SPARK-942: Do not materialize partitions when DISK_ONLY storage level is used

This is a port of a pull request original targeted at incubator-spark: https://github.com/apache/incubator-spark/pull/180

Essentially if a user returns a generative iterator (from a flatMap operation), when trying to persist the data, Spark would first unroll the iterator into an ArrayBuffer, and then try to figure out if it could store the data. In cases where the user provided an iterator that generated more data then available memory, this would case a crash. With this patch, if the user requests a persist with a 'StorageLevel.DISK_ONLY', the iterator will be unrolled as it is inputed into the serializer.

To do this, two changes where made:
1) The type of the 'values' argument in the putValues method of the BlockStore interface was changed from ArrayBuffer to Iterator (and all code interfacing with this method was modified to connect correctly.
2) The JavaSerializer now calls the ObjectOutputStream 'reset' method every 1000 objects. This was done because the ObjectOutputStream caches objects (thus preventing them from being GC'd) to write more compact serialization. If reset is never called, eventually the memory fills up, if it is called too often then the serialization streams become much larger because of redundant class descriptions.

Author: Kyle Ellrott <kellrott@gmail.com>

Closes #50 from kellrott/iterator-to-disk and squashes the following commits:

9ef7cb8 [Kyle Ellrott] Fixing formatting issues.
60e0c57 [Kyle Ellrott] Fixing issues (formatting, variable names, etc.) from review comments
8aa31cd [Kyle Ellrott] Merge ../incubator-spark into iterator-to-disk
33ac390 [Kyle Ellrott] Merge branch 'iterator-to-disk' of github.com:kellrott/incubator-spark into iterator-to-disk
2f684ea [Kyle Ellrott] Refactoring the BlockManager to replace the Either[Either[A,B]] usage. Now using trait 'Values'. Also modified BlockStore.putBytes call to return PutResult, so that it behaves like putValues.
f70d069 [Kyle Ellrott] Adding docs for spark.serializer.objectStreamReset configuration
7ccc74b [Kyle Ellrott] Moving the 'LargeIteratorSuite' to simply test persistance of iterators. It doesn't try to invoke a OOM error any more
16a4cea [Kyle Ellrott] Streamlined the LargeIteratorSuite unit test. It should now run in ~25 seconds. Confirmed that it still crashes an unpatched copy of Spark.
c2fb430 [Kyle Ellrott] Removing more un-needed array-buffer to iterator conversions
627a8b7 [Kyle Ellrott] Wrapping a few long lines
0f28ec7 [Kyle Ellrott] Adding second putValues to BlockStore interface that accepts an ArrayBuffer (rather then an Iterator). This will allow BlockStores to have slightly different behaviors dependent on whether they get an Iterator or ArrayBuffer. In the case of the MemoryStore, it needs to duplicate and cache an Iterator into an ArrayBuffer, but if handed a ArrayBuffer, it can skip the duplication.
656c33e [Kyle Ellrott] Fixing the JavaSerializer to read from the SparkConf rather then the System property.
8644ee8 [Kyle Ellrott] Merge branch 'master' into iterator-to-disk
00c98e0 [Kyle Ellrott] Making the Java ObjectStreamSerializer reset rate configurable by the system variable 'spark.serializer.objectStreamReset', default is not 10000.
40fe1d7 [Kyle Ellrott] Removing rouge space
31fe08e [Kyle Ellrott] Removing un-needed semi-colons
9df0276 [Kyle Ellrott] Added check to make sure that streamed-to-dist RDD actually returns good data in the LargeIteratorSuite
a6424ba [Kyle Ellrott] Wrapping long line
2eeda75 [Kyle Ellrott] Fixing dumb mistake ("||" instead of "&&")
0e6f808 [Kyle Ellrott] Deleting temp output directory when done
95c7f67 [Kyle Ellrott] Simplifying StorageLevel checks
56f71cd [Kyle Ellrott] Merge branch 'master' into iterator-to-disk
44ec35a [Kyle Ellrott] Adding some comments.
5eb2b7e [Kyle Ellrott] Changing the JavaSerializer reset to occur every 1000 objects.
f403826 [Kyle Ellrott] Merge branch 'master' into iterator-to-disk
81d670c [Kyle Ellrott] Adding unit test for straight to disk iterator methods.
d32992f [Kyle Ellrott] Merge remote-tracking branch 'origin/master' into iterator-to-disk
cac1fad [Kyle Ellrott] Fixing MemoryStore, so that it converts incoming iterators to ArrayBuffer objects. This was previously done higher up the stack.
efe1102 [Kyle Ellrott] Changing CacheManager and BlockManager to pass iterators directly to the serializer when a 'DISK_ONLY' persist is called. This is in response to SPARK-942.

