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From "Benjamin Reed (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-196) Pig should use '-reducer NONE' for map-only jobs
Date Fri, 18 Apr 2008 18:40:21 GMT

    [ https://issues.apache.org/jira/browse/PIG-196?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12590558#action_12590558
] 

Benjamin Reed commented on PIG-196:
-----------------------------------

This patch looks like it assumes that splitSpec is not null if there isn't a groupSpec. It
would be nice to add a comment stating that assumption and justifying it. Looking back at
MapReduceLauncher doesn't hint that this is the case. I think it is important so that later
changes do not break the justification for this assumption. (Or at least it will give the
person debugging a hint at where to look.)

> Pig should use '-reducer NONE' for map-only jobs
> ------------------------------------------------
>
>                 Key: PIG-196
>                 URL: https://issues.apache.org/jira/browse/PIG-196
>             Project: Pig
>          Issue Type: Bug
>            Reporter: Arun C Murthy
>            Assignee: Arun C Murthy
>         Attachments: PIG-196_0_20080412.patch
>
>
> Currently, for map-only jobs, Pig writes map-outputs directly to HDFS and then sends
zero data to reducers. The problem with this is two fold:
>  * Reduce slots are unnecessarily wasted on the cluster
>  * Reduces write empty files to HDFS putting pressure on the Namenode
> Both these can we very easily avoided by just calling:
> {noformat}
> job.setNumReduces(0);
> {noformat}
> and letting Hadoop Map-Reduce take care of writing map-outputs directly to HDFS.

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