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https://issues.apache.org/jira/browse/HBASE-7336?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13534149#comment-13534149
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Lars Hofhansl commented on HBASE-7336:
--------------------------------------

The 0.94 tests fail with the same OOM even without this patch, so I am going to 
reapply. Sorry for the noise, but I had to make sure.
                
> 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: Sub-task
>            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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