8 files changed, 226 insertions, 53 deletions

diff --git a/core/src/main/scala/org/apache/spark/CacheManager.scala b/core/src/main/scala/org/apache/spark/CacheManager.scala
index 1daabecf23..872e892c04 100644
--- a/core/src/main/scala/org/apache/spark/CacheManager.scala
+++ b/core/src/main/scala/org/apache/spark/CacheManager.scala
@@ -71,10 +71,30 @@ private[spark] class CacheManager(blockManager: BlockManager) extends Logging {
           val computedValues = rdd.computeOrReadCheckpoint(split, context)
           // Persist the result, so long as the task is not running locally
           if (context.runningLocally) { return computedValues }
-          val elements = new ArrayBuffer[Any]
-          elements ++= computedValues
-          blockManager.put(key, elements, storageLevel, tellMaster = true)
-          elements.iterator.asInstanceOf[Iterator[T]]
+          if (storageLevel.useDisk && !storageLevel.useMemory) {
+            // In the case that this RDD is to be persisted using DISK_ONLY
+            // the iterator will be passed directly to the blockManager (rather then
+            // caching it to an ArrayBuffer first), then the resulting block data iterator
+            // will be passed back to the user. If the iterator generates a lot of data,
+            // this means that it doesn't all have to be held in memory at one time.
+            // This could also apply to MEMORY_ONLY_SER storage, but we need to make sure
+            // blocks aren't dropped by the block store before enabling that.
+            blockManager.put(key, computedValues, storageLevel, tellMaster = true)
+            return blockManager.get(key) match {
+              case Some(values) =>
+                return new InterruptibleIterator(context, values.asInstanceOf[Iterator[T]])
+              case None =>
+                logInfo("Failure to store %s".format(key))
+                throw new Exception("Block manager failed to return persisted valued")
+            }
+          } else {
+            // In this case the RDD is cached to an array buffer. This will save the results
+            // if we're dealing with a 'one-time' iterator
+            val elements = new ArrayBuffer[Any]
+            elements ++= computedValues
+            blockManager.put(key, elements, storageLevel, tellMaster = true)
+            return elements.iterator.asInstanceOf[Iterator[T]]
+          }
         } finally {
           loading.synchronized {
             loading.remove(key)
diff --git a/core/src/main/scala/org/apache/spark/serializer/JavaSerializer.scala b/core/src/main/scala/org/apache/spark/serializer/JavaSerializer.scala
index 33c1705ad7..bfa647f7f0 100644
--- a/core/src/main/scala/org/apache/spark/serializer/JavaSerializer.scala
+++ b/core/src/main/scala/org/apache/spark/serializer/JavaSerializer.scala
@@ -23,9 +23,28 @@ import java.nio.ByteBuffer
 import org.apache.spark.SparkConf
 import org.apache.spark.util.ByteBufferInputStream
 
-private[spark] class JavaSerializationStream(out: OutputStream) extends SerializationStream {
+private[spark] class JavaSerializationStream(out: OutputStream, conf: SparkConf)
+  extends SerializationStream {
   val objOut = new ObjectOutputStream(out)
-  def writeObject[T](t: T): SerializationStream = { objOut.writeObject(t); this }
+  var counter = 0
+  val counterReset = conf.getInt("spark.serializer.objectStreamReset", 10000)
+
+  /**
+   * Calling reset to avoid memory leak:
+   * http://stackoverflow.com/questions/1281549/memory-leak-traps-in-the-java-standard-api
+   * But only call it every 10,000th time to avoid bloated serialization streams (when
+   * the stream 'resets' object class descriptions have to be re-written)
+   */
+  def writeObject[T](t: T): SerializationStream = {
+    objOut.writeObject(t)
+    if (counterReset > 0 && counter >= counterReset) {
+      objOut.reset()
+      counter = 0
+    } else {
+      counter += 1
+    }
+    this
+  }
   def flush() { objOut.flush() }
   def close() { objOut.close() }
 }
@@ -41,7 +60,7 @@ extends DeserializationStream {
   def close() { objIn.close() }
 }
 
