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https://issues.apache.org/jira/browse/HBASE-7336?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13532736#comment-13532736
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Lars Hofhansl commented on HBASE-7336:
--------------------------------------
Note that the previous tests all were scans that did not return anything to the
client.
Did another verification test with scans in blocks and returns data to the
client. Even in that case the performance is better, because of improved
concurrency.
(Three concurrent clients went from 74s to 65s)
> 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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