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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5831) Implement memory-to-memory merge in the reduce
Date Mon, 18 May 2009 05:01:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5831?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12710271#action_12710271
] 

dhruba borthakur commented on HADOOP-5831:
------------------------------------------

This looks a really impressive performance gain! awesome.

> Implement memory-to-memory merge in the reduce
> ----------------------------------------------
>
>                 Key: HADOOP-5831
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5831
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: mapred
>            Reporter: Arun C Murthy
>            Assignee: Arun C Murthy
>             Fix For: 0.21.0
>
>
> HADOOP-3446 fixed the reduce to not flush the in-memory shuffled map-outputs before feeding
to the reduce. However for latency-sensitive applications with lots of memory like the terasort
this hurts performance since the fan-in for the final in-memory merge is too large (all 8000
map-outputs very in-memory) resulting in less than optimal performance.
> When I put in an intermediate memory-to-memory merge for the terasort's reduce (there-by
avoiding disk i/o) to cut the fan-in from 8000 to <100 the 'reduce' phase (including the
local datanode-write) sped-up 250% (from 10s to 4s). 

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