLocalTest.java |  2 +-
 3 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/common/src/main/java/io/netty/util/concurrent/FastThreadLocal.java b/common/src/main/java/io/netty/util/concurrent/FastThreadLocal.java
index e71d6db414..561058abca 100644
--- a/common/src/main/java/io/netty/util/concurrent/FastThreadLocal.java
+++ b/common/src/main/java/io/netty/util/concurrent/FastThreadLocal.java
@@ -125,11 +125,8 @@ public class FastThreadLocal<V> {
 
     private final int index;
 
-    private final int cleanerFlagIndex;
-
     public FastThreadLocal() {
         index = InternalThreadLocalMap.nextVariableIndex();
-        cleanerFlagIndex = InternalThreadLocalMap.nextVariableIndex();
     }
 
     /**
@@ -150,13 +147,11 @@ public class FastThreadLocal<V> {
 
     private void registerCleaner(final InternalThreadLocalMap threadLocalMap) {
         Thread current = Thread.currentThread();
-        if (FastThreadLocalThread.willCleanupFastThreadLocals(current) ||
-            threadLocalMap.indexedVariable(cleanerFlagIndex) != InternalThreadLocalMap.UNSET) {
+        if (FastThreadLocalThread.willCleanupFastThreadLocals(current) || threadLocalMap.isCleanerFlagSet(index)) {
             return;
         }
-        // removeIndexedVariable(cleanerFlagIndex) isn't necessary because the finally cleanup is tied to the lifetime
-        // of the thread, and this Object will be discarded if the associated thread is GCed.
-        threadLocalMap.setIndexedVariable(cleanerFlagIndex, Boolean.TRUE);
+
+        threadLocalMap.setCleanerFlag(index);
 
         // We will need to ensure we will trigger remove(InternalThreadLocalMap) so everything will be released
         // and FastThreadLocal.onRemoval(...) will be called.
diff --git a/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java b/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java
index a0d186e74b..6ab3bdf131 100644
--- a/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java
+++ b/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java
@@ -26,6 +26,7 @@ import java.nio.charset.CharsetDecoder;
 import java.nio.charset.CharsetEncoder;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.BitSet;
 import java.util.IdentityHashMap;
 import java.util.Map;
 import java.util.WeakHashMap;
@@ -45,6 +46,8 @@ public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap
 
     public static final Object UNSET = new Object();
 
+    private BitSet cleanerFlags;
+
     static {
         STRING_BUILDER_INITIAL_SIZE =
                 SystemPropertyUtil.getInt("io.netty.threadLocalMap.stringBuilder.initialSize", 1024);
@@ -331,4 +334,15 @@ public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap
         Object[] lookup = indexedVariables;
         return index < lookup.length && lookup[index] != UNSET;
     }
+
+    public boolean isCleanerFlagSet(int index) {
+        return cleanerFlags != null && cleanerFlags.get(index);
+    }
+
+    public void setCleanerFlag(int index) {
+        if (cleanerFlags == null) {
+            cleanerFlags = new BitSet();
+        }
+        cleanerFlags.set(index);
+    }
 }
diff --git a/common/src/test/java/io/netty/util/concurrent/FastThreadLocalTest.java b/common/src/test/java/io/netty/util/concurrent/FastThreadLocalTest.java
index cc1e69fca7..4551d4f78b 100644
--- a/common/src/test/java/io/netty/util/concurrent/FastThreadLocalTest.java
+++ b/common/src/test/java/io/netty/util/concurrent/FastThreadLocalTest.java
@@ -47,7 +47,7 @@ public class FastThreadLocalTest {
 
         // Initialize a thread-local variable.
         assertThat(var.get(), is(nullValue()));
-        assertThat(FastThreadLocal.size(), is(2));
+        assertThat(FastThreadLocal.size(), is(1));
 
         // And then remove it.
         FastThreadLocal.removeAll();