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From "Olga Natkovich (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-539) unable to control parallelism of Map tasks
Date Thu, 20 Nov 2008 18:14:44 GMT

    [ https://issues.apache.org/jira/browse/PIG-539?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12649436#action_12649436

Olga Natkovich commented on PIG-539:

The current semantics of PARALLEL is that it controls the number of reducers and not the number
of maps. The number of maps is controlled by the slicer. You might need a custom slicer to
make sure that all your data is flowing through a single map.

To change the semantics, we need to have a wide discussion on the user list and get community
support for this.

> unable to control parallelism of Map tasks
> ------------------------------------------
>                 Key: PIG-539
>                 URL: https://issues.apache.org/jira/browse/PIG-539
>             Project: Pig
>          Issue Type: Bug
>          Components: impl
>         Environment: local execution + hadoop execution
>            Reporter: Christopher Olston
> I put "PARALLEL 1" following *every* statement in my pig script, and it still executes
maps with more than 1 parallel task. This is a major problem because for one of my operations
I need to have a serialized (non-parallel) map.
> Probably the semantics of parallelism should be as follows:
>  1. group pig operators into map/reduce stages
>  2. for each stage, take the minimum of the "Parallel" directives given by the user for
statements executed as part of that stage
> (We'll have to decide on a rule for statements that use the combiner, which execute partially
on the map side and partially on the reduce side ...)

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