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https://issues.apache.org/jira/browse/SPARK-10797?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Hyukjin Kwon updated SPARK-10797:
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Labels: bulk-closed (was: )
> RDD's coalesce should not write out the temporary key
> -----------------------------------------------------
>
> Key: SPARK-10797
> URL: https://issues.apache.org/jira/browse/SPARK-10797
> Project: Spark
> Issue Type: Improvement
> Components: Spark Core
> Reporter: Zoltán Zvara
> Priority: Major
> Labels: bulk-closed
>
> It seems that {{RDD.coalesce}} will unnecessarily write out (to shuffle
> files) temporary keys used on the shuffle code path. Consider the following
> code:
> {code:title=RDD.scala|borderStyle=solid}
> if (shuffle) {
> /** Distributes elements evenly across output partitions, starting from
> a random partition. */
> val distributePartition = (index: Int, items: Iterator[T]) => {
> var position = (new Random(index)).nextInt(numPartitions)
> items.map { t =>
> // Note that the hash code of the key will just be the key itself.
> The HashPartitioner
> // will mod it with the number of total partitions.
> position = position + 1
> (position, t)
> }
> } : Iterator[(Int, T)]
> // include a shuffle step so that our upstream tasks are still
> distributed
> new CoalescedRDD(
> new ShuffledRDD[Int, T,
> T](mapPartitionsWithIndex(distributePartition),
> new HashPartitioner(numPartitions)),
> numPartitions).values
> } else {
> {code}
> {{ShuffledRDD}} will hash using {{position}} as keys as in the
> {{distributePartition}} function. After the bucket has been chosen by the
> sorter {{ExternalSorter}} or {{BypassMergeSortShuffleWriter}}, the
> {{DiskBlockObjectWriter}} writes out both the (temporary) key and value to
> the specified partition. On the next stage, after reading we take only the
> values with {{PairRDDFunctions}}.
> This certainly has a performance impact, as we unnecessarily write/read
> {{Int}} and transform the data.
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