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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10015) Major performance improvement: Avoid synchronization in StoreScanner
Date Thu, 21 Nov 2013 22:15:38 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10015?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13829403#comment-13829403
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Lars Hofhansl commented on HBASE-10015:
---------------------------------------

Thanks [~vrodionov], we'll look at RegionScanner next. :)  My evil plans is to eventually
get rid of all synchronization during scanning and provide exclusion by containment instead.

The overall improvement is real. Validated on various different machines. The effect of memory
stalls is probably more pronounced in the running server.

In any case, I think we agree that this patch can't make things worse (provide it is correct,
of course).
I also tried with count(*) queries in Phoenix on tall tables (to rule out some anomalies with
Filters). I see a 90% improvement there - again only on tall tables.


> 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
>         Attachments: 10015-0.94-withtest.txt, 10015-0.94.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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