Memoize preferred locations in ZippedPartitionsBaseRDD so preferred location...

Memoize preferred locations in ZippedPartitionsBaseRDD so preferred location computation doesn't lead to exponential explosion.

(cherry picked from commit e36fe55a)
Signed-off-by: Reynold Xin <rxin@apache.org>

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

@@ -22,7 +22,8 @@ import java.io.{ObjectOutputStream, IOException}
 
 private[spark] class ZippedPartitionsPartition(
     idx: Int,
-    @transient rdds: Seq[RDD[_]])
+    @transient rdds: Seq[RDD[_]],
+    @transient val preferredLocations: Seq[String])
   extends Partition {
 
   override val index: Int = idx
@@ -47,27 +48,21 @@ abstract class ZippedPartitionsBaseRDD[V: ClassManifest](
     if (preservesPartitioning) firstParent[Any].partitioner else None
 
   override def getPartitions: Array[Partition] = {
-    val sizes = rdds.map(x => x.partitions.size)
-    if (!sizes.forall(x => x == sizes(0))) {
+    val numParts = rdds.head.partitions.size
+    if (!rdds.forall(rdd => rdd.partitions.size == numParts)) {
       throw new IllegalArgumentException("Can't zip RDDs with unequal numbers of partitions")
     }
-    val array = new Array[Partition](sizes(0))
-    for (i <- 0 until sizes(0)) {
-      array(i) = new ZippedPartitionsPartition(i, rdds)
+    Array.tabulate[Partition](numParts) { i =>
+      val prefs = rdds.map(rdd => rdd.preferredLocations(rdd.partitions(i)))
+      // Check whether there are any hosts that match all RDDs; otherwise return the union
+      val exactMatchLocations = prefs.reduce((x, y) => x.intersect(y))
+      val locs = if (!exactMatchLocations.isEmpty) exactMatchLocations else prefs.flatten.distinct
+      new ZippedPartitionsPartition(i, rdds, locs)
     }
-    array
   }
 
   override def getPreferredLocations(s: Partition): Seq[String] = {
-    val parts = s.asInstanceOf[ZippedPartitionsPartition].partitions
-    val prefs = rdds.zip(parts).map { case (rdd, p) => rdd.preferredLocations(p) }
-    // Check whether there are any hosts that match all RDDs; otherwise return the union
-    val exactMatchLocations = prefs.reduce((x, y) => x.intersect(y))
-    if (!exactMatchLocations.isEmpty) {
-      exactMatchLocations
-    } else {
-      prefs.flatten.distinct
-    }
+    s.asInstanceOf[ZippedPartitionsPartition].preferredLocations
   }
 
   override def clearDependencies() {