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https://issues.apache.org/jira/browse/HDFS-14535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16855261#comment-16855261
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Zheng Hu commented on HDFS-14535:
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bq. Emm...IIRC, seems our HBase code have a bug here, for Get operation, all of
the query should share the the same reader, which means shouldn't request the
short-circuit fd so frequently.
OK, I considered something wrong here. It's a big HFile and has many blocks
inside, for every block the Get Op wanted, DFS client will
requestFileDescriptors, so the problem occur. The HBase implementation is OK.
Thanks.
> The default 8KB buffer in requestFileDescriptors#BufferedOutputStream is
> causing lots of heap allocation in HBase when using short-circut read
> ----------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: HDFS-14535
> URL: https://issues.apache.org/jira/browse/HDFS-14535
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Zheng Hu
> Assignee: Zheng Hu
> Priority: Major
> Attachments: HDFS-14535.patch
>
>
> Our HBase team are trying to read the blocks from HDFS into pooled offheap
> ByteBuffers directly (HBASE-21879), and recently we had some benchmark,
> found that almost 45% heap allocation from the DFS client. The heap
> allocation flame graph can be see here:
> https://issues.apache.org/jira/secure/attachment/12970295/async-prof-pid-25042-alloc-2.svg
> After checking the code path, we found that when requesting file descriptors
> from a DomainPeer, we allocated huge 8KB buffer for BufferedOutputStream,
> though the protocal content was quite small and just few bytes.
> It made a heavy GC pressure for HBase when cacheHitRatio < 60%, which
> increased the HBase P999 latency. Actually, we can pre-allocate a small
> buffer for the BufferedOutputStream, such as 512 bytes, it's enough to read
> the short-circuit fd protocal content. we've created a patch like that, and
> the allocation flame graph show that after the patch, the heap allocation
> from DFS client dropped from 45% to 27%, that's a very good thing I think.
> see:
> https://issues.apache.org/jira/secure/attachment/12970475/async-prof-pid-24534-alloc-2.svg
> Hope this attached patch can be merged into HDFS trunk, also Hadoop-2.8.x,
> HBase will benifit a lot from this.
> Thanks.
> For more details, can see here:
> https://issues.apache.org/jira/browse/HBASE-22387?focusedCommentId=16851639&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-16851639
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