[GitHub] [hbase] Apache9 commented on a change in pull request #3691: HBASE-26259 Fallback support to pure Java compression

[GitBox <gi...@apache.org>]

Apache9 commented on a change in pull request #3691:
URL: https://github.com/apache/hbase/pull/3691#discussion_r721428211



##########
File path: hbase-common/src/main/java/org/apache/hadoop/hbase/util/UnsafeAccess.java
##########
@@ -471,4 +485,57 @@ public static byte toByte(ByteBuffer buf, int offset) {
   public static byte toByte(Object ref, long offset) {
     return theUnsafe.getByte(ref, offset);
   }
+
+  /**
+   * Zero fill a byte buffer as efficiently as possible.
+   * @param buf the byte buffer
+   */
+  public static void clear(final ByteBuffer buf) {
+    if (buf.isDirect()) {
+      clear(((DirectBuffer)buf).address(), buf.capacity());

Review comment:
       OK, I checked the source code of openjdk, it is not a memset, it will try to use long if the address and length are both 8 bytes aligned. It is a bit strange that they do not use the typical loop unrolling way to implement this function, as what you do here.
   
   https://github.com/adoptium/jdk11u/blob/c9c728849c137e21535cc402664dd54ee421b0d3/src/hotspot/share/utilities/copy.cpp#L238
   
   And the behavior is also well documented in the javadoc of the Unsafe.setMemory method.
   ```
        * <p>The stores are in coherent (atomic) units of a size determined
        * by the address and length parameters.  If the effective address and
        * length are all even modulo 8, the stores take place in 'long' units.
        * If the effective address and length are (resp.) even modulo 4 or 2,
        * the stores take place in units of 'int' or 'short'.
   ```
   
   So let's keep your version here. And could also add some comments to say why we do not use Unsafe.setMemory.




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