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From "David Dreyfus (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PIG-3979) group all performance, garbage collection, and incremental aggregation
Date Thu, 02 Oct 2014 14:28:34 GMT

    [ https://issues.apache.org/jira/browse/PIG-3979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14156611#comment-14156611

David Dreyfus commented on PIG-3979:

Hi Daniel,
Do I need to do anything at this time with the patch?

> group all performance, garbage collection, and incremental aggregation
> ----------------------------------------------------------------------
>                 Key: PIG-3979
>                 URL: https://issues.apache.org/jira/browse/PIG-3979
>             Project: Pig
>          Issue Type: Improvement
>          Components: impl
>    Affects Versions: 0.12.0, 0.11.1
>            Reporter: David Dreyfus
>            Assignee: David Dreyfus
>             Fix For: 0.15.0
>         Attachments: PIG-3979-v1.patch, POPartialAgg.java.patch, SpillableMemoryManager.java.patch
> I have a PIG statement similar to:
> summary = foreach (group data ALL) generate 
> COUNT(data.col1), SUM(data.col2), SUM(data.col2)
> , Moments(col3)
> , Moments(data.col4)
> There are a couple of hundred columns.
> I set the following:
> SET pig.exec.mapPartAgg true;
> SET pig.exec.mapPartAgg.minReduction 3;
> SET pig.cachedbag.memusage 0.05;
> I found that when I ran this on a JVM with insufficient memory, the process eventually
timed out because of an infinite garbage collection loop.
> The problem was invariant to the memusage setting.
> I solved the problem by making changes to:
> org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperator.POPartialAgg.java
> Rather than reading in 10000 records to establish an estimate of the reduction, I make
an estimate after reading in enough tuples to fill pig.cachedbag.memusage percent of Runtime.getRuntime().maxMemory().
> I also made a change to guarantee at least one record allowed in second tier storage.
In the current implementation, if the reduction is very high 1000:1, space in second tier
storage is zero.
> With these changes, I can summarize large data sets with small JVMs. I also find that
setting pig.cachedbag.memusage to a small number such as 0.05 results in much better garbage
collection performance without reducing throughput. I suppose tuning GC would also solve a
problem with excessive garbage collection.
> The performance is sweet. 

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