[GitHub] [hbase] saintstack commented on a change in pull request #3407: HBASE-26018 Perf improvement in L1 cache - Optimistic call to buffer.retain()

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saintstack commented on a change in pull request #3407:
URL: https://github.com/apache/hbase/pull/3407#discussion_r671532481



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File path: hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/LruAdaptiveBlockCache.java
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@@ -646,14 +639,16 @@ private long updateSizeMetrics(LruCachedBlock cb, boolean evict) {
   @Override
   public Cacheable getBlock(BlockCacheKey cacheKey, boolean caching, boolean repeat,
     boolean updateCacheMetrics) {
-    LruCachedBlock cb = map.computeIfPresent(cacheKey, (key, val) -> {
-      // It will be referenced by RPC path, so increase here. NOTICE: Must do the retain inside
-      // this block. because if retain outside the map#computeIfPresent, the evictBlock may remove
-      // the block and release, then we're retaining a block with refCnt=0 which is disallowed.
-      // see HBASE-22422.
-      val.getBuffer().retain();
-      return val;
-    });
+    LruCachedBlock cb = map.get(cacheKey);
+    if (cb != null) {
+      try {
+        cb.getBuffer().retain();

Review comment:
       Looking at this more....  I don't think we can do your trick afterall.
   
   The refcounting is not for the cache, it is for a backing pool of memory used reading data in from hdfs into the cache. When we evict a block from the cache, we call #release on the memory. If the refcount is zero, the memory is released and can be reused in the backing pool. If #release is called and the #refcount is not zero, we just decrement the refcount.
   
   A cached buffer item detached from the cache still needs to have its #release called w/ refcount at zero so the backing memory gets readded to the pool.
   
   So it seems to me. What you think @virajjasani 




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