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https://issues.apache.org/jira/browse/HBASE-5898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13264470#comment-13264470
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Zhihong Yu commented on HBASE-5898:
-----------------------------------
Consider the case where off heap cache is enabled.
>From DoubleBlockCache:
Suppose getBlock() is executed without the lock (first pass in the new loop of
readBlock) and doesn't find cacheKey from onHeapCache but finds it in
offHeapCache - it will call onHeapCache.cacheBlock():
{code}
public Cacheable getBlock(BlockCacheKey cacheKey, boolean caching) {
Cacheable cachedBlock;
if ((cachedBlock = onHeapCache.getBlock(cacheKey, caching)) != null) {
stats.hit(caching);
return cachedBlock;
} else if ((cachedBlock = offHeapCache.getBlock(cacheKey, caching)) !=
null) {
if (caching) {
onHeapCache.cacheBlock(cacheKey, cachedBlock);
}
{code}
Another thread calls cacheBlock() around the same time and executes
onHeapCache.cacheBlock() for the same cacheKey:
{code}
public void cacheBlock(BlockCacheKey cacheKey, Cacheable buf) {
onHeapCache.cacheBlock(cacheKey, buf);
offHeapCache.cacheBlock(cacheKey, buf);
}
{code}
I think there is a race condition which didn't exist before the proposed
change: the entries for the same cacheKey in onHeapCache and offHeapCache would
diverge.
If off heap cache is disabled, I don't see problem with proposed optimization.
> Consider double-checked locking for block cache lock
> ----------------------------------------------------
>
> Key: HBASE-5898
> URL: https://issues.apache.org/jira/browse/HBASE-5898
> Project: HBase
> Issue Type: Improvement
> Components: performance
> Affects Versions: 0.94.1
> Reporter: Todd Lipcon
> Attachments: hbase-5898.txt
>
>
> Running a workload with a high query rate against a dataset that fits in
> cache, I saw a lot of CPU being used in IdLock.getLockEntry, being called by
> HFileReaderV2.readBlock. Even though it was all cache hits, it was wasting a
> lot of CPU doing lock management here. I wrote a quick patch to switch to a
> double-checked locking and it improved throughput substantially for this
> workload.
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