Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/9276#discussion_r43719923
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/ExchangeCoordinator.scala
---
@@ -0,0 +1,260 @@
+/*
+ * 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.sql.execution
+
+import java.util.{Map => JMap, HashMap => JHashMap}
+
+import scala.collection.mutable.ArrayBuffer
+
+import org.apache.spark.{Logging, SimpleFutureAction, ShuffleDependency,
MapOutputStatistics}
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.InternalRow
+
+/**
+ * A coordinator used to determines how we shuffle data between stages
generated by Spark SQL.
+ * Right now, the work of this coordinator is to determine the number of
post-shuffle partitions
+ * for a stage that needs to fetch shuffle data from one or multiple
stages.
+ *
+ * A coordinator is constructed with three parameters, `numExchanges`,
+ * `targetPostShuffleInputSize`, and `minNumPostShufflePartitions`.
+ * - `numExchanges` is used to indicated that how many [[Exchange]]s that
will be registered to
+ * this coordinator. So, when we start to do any actual work, we have a
way to make sure that
+ * we have got expected number of [[Exchange]]s.
+ * - `targetPostShuffleInputSize` is the targeted size of a post-shuffle
partition's
+ * input data size. With this parameter, we can estimate the number of
post-shuffle partitions.
+ * This parameter is configured through
+ * `spark.sql.adaptive.shuffle.targetPostShuffleInputSize`.
+ * - `minNumPostShufflePartitions` is an optional parameter. If it is
defined, this coordinator
+ * will try to make sure that there are at least
`minNumPostShufflePartitions` post-shuffle
+ * partitions.
+ *
+ * The workflow of this coordinator is described as follows:
+ * - Before the execution of a [[SparkPlan]], for an [[Exchange]]
operator,
+ * if an [[ExchangeCoordinator]] is assigned to it, it registers itself
to this coordinator.
+ * This happens in the `doPrepare` method.
+ * - Once we start to execute a physical plan, an [[Exchange]] registered
to this coordinator will
+ * call `postShuffleRDD` to get its corresponding post-shuffle
[[ShuffledRowRDD]].
+ * If this coordinator has made the decision on how to shuffle data,
this [[Exchange]] will
+ * immediately get its corresponding post-shuffle [[ShuffledRowRDD]].
+ * - If this coordinator has not made the decision on how to shuffle
data, it will ask those
+ * registered [[Exchange]]s to submit their pre-shuffle stages. Then,
based on the the size
+ * statistics of pre-shuffle partitions, this coordinator will
determine the number of
+ * post-shuffle partitions and pack multiple pre-shuffle partitions
with continuous indices
+ * to a single post-shuffle partition whenever necessary.
+ * - Finally, this coordinator will create post-shuffle
[[ShuffledRowRDD]]s for all registered
+ * [[Exchange]]s. So, when an [[Exchange]] calls `postShuffleRDD`, this
coordinator can
+ * lookup the corresponding [[RDD]].
+ *
+ * The strategy used to determine the number of post-shuffle partitions is
described as follows.
+ * To determine the number of post-shuffle partitions, we have a target
input size for a
+ * post-shuffle partition. Once we have size statistics of pre-shuffle
partitions from stages
+ * corresponding to the registered [[Exchange]]s, we will do a pass of
those statistics and
+ * pack pre-shuffle partitions with continuous indices to a single
post-shuffle partition until
+ * the size of a post-shuffle partition is equal or greater than the
target size.
+ * For example, we have two stages with the following pre-shuffle
partition size statistics:
+ * stage 1: [100 MB, 20 MB, 100 MB, 10MB, 30 MB]
+ * stage 2: [10 MB, 10 MB, 70 MB, 5 MB, 5 MB]
+ * assuming the target input size is 128 MB, we will have three
post-shuffle partitions,
+ * which are:
+ * - post-shuffle partition 0: pre-shuffle partition 0 and 1
+ * - post-shuffle partition 1: pre-shuffle partition 2
+ * - post-shuffle partition 2: pre-shuffle partition 3 and 4
+ */
+private[sql] class ExchangeCoordinator(
+ numExchanges: Int,
+ advisoryTargetPostShuffleInputSize: Long,
+ minNumPostShufflePartitions: Option[Int] = None)
+ extends Logging {
+
+ // The registered Exchange operators.
+ private[this] val exchanges = ArrayBuffer[Exchange]()
--- End diff --
Should this be a set instead of an array buffer in order to guard against
double-registration?
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