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https://issues.apache.org/jira/browse/HBASE-10015?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13831814#comment-13831814
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Lars Hofhansl commented on HBASE-10015:
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So it seems this is universal.
Our performance dude has actually confirmed that this is an expected outcome.
I'll gather some more CPU metrics as I find time today.
[~stack], these locks are taken in StoreScanner, which is "row" based (unlike
StoreFileScanner and MemstoreScanner). So the frequent locks/unlocks there we'd
mostly see per row. Wider rows won't be slower, but the speedup effect would be
proportionally less. To be sure I'll validate with wider tables (maybe 20 CQs).
I will also double check that biased locking is in effect.
> Major performance improvement: Avoid synchronization in StoreScanner
> --------------------------------------------------------------------
>
> Key: HBASE-10015
> URL: https://issues.apache.org/jira/browse/HBASE-10015
> Project: HBase
> Issue Type: Bug
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Attachments: 10015-0.94-lock.txt, 10015-0.94-new-sample.txt,
> 10015-0.94-v2.txt, 10015-0.94-v3.txt, 10015-0.94-v4.txt,
> 10015-0.94-withtest.txt, 10015-0.94.txt, 10015-trunk-v2.txt,
> 10015-trunk-v3.txt, 10015-trunk-v4.txt, 10015-trunk-v4.txt,
> 10015-trunk-v4.txt, 10015-trunk.txt, TestLoad.java
>
>
> Did some more profiling (this time with a sampling profiler) and
> StoreScanner.peek() showed up a lot in the samples. At first that was
> surprising, but peek is synchronized, so it seems a lot of the sync'ing cost
> is eaten there.
> It seems the only reason we have to synchronize all these methods is because
> a concurrent flush or compaction can change the scanner stack, other than
> that only a single thread should access a StoreScanner at any given time.
> So replaced updateReaders() with some code that just indicates to the scanner
> that the readers should be updated and then make it the using thread's
> responsibility to do the work.
> The perf improvement from this is staggering. I am seeing somewhere around 3x
> scan performance improvement across all scenarios.
> Now, the hard part is to reason about whether this is 100% correct. I ran
> TestAtomicOperation and TestAcidGuarantees a few times in a loop, all still
> pass.
> Will attach a sample patch.
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