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Date: Wed, 12 Dec 2012 07:03:22 +0000 (UTC)
From: "Lars Hofhansl (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: <JIRA.12623489.1355291472777.21825.1355295802172@arcas>
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Subject: [jira] [Commented] (HBASE-7336) HFileBlock.readAtOffset does not
 work well with multiple threads
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    [ https://issues.apache.org/jira/browse/HBASE-7336?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13529682#comment-13529682 ] 

Lars Hofhansl commented on HBASE-7336:
--------------------------------------

tryLock without a timeout returns immediately. I think in this case it should, waiting here would potentially wait (at least a threads context switch) on each single next or get request.

I think this issue would be most prominent in long scans, where the scans typically do not want to mess up the block cache.

Multiple readers is probably the right solution. How many, though? And I am not sure how this would interact with the block cache (would probably be OK).

                
> HFileBlock.readAtOffset does not work well with multiple threads
> ----------------------------------------------------------------
>
>                 Key: HBASE-7336
>                 URL: https://issues.apache.org/jira/browse/HBASE-7336
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Lars Hofhansl
>            Assignee: Lars Hofhansl
>            Priority: Critical
>             Fix For: 0.96.0, 0.94.4
>
>         Attachments: 7336-0.94.txt, 7336-0.96.txt
>
>
> HBase grinds to a halt when many threads scan along the same set of blocks and neither read short circuit is nor block caching is enabled for the dfs client ... disabling the block cache makes sense on very large scans.
> It turns out that synchronizing in istream in HFileBlock.readAtOffset is the culprit.

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