-private[spark] class JavaSerializerInstance extends SerializerInstance {
+private[spark] class JavaSerializerInstance(conf: SparkConf) extends SerializerInstance {
   def serialize[T](t: T): ByteBuffer = {
     val bos = new ByteArrayOutputStream()
     val out = serializeStream(bos)
@@ -63,7 +82,7 @@ private[spark] class JavaSerializerInstance extends SerializerInstance {
   }
 
   def serializeStream(s: OutputStream): SerializationStream = {
-    new JavaSerializationStream(s)
+    new JavaSerializationStream(s, conf)
   }
 
   def deserializeStream(s: InputStream): DeserializationStream = {
@@ -79,5 +98,5 @@ private[spark] class JavaSerializerInstance extends SerializerInstance {
  * A Spark serializer that uses Java's built-in serialization.
  */
 class JavaSerializer(conf: SparkConf) extends Serializer {
-  def newInstance(): SerializerInstance = new JavaSerializerInstance
+  def newInstance(): SerializerInstance = new JavaSerializerInstance(conf)
 }
diff --git a/core/src/main/scala/org/apache/spark/storage/BlockManager.scala b/core/src/main/scala/org/apache/spark/storage/BlockManager.scala
index a734ddc1ef..977c24687c 100644
--- a/core/src/main/scala/org/apache/spark/storage/BlockManager.scala
+++ b/core/src/main/scala/org/apache/spark/storage/BlockManager.scala
@@ -35,6 +35,12 @@ import org.apache.spark.network._
 import org.apache.spark.serializer.Serializer
 import org.apache.spark.util._
 
+sealed trait Values
+
+case class ByteBufferValues(buffer: ByteBuffer) extends Values
+case class IteratorValues(iterator: Iterator[Any]) extends Values
+case class ArrayBufferValues(buffer: ArrayBuffer[Any]) extends Values
+
 private[spark] class BlockManager(
     executorId: String,
     actorSystem: ActorSystem,
@@ -455,9 +461,7 @@ private[spark] class BlockManager(
 
   def put(blockId: BlockId, values: Iterator[Any], level: StorageLevel, tellMaster: Boolean)
     : Long = {
-    val elements = new ArrayBuffer[Any]
-    elements ++= values
-    put(blockId, elements, level, tellMaster)
+    doPut(blockId, IteratorValues(values), level, tellMaster)
   }
 
   /**
@@ -479,7 +483,7 @@ private[spark] class BlockManager(
   def put(blockId: BlockId, values: ArrayBuffer[Any], level: StorageLevel,
           tellMaster: Boolean = true) : Long = {
     require(values != null, "Values is null")
-    doPut(blockId, Left(values), level, tellMaster)
+    doPut(blockId, ArrayBufferValues(values), level, tellMaster)
   }
 
   /**
@@ -488,10 +492,11 @@ private[spark] class BlockManager(
   def putBytes(blockId: BlockId, bytes: ByteBuffer, level: StorageLevel,
                tellMaster: Boolean = true) {
     require(bytes != null, "Bytes is null")
-    doPut(blockId, Right(bytes), level, tellMaster)
+    doPut(blockId, ByteBufferValues(bytes), level, tellMaster)
   }
 
-  private def doPut(blockId: BlockId, data: Either[ArrayBuffer[Any], ByteBuffer],
+  private def doPut(blockId: BlockId,
+                    data: Values,
                     level: StorageLevel, tellMaster: Boolean = true): Long = {
     require(blockId != null, "BlockId is null")
     require(level != null && level.isValid, "StorageLevel is null or invalid")
@@ -534,8 +539,9 @@ private[spark] class BlockManager(
 
     // If we're storing bytes, then initiate the replication before storing them locally.
     // This is faster as data is already serialized and ready to send.
-    val replicationFuture = if (data.isRight && level.replication > 1) {
-      val bufferView = data.right.get.duplicate() // Doesn't copy the bytes, just creates a wrapper
+    val replicationFuture = if (data.isInstanceOf[ByteBufferValues] && level.replication > 1) {
+      // Duplicate doesn't copy the bytes, just creates a wrapper
+      val bufferView = data.asInstanceOf[ByteBufferValues].buffer.duplicate()
       Future {
         replicate(blockId, bufferView, level)
       }
@@ -549,34 +555,43 @@ private[spark] class BlockManager(
 
