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From "Kannan Muthukkaruppan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5979) Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers
Date Tue, 15 May 2012 17:16:03 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13275996#comment-13275996
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Kannan Muthukkaruppan commented on HBASE-5979:
----------------------------------------------

Todd: Nice catch! Your suggestion makes sense.
                
> Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers
> -------------------------------------------------------------------------------
>
>                 Key: HBASE-5979
>                 URL: https://issues.apache.org/jira/browse/HBASE-5979
>             Project: HBase
>          Issue Type: Improvement
>          Components: performance, regionserver
>            Reporter: Todd Lipcon
>
> Currently, every HFile.Reader has a single DFSInputStream, which it uses to service all
gets and scans. For gets, we use the positional read API (aka "pread") and for scans we use
a synchronized block to seek, then read. The advantage of pread is that it doesn't hold any
locks, so multiple gets can proceed at the same time. The advantage of seek+read for scans
is that the datanode starts to send the entire rest of the HDFS block, rather than just the
single hfile block necessary. So, in a single thread, pread is faster for gets, and seek+read
is faster for scans since you get a strong pipelining effect.
> However, in a multi-threaded case where there are multiple scans (including scans which
are actually part of compactions), the seek+read strategy falls apart, since only one scanner
may be reading at a time. Additionally, a large amount of wasted IO is generated on the datanode
side, and we get none of the earlier-mentioned advantages.
> In one test, I switched scans to always use pread, and saw a 5x improvement in throughput
of the YCSB scan-only workload, since it previously was completely blocked by contention on
the DFSIS lock.

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