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From "Hong Tang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1220) Implement an in-cluster LocalJobRunner
Date Thu, 19 Nov 2009 03:36:40 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12779812#action_12779812
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Hong Tang commented on MAPREDUCE-1220:
--------------------------------------

+1 on the direction.

Agree with Dhruba that this should be explicitly enabled until we have more experience.

Not sure we should mandate the serialization of task execution, maybe we can use multiple
slots if available on a particular TT? We probably should defer such optimizations after JT
refactor is done though.

> Implement an in-cluster LocalJobRunner
> --------------------------------------
>
>                 Key: MAPREDUCE-1220
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1220
>             Project: Hadoop Map/Reduce
>          Issue Type: New Feature
>          Components: client, jobtracker
>            Reporter: Arun C Murthy
>            Assignee: Arun C Murthy
>             Fix For: 0.22.0
>
>
> Currently very small map-reduce jobs suffer from latency issues due to overheads in Hadoop
Map-Reduce such as scheduling, jvm startup etc. We've periodically tried to optimize all parts
of framework to achieve lower latencies.
> I'd like to turn the problem around a little bit. I propose we allow very small jobs
to run as a single task job with multiple maps and reduces i.e. similar to our current implementation
of the LocalJobRunner. Thus, under certain conditions (maybe user-set configuration, or if
input data is small i.e. less a DFS blocksize) we could launch a special task which will run
all maps in a serial manner, followed by the reduces. This would really help small jobs achieve
significantly smaller latencies, thanks to lesser scheduling overhead, jvm startup, lack of
shuffle over the network etc. 
> This would be a huge benefit, especially on large clusters, to small Hive/Pig queries.
> Thoughts?

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