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From "Pradeep Kamath (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-729) Use of default parallelism
Date Tue, 24 Mar 2009 17:09:52 GMT

    [ https://issues.apache.org/jira/browse/PIG-729?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12688755#action_12688755
] 

Pradeep Kamath commented on PIG-729:
------------------------------------

Another option maybe to detect mapreduce boundaries in the script which do not have a parallel
specification and prompt the user to input a parallel number they want to use for all such
mapreduce boundaries (default being 1). This way users are given an opportunity at submit
time to specify parallelism if they forgot to do so in the script. 

> Use of default parallelism
> --------------------------
>
>                 Key: PIG-729
>                 URL: https://issues.apache.org/jira/browse/PIG-729
>             Project: Pig
>          Issue Type: Bug
>          Components: impl
>    Affects Versions: 1.0.1
>         Environment: Hadoop 0.20
>            Reporter: Santhosh Srinivasan
>             Fix For: 1.0.1
>
>
> Currently, if the user does not specify the number of reduce slots using the parallel
keyword, Pig lets Hadoop decide on the default number of reducers. This model worked well
with dynamically allocated clusters using HOD and for static clusters where the default number
of reduce slots was explicitly set. With Hadoop 0.20, a single static cluster will be shared
amongst a number of queues. As a result, a common scenario is to end up with default number
of reducers set to one (1).
> When users migrate to Hadoop 0.20, they might see a dramatic change in the performance
of their queries if they had not used the parallel keyword to specify the number of reducers.
In order to mitigate such circumstances, Pig can support one of the following:
> 1. Specify a default parallelism for the entire script.
> This option will allow users to use the same parallelism for all operators that do not
have the explicit parallel keyword. This will ensure that the scripts utilize more reducers
than the default of one reducer. On the down side, due to data transformations, usually operations
that are performed towards the end of the script will need smaller number of reducers compared
to the operators that appear at the beginning of the script.
> 2. Display a warning message for each reduce side operator that does have the use of
the explicit parallel keyword. Proceed with the execution.
> 3. Display an error message indicating the operator that does not have the explicit use
of the parallel keyword. Stop the execution.
> Other suggestions/thoughts/solutions are welcome.

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