[jira] Commented: (HADOOP-4749) reducer should output input data size when shuffling is done

["Zheng Shao (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-4749?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12654333#action_12654333 ] 

Zheng Shao commented on HADOOP-4749:
------------------------------------

Thanks for the patch.

I think we can ignore the case that mapper is local, because load balance problem would not be interesting in that case.


Several suggestions:
1. Please use a new variable instead of moving bytesTransferred to class member level. There are other places that referenced bytesTransferred.
2. Make the patch into a single file (svn diff in the trunk directory);
3. Please click on "Submit patch" after "Attach file" is done;


> reducer should output input data size when shuffling is done
> ------------------------------------------------------------
>
>                 Key: HADOOP-4749
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4749
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: mapred
>            Reporter: Zheng Shao
>         Attachments: ReduceTask.patch, Task.patch
>
>
> Sometimes we see a single slow reducer because of the load balancing problem. This information will be very useful to understand how imbalanced the load is.
> Should be easy to fix I guess, since reducer should have all information needed at the end of the shuffling phase.

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