Spark源码解析之小分区合并
2016-12-13 13:53
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coalesce 函数来减少分区
小分区合并,不需要 Shuffle 过程/** * Return a new RDD that is reduced into `numPartitions` partitions. * * This results in a narrow dependency, e.g. if you go from 1000 partitions * to 100 partitions, there will not be a shuffle, instead each of the 100 * new partitions will claim 10 of the current partitions. * * However, if you're doing a drastic coalesce, e.g. to numPartitions = 1, * this may result in your computation taking place on fewer nodes than * you like (e.g. one node in the case of numPartitions = 1). To avoid this, * you can pass shuffle = true. This will add a shuffle step, but means the * current upstream partitions will be executed in parallel (per whatever * the current partitioning is). * * Note: With shuffle = true, you can actually coalesce to a larger number * of partitions. This is useful if you have a small number of partitions, * say 100, potentially with a few partitions being abnormally large. Calling * coalesce(1000, shuffle = true) will result in 1000 partitions with the * data distributed using a hash partitioner. */ def coalesce(numPartitions: Int, shuffle: Boolean = false)(implicit ord: Ordering[T] = null) : RDD[T] = withScope { 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 b207 ) } } : 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 { new CoalescedRDD(this, numPartitions) } }
repartition 重分区
增加分区数,需要 Shuffle 过程/** * Return a new RDD that has exactly numPartitions partitions. * * Can increase or decrease the level of parallelism in this RDD. Internally, this uses * a shuffle to redistribute data. * * If you are decreasing the number of partitions in this RDD, consider using `coalesce`, * which can avoid performing a shuffle. */ def repartition(numPartitions: Int)(implicit ord: Ordering[T] = null): RDD[T] = withScope { coalesce(numPartitions, shuffle = true) }
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