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https://issues.apache.org/jira/browse/SPARK-4550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14302326#comment-14302326
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Patrick Wendell commented on SPARK-4550:
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Yeah, this is a good idea. I don't see why we don't serialize these immediately.
> In sort-based shuffle, store map outputs in serialized form
> -----------------------------------------------------------
>
> Key: SPARK-4550
> URL: https://issues.apache.org/jira/browse/SPARK-4550
> Project: Spark
> Issue Type: Improvement
> Components: Shuffle, Spark Core
> Affects Versions: 1.2.0
> Reporter: Sandy Ryza
> Priority: Critical
> Attachments: SPARK-4550-design-v1.pdf
>
>
> One drawback with sort-based shuffle compared to hash-based shuffle is that
> it ends up storing many more java objects in memory. If Spark could store
> map outputs in serialized form, it could
> * spill less often because the serialized form is more compact
> * reduce GC pressure
> This will only work when the serialized representations of objects are
> independent from each other and occupy contiguous segments of memory. E.g.
> when Kryo reference tracking is left on, objects may contain pointers to
> objects farther back in the stream, which means that the sort can't relocate
> objects without corrupting them.
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