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https://issues.apache.org/jira/browse/PIG-2829?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13423602#comment-13423602
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Thejas M Nair commented on PIG-2829:
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I will review the patch soon. Some comments regarding the default configuration
-
bq. 2: changes existing default values:
After thinking of the multi-query use case, where you can have multiple
POPartialAgg operators in a map task, I am having second thoughts on turning
partial agg on by default. Can you try these settings queries where there are
around 10+ group+agg that get combined into single MR job ? Maybe we should
address the potential OOM issues for this use case before we change the
defaults. This is likely to be become a bigger issue when we use 100k records
to decide to turn on/off the partial aggregation.
bq. 3: adds a property pig.exec.mapPartAgg.reduction.checkinterval which
defaults to 100k, so after processing every 100k records mapagg will check the
reduction rate to see if it should be disabled. Previously we only look at
first 1000 records.
Can you do some benchmarks to see if there is any noticeable difference in
runtime because of the delay in turning mapPartAgg off ?
> Use partial aggregation more aggresively
> ----------------------------------------
>
> Key: PIG-2829
> URL: https://issues.apache.org/jira/browse/PIG-2829
> Project: Pig
> Issue Type: Improvement
> Affects Versions: 0.10.0
> Reporter: Jie Li
> Attachments: 2829.1.patch, 2829.separate.options.patch,
> pigmix-10G.png, tpch-10G.png
>
>
> Partial aggregation (Hash Aggregation, aka in-map combiner) is a new feature
> in Pig 0.10 that will perform aggregation within map function. The main
> advantage against combiner is it avoids de/serializing and sorting the data,
> and it can auto disable itself if the data reduction rate is low. Currently
> it's disabled by default.
> To leverage the power of PartialAgg more aggressively, several things need to
> be revisited:
> 1. The threshold of auto-disabling. Currently each mapper looks at first 1k
> (hard-coded) records to see if there's enough data size reduction (defaults
> to 10x, configurable). The check would happen earlier if the hash table gets
> full before processing the 1k records (hash table size is controlled by
> pig.cachedbag.memusage). We might want to relax these thresholds.
> 2. Dependency on the combiner. Currently the PartialAgg won't work without a
> combiner following it, so we need to provide separate options to enable each
> independently.
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