Fix ZippedRDD as part Matei's suggestion

core/src/main/scala/spark/rdd/ZippedRDD.scala

 package spark.rdd
 
-import spark.{OneToOneDependency, RDD, SparkContext, Partition, TaskContext}
+import spark.{Utils, OneToOneDependency, RDD, SparkContext, Partition, TaskContext}
 import java.io.{ObjectOutputStream, IOException}
 
 
@@ -49,9 +49,20 @@ class ZippedRDD[T: ClassManifest, U: ClassManifest](
 
   override def getPreferredLocations(s: Partition): Seq[String] = {
     val (partition1, partition2) = s.asInstanceOf[ZippedPartition[T, U]].partitions
-    // TODO: becomes complicated - intersect on hostPort if available, else fallback to host (removing intersected hostPort's).
-    // Since I am not very sure about this RDD, leaving it to others to comment better !
-    rdd1.preferredLocations(partition1).intersect(rdd2.preferredLocations(partition2))
+    val pref1 = rdd1.preferredLocations(partition1)
+    val pref2 = rdd2.preferredLocations(partition2)
+
+    // both partitions are instance local.
+    val instanceLocalLocations = pref1.intersect(pref2)
+
+    // remove locations which are already handled via instanceLocalLocations, and intersect where both partitions are node local.
+    val nodeLocalPref1 = pref1.filter(loc => ! instanceLocalLocations.contains(loc)).map(loc => Utils.parseHostPort(loc)._1)
+    val nodeLocalPref2 = pref2.filter(loc => ! instanceLocalLocations.contains(loc)).map(loc => Utils.parseHostPort(loc)._1)
+    val nodeLocalLocations = nodeLocalPref1.intersect(nodeLocalPref2)
+
+
+    // Can have mix of instance local (hostPort) and node local (host) locations as preference !
+    instanceLocalLocations ++ nodeLocalLocations
   }
 
   override def clearDependencies() {