Merge pull request apache#297 from tdas/window-improvement

Improvements to DStream window ops and refactoring of Spark's CheckpointSuite

- Added a new RDD - PartitionerAwareUnionRDD. Using this RDD, one can take multiple RDDs partitioned by the same partitioner and unify them into a single RDD while preserving the partitioner. So m RDDs with p partitions each will be unified to a single RDD with p partitions and the same partitioner. The preferred location for each partition of the unified RDD will be the most common preferred location of the corresponding partitions of the parent RDDs. For example, location of partition 0 of the unified RDD will be where most of partition 0 of the parent RDDs are located.
- Improved the performance of DStream's reduceByKeyAndWindow and groupByKeyAndWindow. Both these operations work by doing per-batch reduceByKey/groupByKey and then using PartitionerAwareUnionRDD to union the RDDs across the window. This eliminates a shuffle related to the window operation, which can reduce batch processing time by 30-40% for simple workloads.
- Fixed bugs and simplified Spark's CheckpointSuite. Some of the tests were incorrect and unreliable. Added missing tests for ZippedRDD. I can go into greater detail if necessary.
- Added mapSideCombine option to combineByKeyAndWindow.

core/src/main/scala/org/apache/spark/rdd/PartitionerAwareUnionRDD.scala

@@ -0,0 +1,110 @@
+/*
+ * 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.rdd
+
+import scala.reflect.ClassTag
+import java.io.{ObjectOutputStream, IOException}
+import org.apache.spark.{TaskContext, OneToOneDependency, SparkContext, Partition}
+
+
+/**
+ * Class representing partitions of PartitionerAwareUnionRDD, which maintains the list of corresponding partitions
+ * of parent RDDs.
+ */
+private[spark]
+class PartitionerAwareUnionRDDPartition(
+    @transient val rdds: Seq[RDD[_]],
+    val idx: Int
+  ) extends Partition {
+  var parents = rdds.map(_.partitions(idx)).toArray
+
+  override val index = idx
+  override def hashCode(): Int = idx
+
+  @throws(classOf[IOException])
+  private def writeObject(oos: ObjectOutputStream) {
+    // Update the reference to parent partition at the time of task serialization
+    parents = rdds.map(_.partitions(index)).toArray
+    oos.defaultWriteObject()
+  }
+}
+
+/**
+ * Class representing an RDD that can take multiple RDDs partitioned by the same partitioner and
+ * unify them into a single RDD while preserving the partitioner. So m RDDs with p partitions each
+ * will be unified to a single RDD with p partitions and the same partitioner. The preferred
+ * location for each partition of the unified RDD will be the most common preferred location
+ * of the corresponding partitions of the parent RDDs. For example, location of partition 0
+ * of the unified RDD will be where most of partition 0 of the parent RDDs are located.
+ */
+private[spark]
+class PartitionerAwareUnionRDD[T: ClassTag](
+    sc: SparkContext,
+    var rdds: Seq[RDD[T]]
+  ) extends RDD[T](sc, rdds.map(x => new OneToOneDependency(x))) {
+  require(rdds.length > 0)
+  require(rdds.flatMap(_.partitioner).toSet.size == 1,
+    "Parent RDDs have different partitioners: " + rdds.flatMap(_.partitioner))
+
+  override val partitioner = rdds.head.partitioner
+
+  override def getPartitions: Array[Partition] = {
+    val numPartitions = partitioner.get.numPartitions
+    (0 until numPartitions).map(index => {
+      new PartitionerAwareUnionRDDPartition(rdds, index)
+    }).toArray
+  }
+
+  // Get the location where most of the partitions of parent RDDs are located
+  override def getPreferredLocations(s: Partition): Seq[String] = {
+    logDebug("Finding preferred location for " + this + ", partition " + s.index)
+    val parentPartitions = s.asInstanceOf[PartitionerAwareUnionRDDPartition].parents
+    val locations = rdds.zip(parentPartitions).flatMap {
+      case (rdd, part) => {
+        val parentLocations = currPrefLocs(rdd, part)
+        logDebug("Location of " + rdd + " partition " + part.index + " = " + parentLocations)
+        parentLocations
+      }
+    }
+    val location = if (locations.isEmpty) {
+      None
+    } else  {
+      // Find the location that maximum number of parent partitions prefer
+      Some(locations.groupBy(x => x).maxBy(_._2.length)._1)
+    }
+    logDebug("Selected location for " + this + ", partition " + s.index + " = " + location)
+    location.toSeq
+  }
+
+  override def compute(s: Partition, context: TaskContext): Iterator[T] = {
+    val parentPartitions = s.asInstanceOf[PartitionerAwareUnionRDDPartition].parents
+    rdds.zip(parentPartitions).iterator.flatMap {
+      case (rdd, p) => rdd.iterator(p, context)
+    }
+  }
+
+  override def clearDependencies() {
+    super.clearDependencies()
+    rdds = null
+  }
+
+  // Get the *current* preferred locations from the DAGScheduler (as opposed to the static ones)
+  private def currPrefLocs(rdd: RDD[_], part: Partition): Seq[String] = {
+    rdd.context.getPreferredLocs(rdd, part.index).map(tl => tl.host)
+  }
+}

core/src/main/scala/org/apache/spark/rdd/RDDCheckpointData.scala

@@ -40,7 +40,7 @@ private[spark] object CheckpointState extends Enumeration {
  * manages the post-checkpoint state by providing the updated partitions, iterator and preferred locations
  * of the checkpointed RDD.
  */
-private[spark] class RDDCheckpointData[T: ClassTag](rdd: RDD[T])
+private[spark] class RDDCheckpointData[T: ClassTag](@transient rdd: RDD[T])
   extends Logging with Serializable {
 
   import CheckpointState._