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https://issues.apache.org/jira/browse/SPARK-4550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14302999#comment-14302999
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Patrick Wendell commented on SPARK-4550:
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The doc alludes to having to (at some point) deal with comparing serialized
objects. In the future one approach would be to restrict this only to
SchemaRDD's where we can have more control over the serialized format. This is
effectively what Flink and other systems do (they basically only have
SchemaRDD's).
> 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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