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From "Hong Tang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-7339) Introduce a buffered checksum for avoiding frequently calls on Checksum.update()
Date Mon, 30 May 2011 19:21:47 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7339?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13041242#comment-13041242

Hong Tang commented on HADOOP-7339:

@min, you may minimize the impact of shuffle seek overhead by limiting the amount of input
data, and choosing a small reducer number. The effect of CPU saturation may be more pronounced
if you set the # of map slots > # of cores, and you use GZIP for intermediate data compression.

> Introduce a buffered checksum for avoiding frequently calls on Checksum.update()
> --------------------------------------------------------------------------------
>                 Key: HADOOP-7339
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7339
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: util
>            Reporter: Min Zhou
>             Fix For: 0.22.0
>         Attachments: HADOOP-7339-v1.diff, HADOOP-7339-v2.diff
> We found that PureJavaCRC32/CRC32.update() is the TOP 1 of the methods consuming CPU
in a map side, and in reduce side, it cost a lots of CPU too.
> IFileOutputStream would frequently call Checksum.update() during writing a record. It's
very common a MR key/value less than 512 bytes. Checksum.update() would be called every time
writing a key/value. 
> Test case: terasort 100MB. 
> Checksum.update() calls has be reduced from 4030348 to 28069.  This method is not a hotspot

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