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https://issues.apache.org/jira/browse/HBASE-5898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13485348#comment-13485348
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Lars Hofhansl commented on HBASE-5898:
--------------------------------------
Something like this. I don't think the patch works around or even affects the
2nd issue.
The most likely explanation for the 2nd issue seems to be HDFS slowness:
The block is not in the cache, the first thread tries to load it, and while
that is happening all other threads have to (and should) wait.
If there is a temporary network hickup that will take a bit... And it would
look exactly like these stack traces, where many threads are queued up behind
this lock.
Now that you say it, though... On further consideration I am not sure I buy
there even *is* a contention issue here. We either:
* have the block in the cache, in which case we'll return it very quickly.
* do not have the block, in that case we have to load it and all other threads
must wait.
> 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
> Assignee: Todd Lipcon
> Priority: Critical
> Fix For: 0.94.3, 0.96.0
>
> Attachments: 5898-TestBlocksRead.txt, HBASE-5898-0.patch,
> HBASE-5898-1.patch, 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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