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https://issues.apache.org/jira/browse/HBASE-7336?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14050768#comment-14050768
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Lars Hofhansl commented on HBASE-7336:
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The trylock stuff is from the patch here:
# try to do seek + read (if requested, i.e. a scan)
# if that is not possible rather than locking, do a pread immediately.
My tests showed a significant improvement, you can always verify yourself. Note
that this was 1.5 years ago.
Curious... If what you say is true there would be no difference at all between
seek+read and pread. That would indeed be bad. Hmm.
> 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
> Components: Performance
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Priority: Critical
> Fix For: 0.94.4, 0.95.0
>
> 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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