Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/9276#discussion_r49286654
--- 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.
--- End diff --
According to the implementation, we can't guarantee to satisfy
`minNumPostShufflePartitions` right?
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