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From "CanBin Zheng (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (SPARK-20943) Correct BypassMergeSortShuffleWriter's comment
Date Fri, 02 Jun 2017 07:00:19 GMT

    [ https://issues.apache.org/jira/browse/SPARK-20943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16033988#comment-16033988
] 

CanBin Zheng edited comment on SPARK-20943 at 6/2/17 7:00 AM:
--------------------------------------------------------------

Look at these two cases, either Aggregator or Ordering is defined but mapsideCombine is false,
 they both run with BypassMergeSortShuffleWriter,
 {code}
 //Has Aggregator defined
  @Test
  def testGroupByKeyUsingBypassMergeSort(): Unit = {
    val data = List("Hello", "World", "Hello", "One", "Two")
    val rdd = sc.parallelize(data).map((_, 1)).groupByKey(2)
    rdd.collect()
  }

  //Has Ordering defined
  @Test
  def testShuffleWithKeyOrderingUsingBypassMergeSort(): Unit = {
    val data = List("Hello", "World", "Hello", "One", "Two")
    val rdd = sc.parallelize(data).map((_, 1))
    val ord = implicitly[Ordering[String]]
    val shuffledRDD = new ShuffledRDD[String, Int, Int](rdd, new HashPartitioner(2)).setKeyOrdering(ord)
    shuffledRDD.collect()
  }
{code}


was (Author: canbinzheng):
Look at there two cases.
 {code}
 //Has Aggregator defined
  @Test
  def testGroupByKeyUsingBypassMergeSort(): Unit = {
    val data = List("Hello", "World", "Hello", "One", "Two")
    val rdd = sc.parallelize(data).map((_, 1)).groupByKey(2)
    rdd.collect()
  }

  //Has Ordering defined
  @Test
  def testShuffleWithKeyOrderingUsingBypassMergeSort(): Unit = {
    val data = List("Hello", "World", "Hello", "One", "Two")
    val rdd = sc.parallelize(data).map((_, 1))
    val ord = implicitly[Ordering[String]]
    val shuffledRDD = new ShuffledRDD[String, Int, Int](rdd, new HashPartitioner(2)).setKeyOrdering(ord)
    shuffledRDD.collect()
  }
{code}

> Correct BypassMergeSortShuffleWriter's comment
> ----------------------------------------------
>
>                 Key: SPARK-20943
>                 URL: https://issues.apache.org/jira/browse/SPARK-20943
>             Project: Spark
>          Issue Type: Improvement
>          Components: Documentation, Shuffle
>    Affects Versions: 2.1.1
>            Reporter: CanBin Zheng
>            Priority: Trivial
>              Labels: starter
>
> There are some comments written in BypassMergeSortShuffleWriter.java about when to select
this write path´╝î the three required conditions are described as follows:  
> 1. no Ordering is specified, and
> 2. no Aggregator is specified, and
> 3. the number of partitions is less than 
>  spark.shuffle.sort.bypassMergeThreshold
> Obviously, the conditions written are partially wrong and misleading, the right conditions
should be:
> 1. map-side combine is false, and
> 2. the number of partitions is less than 
>  spark.shuffle.sort.bypassMergeThreshold



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