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https://issues.apache.org/jira/browse/SPARK-31948?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17132569#comment-17132569
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L. C. Hsieh commented on SPARK-31948:
-------------------------------------
I guess I have similar question. When the reduction factor isn't high, map-side
combining will be any disadvantage to run? I think it still can reduce shuffle
data, although it might not be too much.
> expose mapSideCombine in aggByKey/reduceByKey/foldByKey
> -------------------------------------------------------
>
> Key: SPARK-31948
> URL: https://issues.apache.org/jira/browse/SPARK-31948
> Project: Spark
> Issue Type: Improvement
> Components: ML, Spark Core
> Affects Versions: 3.1.0
> Reporter: zhengruifeng
> Priority: Minor
>
> 1. {{aggregateByKey}}, {{reduceByKey}} and {{foldByKey}} will always perform
> {{mapSideCombine}};
> However, this can be skiped sometime, specially in ML (RobustScaler):
> {code:java}
> vectors.mapPartitions { iter =>
> if (iter.hasNext) {
> val summaries = Array.fill(numFeatures)(
> new QuantileSummaries(QuantileSummaries.defaultCompressThreshold,
> relativeError))
> while (iter.hasNext) {
> val vec = iter.next
> vec.foreach { (i, v) => if (!v.isNaN) summaries(i) =
> summaries(i).insert(v) }
> }
> Iterator.tabulate(numFeatures)(i => (i, summaries(i).compress))
> } else Iterator.empty
> }.reduceByKey { case (s1, s2) => s1.merge(s2) } {code}
>
> This {{reduceByKey}} in {{RobustScaler}} does not need {{mapSideCombine}} at
> all, similar places exist in {{KMeans}}, {{GMM}}, etc;
> To my knowledge, we do not need {{mapSideCombine}} if the reduction factor
> isn't high;
>
> 2. {{treeAggregate}} and {{treeReduce}} are based on {{foldByKey}}, the
> {{mapSideCombine}} in the first call of {{foldByKey}} can also be avoided.
>
> SPARK-772:
> {quote}
> Map side combine in group by key case does not reduce the amount of data
> shuffled. Instead, it forces a lot more objects to go into old gen, and leads
> to worse GC.
> {quote}
>
> So what about:
> 1. exposing mapSideCombine in {{aggByKey}}/{{reduceByKey}}/{{foldByKey}}, so
> that user can disable unnecessary mapSideCombine
> 2. disabling the {{mapSideCombine}} in the first call of {{foldByKey}} in
> {{treeAggregate}} and {{treeReduce}}
> 3. disabling the unnecessary {{mapSideCombine}} in ML;
> Friendly ping [~srowen] [~huaxingao] [~weichenxu123] [~hyukjin.kwon]
> [~viirya]
>
>
>
>
>
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