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From "Thejas M Nair (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-729) Use of default parallelism
Date Mon, 23 Mar 2009 22:39:50 GMT

    [ https://issues.apache.org/jira/browse/PIG-729?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12688447#action_12688447
] 

Thejas M Nair commented on PIG-729:
-----------------------------------

Pig users might not know enough to decide on a good default parallelism,specially when running
adhoc queries.

Instead of defaulting to 1 , if a user does not specify the parallelism , we should use as
default a higher number which does not have negative impact on the throughput of the system.

Hadoop-dev might be able to guide us on the extent to which hadoop scales linearly with increasing
number of reducers. For example, if we are able to linearly scale upto x reducers, we can
use a default of min(max_reducers_possible, max_reducers_linear) .


> 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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