From cb3819a6176a109689eb5f28a7143c17f71f2e8f Mon Sep 17 00:00:00 2001
From: amcreynolds <amcreynolds@ebay.com>
Date: Mon, 6 Nov 2017 15:37:15 -0800
Subject: [PATCH] Optimize AbstractListenableFuture Memory Allocations

Problem: If an empty ArrayList has addAll() invoked with a Collection
argument that has a size less than 10, the underlying elementData
Object[] is still grown to 10 via an Array copy. In the case of
AbstractListenableFuture, the number of listeners is often 0 or 1,
and AbstractListenableFuture is in the critical path, meaning this
behavior causes unnecessary GC pressure for high throughput
applications.

Therefore, this commit enhances the logic to avoid the construction
of the listener copy ArrayList and its iterator if possible, and if
not, to use the more efficient constructor vs. addAll().
---
 .../internal/AbstractListenableFuture.java    | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/src/main/java/net/spy/memcached/internal/AbstractListenableFuture.java b/src/main/java/net/spy/memcached/internal/AbstractListenableFuture.java
index fd0b4e970..23c54bdce 100644
--- a/src/main/java/net/spy/memcached/internal/AbstractListenableFuture.java
+++ b/src/main/java/net/spy/memcached/internal/AbstractListenableFuture.java
@@ -141,15 +141,22 @@ protected void notifyListeners() {
    * @param future the future to pass on to the listeners.
    */
   protected void notifyListeners(final Future<?> future) {
-    final List<GenericCompletionListener<? extends Future<T>>> copy =
-      new ArrayList<GenericCompletionListener<? extends Future<T>>>();
+    final List<GenericCompletionListener<? extends Future<T>>> copy;
     synchronized(this) {
-      copy.addAll(listeners);
+      if (listeners.isEmpty()) {
+        copy = null;
+      }
+      else {
+        copy = new ArrayList<GenericCompletionListener
+          <? extends Future<T>>>(listeners);
+      }
       listeners = new ArrayList<GenericCompletionListener<? extends Future<T>>>();
     }
-    for (GenericCompletionListener<? extends Future<? super T>> listener
-      : copy) {
-      notifyListener(executor(), future, listener);
+    if (copy != null) {
+      for (GenericCompletionListener<? extends Future<? super T>> listener
+        : copy) {
+        notifyListener(executor(), future, listener);
+      }
     }
   }