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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5979) Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers
Date Tue, 22 May 2012 00:17:41 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13280640#comment-13280640
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Todd Lipcon commented on HBASE-5979:
------------------------------------

Hey Kannan,

Sorry, let me elaborate on that suggestion:

The idea is to make a new FSReader implementation, which only has one API. That API would
look like the current positional read call (i.e take a position and length).

Internally, it would have a pool of cached DFSInputStreams, and remember the position for
each of them. Each of the input streams would be referencing the same file. When a read request
comes in, it is matched against the pooled streams: if it is within N bytes forward from the
current position of one of the streams, then a seek and read would be issued, synchronized
on that stream. Otherwise, any random stream would be chosen and a position read would be
chosen. Separately, we can track the last N positional reads: if we detect a sequential pattern
in the position reads, we can take one of the pooled input streams and seek to the next predicted
offset, so that future reads get the sequential benefit.
                
> 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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