"By default, this uses only 8 parallel tasks to do the grouping." is a big misleading. Please refer to https://github.com/apache/spark/pull/389

detail is as following code :

  def defaultPartitioner(rdd: RDD[_], others: RDD[_]*): Partitioner = {
    val bySize = (Seq(rdd) ++ others).sortBy(_.partitions.size).reverse
    for (r <- bySize if r.partitioner.isDefined) {
      return r.partitioner.get
    }
    if (rdd.context.conf.contains("spark.default.parallelism")) {
      new HashPartitioner(rdd.context.defaultParallelism)
    } else {
      new HashPartitioner(bySize.head.partitions.size)
    }
  }

Author: Chen Chao <crazyjvm@gmail.com>

Closes #403 from CrazyJvm/patch-4 and squashes the following commits:

42f6c9e [Chen Chao] fix format
829a995 [Chen Chao] fix format
1568336 [Chen Chao] misleading task number of groupByKey