       var marked = false
       try {
-        data match {
-          case Left(values) => {
-            if (level.useMemory) {
-              // Save it just to memory first, even if it also has useDisk set to true; we will
-              // drop it to disk later if the memory store can't hold it.
-              val res = memoryStore.putValues(blockId, values, level, true)
-              size = res.size
-              res.data match {
-                case Right(newBytes) => bytesAfterPut = newBytes
-                case Left(newIterator) => valuesAfterPut = newIterator
-              }
-            } else {
-              // Save directly to disk.
-              // Don't get back the bytes unless we replicate them.
-              val askForBytes = level.replication > 1
-              val res = diskStore.putValues(blockId, values, level, askForBytes)
-              size = res.size
-              res.data match {
-                case Right(newBytes) => bytesAfterPut = newBytes
-                case _ =>
-              }
+        if (level.useMemory) {
+          // Save it just to memory first, even if it also has useDisk set to true; we will
+          // drop it to disk later if the memory store can't hold it.
+          val res = data match {
+            case IteratorValues(iterator) =>
+              memoryStore.putValues(blockId, iterator, level, true)
+            case ArrayBufferValues(array) =>
+              memoryStore.putValues(blockId, array, level, true)
+            case ByteBufferValues(bytes) => {
+              bytes.rewind();
+              memoryStore.putBytes(blockId, bytes, level)
+            }
+          }
+          size = res.size
+          res.data match {
+            case Right(newBytes) => bytesAfterPut = newBytes
+            case Left(newIterator) => valuesAfterPut = newIterator
+          }
+        } else {
+          // Save directly to disk.
+          // Don't get back the bytes unless we replicate them.
+          val askForBytes = level.replication > 1
+
+          val res = data match {
+            case IteratorValues(iterator) =>
+              diskStore.putValues(blockId, iterator, level, askForBytes)
+            case ArrayBufferValues(array) =>
+              diskStore.putValues(blockId, array, level, askForBytes)
+            case ByteBufferValues(bytes) => {
+              bytes.rewind();
+              diskStore.putBytes(blockId, bytes, level)
             }
           }
-          case Right(bytes) => {
-            bytes.rewind()
-            // Store it only in memory at first, even if useDisk is also set to true
-            (if (level.useMemory) memoryStore else diskStore).putBytes(blockId, bytes, level)
-            size = bytes.limit
+          size = res.size
+          res.data match {
+            case Right(newBytes) => bytesAfterPut = newBytes
+            case _ =>
           }
         }
 
@@ -605,8 +620,8 @@ private[spark] class BlockManager(
     // values and need to serialize and replicate them now:
     if (level.replication > 1) {
       data match {
-        case Right(bytes) => Await.ready(replicationFuture, Duration.Inf)
-        case Left(values) => {
+        case ByteBufferValues(bytes) => Await.ready(replicationFuture, Duration.Inf)
+        case _ => {
           val remoteStartTime = System.currentTimeMillis
           // Serialize the block if not already done
           if (bytesAfterPut == null) {
diff --git a/core/src/main/scala/org/apache/spark/storage/BlockStore.scala b/core/src/main/scala/org/apache/spark/storage/BlockStore.scala
index b047644b88..9a9be047c7 100644
--- a/core/src/main/scala/org/apache/spark/storage/BlockStore.scala
+++ b/core/src/main/scala/org/apache/spark/storage/BlockStore.scala
@@ -28,7 +28,7 @@ import org.apache.spark.Logging
  */
 private[spark]
 abstract class BlockStore(val blockManager: BlockManager) extends Logging {
-  def putBytes(blockId: BlockId, bytes: ByteBuffer, level: StorageLevel)
+  def putBytes(blockId: BlockId, bytes: ByteBuffer, level: StorageLevel) : PutResult
 
   /**
    * Put in a block and, possibly, also return its content as either bytes or another Iterator.
@@ -37,6 +37,9 @@ abstract class BlockStore(val blockManager: BlockManager) extends Logging {
    * @return a PutResult that contains the size of the data, as well as the values put if
    *         returnValues is true (if not, the result's data field can be null)
    */
+  def putValues(blockId: BlockId, values: Iterator[Any], level: StorageLevel,
+    returnValues: Boolean) : PutResult
+
   def putValues(blockId: BlockId, values: ArrayBuffer[Any], level: StorageLevel,
     returnValues: Boolean) : PutResult
 
diff --git a/core/src/main/scala/org/apache/spark/storage/DiskStore.scala b/core/src/main/scala/org/apache/spark/storage/DiskStore.scala
index d1f07ddb24..36ee4bcc41 100644
--- a/core/src/main/scala/org/apache/spark/storage/DiskStore.scala
+++ b/core/src/main/scala/org/apache/spark/storage/DiskStore.scala
@@ -37,7 +37,7 @@ private class DiskStore(blockManager: BlockManager, diskManager: DiskBlockManage
     diskManager.getBlockLocation(blockId).length
   }
 
