Reduce the initial size of the buffer to 512 bytes.

The allocateDirect() reduce the churn of passing a byte[] from Java to native. However, it's only useful for pushed atoms. In the case of pulled atom, the allocateDirect doesn't help anything as the data is later copied to a new array in build().

Unfortunately, we cannot distinguish pushed from pulled case in StatsEvent unless we introduce a new API. For now, lower the initial buffer size to limit the impact on the non movable memory space.

Bug:366165284
Change-Id: I4e971902f98414a88df6cf033a21aa9da95c24b6

framework/java/android/util/StatsEvent.java

diff --git a/framework/java/android/util/StatsEvent.java b/framework/java/android/util/StatsEvent.java
index 9b7c824..ab49ae9 100644
--- a/framework/java/android/util/StatsEvent.java
+++ b/framework/java/android/util/StatsEvent.java
@@ -856,6 +856,10 @@
         private boolean mOverflow = false;
         private int mMaxSize = MAX_PULL_PAYLOAD_SIZE;
 
+        // The initial size of the buffer 512 bytes. The buffer will be expanded
+        // if needed up to mMaxSize.
+        private static final int INITIAL_BUFFER_SIZE = 512;
+
         @NonNull
         private static Buffer obtain() {
             Buffer buffer = sPool.getAndSet(null);
@@ -868,8 +872,14 @@
         }
 
         private Buffer() {
-            final ByteBuffer tempBuffer = ByteBuffer.allocateDirect(MAX_PUSH_PAYLOAD_SIZE);
-            mBytes = tempBuffer.hasArray() ? tempBuffer.array() : new byte [MAX_PUSH_PAYLOAD_SIZE];
+            // b/366165284, b/192105193 - the allocateDirect() reduces the churn
+            // of passing a byte[] from Java to native. However, it's only
+            // useful for pushed atoms. In the case of pulled atom, the
+            // allocateDirect doesn't help anything as the data is later copied
+            // to a new array in build(). In addition, when the buffer is to be expanded, it
+            // also allocates a new array.
+            final ByteBuffer tempBuffer = ByteBuffer.allocateDirect(INITIAL_BUFFER_SIZE);
+            mBytes = tempBuffer.hasArray() ? tempBuffer.array() : new byte [INITIAL_BUFFER_SIZE];
         }
 
         @NonNull