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From "Benjamin Reed (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-16) setting parallel from grunt via set command
Date Fri, 02 Nov 2007 14:40:51 GMT

    [ https://issues.apache.org/jira/browse/PIG-16?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12539597
] 

Benjamin Reed commented on PIG-16:
----------------------------------

set parallelism would probably be better than set reduce_parallelism because parallelism can
be used in other places besides reduce. For example, hopefully, soon we will detect that a
dataset is already sorted and do the group by in the map rather than reduce.

It would probably also be better to take a number rather than TRUE or FALSE so that you can
deviate from the default when need. I would think it should be

set parallelism number|DEFAULT

In reality, the hadoop configuration file should have the optimal number of reducer tasks
for a given configuration. (That is the default, not false.) If you want to override it, you
could provide a specific number.

I also don't think we should assume the use of an AlgebraicFunction would want the parallelism
set to 1. In general I would think that is not the case. The only case I can think of for
automatically setting to 1 would be a group all.

> setting parallel from grunt via set command
> -------------------------------------------
>
>                 Key: PIG-16
>                 URL: https://issues.apache.org/jira/browse/PIG-16
>             Project: Pig
>          Issue Type: Improvement
>          Components: grunt
>            Reporter: Olga Natkovich
>            Priority: Minor
>
> I'd like to propose a different model which uses the grunt "set" option and/or a command
line option which sets reduce
> parallelism to the be true and automatic.
> 	set reduce_parallelism TRUE
> 	set reduce_parallelism FALSE [Default - BTW, why is this the default?]
> This way I won't have to update my script every single time I try playing with -D"hod=-m
N", parallelism for reduce
> statements will default, appropriately, to 2*(N-1).
> Alternatively, could I just specify PARALLEL with no value or PARALLEL DEFAULT;  And
any time I needed to force reduce
> to be single job, I could write PARALLEL 1.
> Basically, this whole thing tripped me up for a long time and I just haven't understood
if there is a really good
> reason to not make parallelism.
> I guess it might be if you have aggregation functions that do not parallelize.
> If this is the case, then it seems to me that this should be detectable automagically
based on whether the function is
> a vanilla EvalFunction or if it is an AlgebraicFunction.

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