Mridul Muralidharan commented on PIG-539:

Hi Chris,

  We had a similar requirement - where the number of map tasks was high 
(because of a large number of small files created as part of the pipeline 
prefix) and we wanted a small & fixed number of map tasks (== number of mappers 
in cluster).

The only way I found to control the behavior in this case was something 
extremely heavyweight - do :
LOAD, GROUP (with parallel), FOREACH/FLATTEN and rest of pipeline.

Apparently, there is no other way to do this in pig currently ...

> 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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