[GitHub] [hbase] saintstack commented on a change in pull request #3215: HBASE-25698 Fixing IllegalReferenceCountException when using TinyLfuBlockCache

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

saintstack commented on a change in pull request #3215:
URL: https://github.com/apache/hbase/pull/3215#discussion_r638044447



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File path: hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/TinyLfuBlockCache.java
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@@ -158,7 +158,13 @@ public boolean containsBlock(BlockCacheKey cacheKey) {
   @Override
   public Cacheable getBlock(BlockCacheKey cacheKey,
       boolean caching, boolean repeat, boolean updateCacheMetrics) {
-    Cacheable value = cache.getIfPresent(cacheKey);
+    Cacheable value = cache.asMap().computeIfPresent(cacheKey, (blockCacheKey, cacheable) -> {
+      // 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.
+      cacheable.retain();
+      return cacheable;
+    });

Review comment:
       Smile. Related. I've been studying at this bit of code here.. the flip to computeIfPresent. #get in CHM is lockless but computeIfPresent takes a lock on the bucket (of keys) so seems to slow us down especially if high read load with most data in cache; e.g. meta hits.




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