-  override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel) {
+  override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel) : PutResult = {
     // So that we do not modify the input offsets !
     // duplicate does not copy buffer, so inexpensive
     val bytes = _bytes.duplicate()
@@ -52,6 +52,7 @@ private class DiskStore(blockManager: BlockManager, diskManager: DiskBlockManage
     val finishTime = System.currentTimeMillis
     logDebug("Block %s stored as %s file on disk in %d ms".format(
       file.getName, Utils.bytesToString(bytes.limit), (finishTime - startTime)))
+    return PutResult(bytes.limit(), Right(bytes.duplicate()))
   }
 
   override def putValues(
@@ -59,13 +60,22 @@ private class DiskStore(blockManager: BlockManager, diskManager: DiskBlockManage
       values: ArrayBuffer[Any],
       level: StorageLevel,
       returnValues: Boolean)
+  : PutResult = {
+    return putValues(blockId, values.toIterator, level, returnValues)
+  }
+
+  override def putValues(
+      blockId: BlockId,
+      values: Iterator[Any],
+      level: StorageLevel,
+      returnValues: Boolean)
     : PutResult = {
 
     logDebug("Attempting to write values for block " + blockId)
     val startTime = System.currentTimeMillis
     val file = diskManager.getFile(blockId)
     val outputStream = new FileOutputStream(file)
-    blockManager.dataSerializeStream(blockId, outputStream, values.iterator)
+    blockManager.dataSerializeStream(blockId, outputStream, values)
     val length = file.length
 
     val timeTaken = System.currentTimeMillis - startTime
diff --git a/core/src/main/scala/org/apache/spark/storage/MemoryStore.scala b/core/src/main/scala/org/apache/spark/storage/MemoryStore.scala
index 1814175651..b89212eaab 100644
--- a/core/src/main/scala/org/apache/spark/storage/MemoryStore.scala
+++ b/core/src/main/scala/org/apache/spark/storage/MemoryStore.scala
@@ -49,7 +49,7 @@ private class MemoryStore(blockManager: BlockManager, maxMemory: Long)
     }
   }
 
-  override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel) {
+  override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel) : PutResult = {
     // Work on a duplicate - since the original input might be used elsewhere.
     val bytes = _bytes.duplicate()
     bytes.rewind()
@@ -59,8 +59,10 @@ private class MemoryStore(blockManager: BlockManager, maxMemory: Long)
       elements ++= values
       val sizeEstimate = SizeEstimator.estimate(elements.asInstanceOf[AnyRef])
       tryToPut(blockId, elements, sizeEstimate, true)
+      PutResult(sizeEstimate, Left(values.toIterator))
     } else {
       tryToPut(blockId, bytes, bytes.limit, false)
+      PutResult(bytes.limit(), Right(bytes.duplicate()))
     }
   }
 
