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From "Bill Graham (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PIG-2779) Refactoring the code for setting number of reducers
Date Wed, 25 Jul 2012 04:56:35 GMT

    [ https://issues.apache.org/jira/browse/PIG-2779?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13422001#comment-13422001
] 

Bill Graham commented on PIG-2779:
----------------------------------

Jie, as part of this clean up it would be really useful to record the various parallelism
values in the job conf for later analysis. I was thinking we capture defaultParallel, requestedParallelism
and runtimeParallelism (which should == {{mapred.reduce.tasks}}, right). That way we can see
later which values were set and which was used. It would be great to know whether parallelism
was determined by a {{PARALLEL}} statement, via estimation, or via a default. This would be
in addition to the following related params we currently capture:

{noformat}
pig.exec.reducers.max
pig.exec.reducers.bytes.per.reducer
{noformat}

Do you want to add this to this issue or do you think we should we do this in a separate JIRA?


I use IntelliJ and I've just set the syntax to match Apaches.
                
> Refactoring the code for setting number of reducers
> ---------------------------------------------------
>
>                 Key: PIG-2779
>                 URL: https://issues.apache.org/jira/browse/PIG-2779
>             Project: Pig
>          Issue Type: Bug
>            Reporter: Jie Li
>            Assignee: Jie Li
>             Fix For: 0.11
>
>         Attachments: PIG-2779.0.patch, PIG-2779.1.patch, PIG-2779.2.patch, TestNumberOfReducers.java,
TestNumberOfReducers.java
>
>
> As PIG-2652 observed, currently the code for setting number of reducers is a little messy.
MapReduceOper.requestedParallelism seems being misused in some plases, and now we support
runtime estimation of #reducer which further complicates the problem.
> For example, if we specify parallel 1 for the order-by, the estimated #reducer will be
used. If we specify parallel 2 while it estimates 4, order-by will fail due to "Illegal partition
for Null". If we specify parallel 4 while it estimates 2, then some reducers will have nothing
to do. 

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