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From JoshRosen <...@git.apache.org>
Subject [GitHub] spark pull request: [WIP][SPARK-7081]Faster sort-based shuffle pat...
Date Mon, 11 May 2015 23:49:55 GMT
Github user JoshRosen commented on a diff in the pull request:

    https://github.com/apache/spark/pull/5868#discussion_r30093761
  
    --- Diff: core/src/main/scala/org/apache/spark/shuffle/unsafe/UnsafeShuffleManager.scala
---
    @@ -0,0 +1,178 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one or more
    + * contributor license agreements.  See the NOTICE file distributed with
    + * this work for additional information regarding copyright ownership.
    + * The ASF licenses this file to You under the Apache License, Version 2.0
    + * (the "License"); you may not use this file except in compliance with
    + * the License.  You may obtain a copy of the License at
    + *
    + *    http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.spark.shuffle.unsafe
    +
    +import org.apache.spark._
    +import org.apache.spark.serializer.Serializer
    +import org.apache.spark.shuffle._
    +import org.apache.spark.shuffle.sort.SortShuffleManager
    +
    +/**
    + * Subclass of [[BaseShuffleHandle]], used to identify when we've chosen to use the new
shuffle.
    + */
    +private class UnsafeShuffleHandle[K, V](
    +    shuffleId: Int,
    +    override val numMaps: Int,
    +    override val dependency: ShuffleDependency[K, V, V])
    +  extends BaseShuffleHandle(shuffleId, numMaps, dependency) {
    +}
    +
    +private[spark] object UnsafeShuffleManager extends Logging {
    +  /**
    +   * Helper method for determining whether a shuffle should use the optimized unsafe
shuffle
    +   * path or whether it should fall back to the original sort-based shuffle.
    +   */
    +  def canUseUnsafeShuffle[K, V, C](dependency: ShuffleDependency[K, V, C]): Boolean =
{
    +    val shufId = dependency.shuffleId
    +    val serializer = Serializer.getSerializer(dependency.serializer)
    +    if (!serializer.supportsRelocationOfSerializedObjects) {
    +      log.debug(s"Can't use UnsafeShuffle for shuffle $shufId because the serializer,
" +
    --- End diff --
    
    I considered this, but I worry that this will result in extremely chatty logs because
many operations won't be able to use this new shuffle yet.  For example, this would trigger
a warning whenever `reduceByKey` is used.
    
    This is a tricky issue, especially as the number of special-case shuffle optimizations
grows.  It will be very easy for users to slightly change their programs in ways that trigger
slower code paths (e.g. by switching from LZF to LZ4 compression).  Conversely, this also
creates the potential for small changes to result in huge secondary performance benefits in
non-obvious ways: if a user were to switch from LZ4 to LZF, then the current code would hit
a more efficient shuffle merge path and might exhibit huge speed-ups, but a user might misattribute
this to LZF being faster / offering better compression in general, whereas it's really the
optimized merge path that's activated by LZF's concatenatibility that is responsible for the
speed up.  This is a general issue that's probably worth exploring as part of a broader discussion
of how to expose internal knowledge of performance optimizations back to end users.


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