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Posted to issues@hbase.apache.org by "Lars Hofhansl (JIRA)" <ji...@apache.org> on 2012/12/14 01:26:12 UTC
[jira] [Commented] (HBASE-5979) Non-pread DFSInputStreams should be
associated with scanners, not HFile.Readers
[ https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13531690#comment-13531690 ]
Lars Hofhansl commented on HBASE-5979:
--------------------------------------
Can we use a scan's cacheBlocks flag as an indicator for whether it would benefit from its own input stream? Presumably only large scans would have caching disabled.
HBASE-7336 has a simple adhoc fix for the truly terrible read performance when two long running scanners scan the same store file.
> 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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