@@ -69,14 +71,33 @@ private class MemoryStore(blockManager: BlockManager, maxMemory: Long)
       values: ArrayBuffer[Any],
       level: StorageLevel,
       returnValues: Boolean)
-    : PutResult = {
-
+  : PutResult = {
     if (level.deserialized) {
       val sizeEstimate = SizeEstimator.estimate(values.asInstanceOf[AnyRef])
       tryToPut(blockId, values, sizeEstimate, true)
-      PutResult(sizeEstimate, Left(values.iterator))
+      PutResult(sizeEstimate, Left(values.toIterator))
+    } else {
+      val bytes = blockManager.dataSerialize(blockId, values.toIterator)
+      tryToPut(blockId, bytes, bytes.limit, false)
+      PutResult(bytes.limit(), Right(bytes.duplicate()))
+    }
+  }
+
+  override def putValues(
+      blockId: BlockId,
+      values: Iterator[Any],
+      level: StorageLevel,
+      returnValues: Boolean)
+    : PutResult = {
+
+    if (level.deserialized) {
+      val valueEntries = new ArrayBuffer[Any]()
+      valueEntries ++= values
+      val sizeEstimate = SizeEstimator.estimate(valueEntries.asInstanceOf[AnyRef])
+      tryToPut(blockId, valueEntries, sizeEstimate, true)
+      PutResult(sizeEstimate, Left(valueEntries.toIterator))
     } else {
-      val bytes = blockManager.dataSerialize(blockId, values.iterator)
+      val bytes = blockManager.dataSerialize(blockId, values)
       tryToPut(blockId, bytes, bytes.limit, false)
       PutResult(bytes.limit(), Right(bytes.duplicate()))
     }
diff --git a/core/src/test/scala/org/apache/spark/storage/FlatmapIteratorSuite.scala b/core/src/test/scala/org/apache/spark/storage/FlatmapIteratorSuite.scala
new file mode 100644
index 0000000000..b843b4c629
--- /dev/null
+++ b/core/src/test/scala/org/apache/spark/storage/FlatmapIteratorSuite.scala
@@ -0,0 +1,74 @@
+/*
+ * 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.storage
+
+import org.scalatest.FunSuite
+import org.apache.spark.{SharedSparkContext, SparkConf, LocalSparkContext, SparkContext}
+
+
+class FlatmapIteratorSuite extends FunSuite with LocalSparkContext {
+  /* Tests the ability of Spark to deal with user provided iterators from flatMap
+   * calls, that may generate more data then available memory. In any
+   * memory based persistance Spark will unroll the iterator into an ArrayBuffer
+   * for caching, however in the case that the use defines DISK_ONLY persistance,
+   * the iterator will be fed directly to the serializer and written to disk.
+   *
+   * This also tests the ObjectOutputStream reset rate. When serializing using the
+   * Java serialization system, the serializer caches objects to prevent writing redundant
+   * data, however that stops GC of those objects. By calling 'reset' you flush that
+   * info from the serializer, and allow old objects to be GC'd
+   */
+  test("Flatmap Iterator to Disk") {
+    val sconf = new SparkConf().setMaster("local-cluster[1,1,512]")
+      .setAppName("iterator_to_disk_test")
+    sc = new SparkContext(sconf)
+    val expand_size = 100
+    val data = sc.parallelize((1 to 5).toSeq).
+      flatMap( x => Stream.range(0, expand_size))
+    var persisted = data.persist(StorageLevel.DISK_ONLY)
+    println(persisted.count())
+    assert(persisted.count()===500)
+    assert(persisted.filter(_==1).count()===5)
+  }
+
+  test("Flatmap Iterator to Memory") {
+    val sconf = new SparkConf().setMaster("local-cluster[1,1,512]")
+      .setAppName("iterator_to_disk_test")
+    sc = new SparkContext(sconf)
+    val expand_size = 100
+    val data = sc.parallelize((1 to 5).toSeq).
+      flatMap(x => Stream.range(0, expand_size))
+    var persisted = data.persist(StorageLevel.MEMORY_ONLY)
+    println(persisted.count())
+    assert(persisted.count()===500)
+    assert(persisted.filter(_==1).count()===5)
+  }
+
+  test("Serializer Reset") {
+    val sconf = new SparkConf().setMaster("local-cluster[1,1,512]")
+      .setAppName("serializer_reset_test")
+      .set("spark.serializer.objectStreamReset", "10")
+    sc = new SparkContext(sconf)
+    val expand_size = 500
+    val data = sc.parallelize(Seq(1,2)).
+      flatMap(x => Stream.range(1, expand_size).
+      map(y => "%d: string test %d".format(y,x)))
+    var persisted = data.persist(StorageLevel.MEMORY_ONLY_SER)
+    assert(persisted.filter(_.startsWith("1:")).count()===2)
+  }
+
+}
diff --git a/docs/configuration.md b/docs/configuration.md
index dc5553f3da..017d509854 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -245,6 +245,17 @@ Apart from these, the following properties are also available, and may be useful
   </td>
 </tr>
 <tr>
+  <td>spark.serializer.objectStreamReset</td>
+  <td>10000</td>
+  <td>
+    When serializing using org.apache.spark.serializer.JavaSerializer, the serializer caches 
+    objects to prevent writing redundant data, however that stops garbage collection of those 
+    objects. By calling 'reset' you flush that info from the serializer, and allow old 
+    objects to be collected. To turn off this periodic reset set it to a value of <= 0. 
+    By default it will reset the serializer every 10,000 objects.
+  </td>
+</tr>
+<tr>
   <td>spark.broadcast.factory</td>
   <td>org.apache.spark.broadcast.<br />HttpBroadcastFactory</td>
   <td>