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Date: Tue, 26 Nov 2013 01:21:35 +0000 (UTC)
From: "Lars Hofhansl (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: <JIRA.12680362.1384978258355.27894.1385428895488@arcas>
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Subject: [jira] [Commented] (HBASE-10015) Major performance improvement:
 Avoid synchronization in StoreScanner
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    [ https://issues.apache.org/jira/browse/HBASE-10015?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13832162#comment-13832162 ] 

Lars Hofhansl commented on HBASE-10015:
---------------------------------------

Yeah, that's the idea. We'd delay archiving any HFile until no scanners are referring to it any more.

> 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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