Updated according to Matei's code review comment.

core/src/main/scala/spark/ShuffleFetcher.scala

@@ -10,7 +10,7 @@ private[spark] abstract class ShuffleFetcher {
    * @return An iterator over the elements of the fetched shuffle outputs.
    */
   def fetch[K, V](shuffleId: Int, reduceId: Int, metrics: TaskMetrics,
-    serializer: Serializer = Serializer.default): Iterator[(K,V)]
+    serializer: Serializer = SparkEnv.get.serializerManager.default): Iterator[(K,V)]
 
   /** Stop the fetcher */
   def stop() {}

core/src/main/scala/spark/SparkEnv.scala

@@ -7,7 +7,7 @@ import spark.broadcast.BroadcastManager
 import spark.storage.BlockManager
 import spark.storage.BlockManagerMaster
 import spark.network.ConnectionManager
-import spark.serializer.Serializer
+import spark.serializer.{Serializer, SerializerManager}
 import spark.util.AkkaUtils
 
 
@@ -21,6 +21,7 @@ import spark.util.AkkaUtils
 class SparkEnv (
     val executorId: String,
     val actorSystem: ActorSystem,
+    val serializerManager: SerializerManager,
     val serializer: Serializer,
     val closureSerializer: Serializer,
     val cacheManager: CacheManager,
@@ -92,10 +93,12 @@ object SparkEnv extends Logging {
       Class.forName(name, true, classLoader).newInstance().asInstanceOf[T]
     }
 
-    val serializer = Serializer.setDefault(
+    val serializerManager = new SerializerManager
+
+    val serializer = serializerManager.setDefault(
       System.getProperty("spark.serializer", "spark.JavaSerializer"))
 
-    val closureSerializer = Serializer.get(
+    val closureSerializer = serializerManager.get(
       System.getProperty("spark.closure.serializer", "spark.JavaSerializer"))
 
     def registerOrLookup(name: String, newActor: => Actor): ActorRef = {
@@ -155,6 +158,7 @@ object SparkEnv extends Logging {
     new SparkEnv(
       executorId,
       actorSystem,
+      serializerManager,
       serializer,
       closureSerializer,
       cacheManager,

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

@@ -8,7 +8,6 @@ import scala.collection.mutable.ArrayBuffer
 
 import spark.{Aggregator, Logging, Partition, Partitioner, RDD, SparkEnv, TaskContext}
 import spark.{Dependency, OneToOneDependency, ShuffleDependency}
-import spark.serializer.Serializer
 
 
 private[spark] sealed trait CoGroupSplitDep extends Serializable
@@ -114,7 +113,7 @@ class CoGroupedRDD[K](
       }
     }
 
-    val ser = Serializer.get(serializerClass)
+    val ser = SparkEnv.get.serializerManager.get(serializerClass)
     for ((dep, depNum) <- split.deps.zipWithIndex) dep match {
       case NarrowCoGroupSplitDep(rdd, _, itsSplit) => {
         // Read them from the parent

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

@@ -2,7 +2,6 @@ package spark.rdd
 
 import spark.{Partitioner, RDD, SparkEnv, ShuffleDependency, Partition, TaskContext}
 import spark.SparkContext._
-import spark.serializer.Serializer
 
 
 private[spark] class ShuffledRDDPartition(val idx: Int) extends Partition {
@@ -32,7 +31,7 @@ class ShuffledRDD[K, V](
 
   override def compute(split: Partition, context: TaskContext): Iterator[(K, V)] = {
     val shuffledId = dependencies.head.asInstanceOf[ShuffleDependency[K, V]].shuffleId
-    SparkEnv.get.shuffleFetcher.fetch[K, V](
-      shuffledId, split.index, context.taskMetrics, Serializer.get(serializerClass))
+    SparkEnv.get.shuffleFetcher.fetch[K, V](shuffledId, split.index, context.taskMetrics,
+      SparkEnv.get.serializerManager.get(serializerClass))
   }
 }

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

@@ -11,7 +11,7 @@ import spark.Partition
 import spark.SparkEnv
 import spark.ShuffleDependency
 import spark.OneToOneDependency
-import spark.serializer.Serializer
+
 
 /**
  * An optimized version of cogroup for set difference/subtraction.
@@ -68,7 +68,7 @@ private[spark] class SubtractedRDD[K: ClassManifest, V: ClassManifest, W: ClassM
 
   override def compute(p: Partition, context: TaskContext): Iterator[(K, V)] = {
     val partition = p.asInstanceOf[CoGroupPartition]
-    val serializer = Serializer.get(serializerClass)
+    val serializer = SparkEnv.get.serializerManager.get(serializerClass)
     val map = new JHashMap[K, ArrayBuffer[V]]
     def getSeq(k: K): ArrayBuffer[V] = {
       val seq = map.get(k)

core/src/main/scala/spark/scheduler/ShuffleMapTask.scala

@@ -14,7 +14,6 @@ import com.ning.compress.lzf.LZFOutputStream
 
 import spark._
 import spark.executor.ShuffleWriteMetrics
-import spark.serializer.Serializer
 import spark.storage._
 import spark.util.{TimeStampedHashMap, MetadataCleaner}
 
@@ -139,12 +138,12 @@ private[spark] class ShuffleMapTask(
     metrics = Some(taskContext.taskMetrics)
 
     val blockManager = SparkEnv.get.blockManager
-    var shuffle: ShuffleBlockManager#Shuffle = null
+    var shuffle: ShuffleBlocks = null
     var buckets: ShuffleWriterGroup = null
 
     try {
       // Obtain all the block writers for shuffle blocks.
-      val ser = Serializer.get(dep.serializerClass)
+      val ser = SparkEnv.get.serializerManager.get(dep.serializerClass)
       shuffle = blockManager.shuffleBlockManager.forShuffle(dep.shuffleId, numOutputSplits, ser)
       buckets = shuffle.acquireWriters(partition)
 

core/src/main/scala/spark/serializer/Serializer.scala

@@ -2,7 +2,6 @@ package spark.serializer
 
 import java.io.{EOFException, InputStream, OutputStream}
 import java.nio.ByteBuffer
-import java.util.concurrent.ConcurrentHashMap
 
 import it.unimi.dsi.fastutil.io.FastByteArrayOutputStream
 
@@ -19,47 +18,6 @@ trait Serializer {
 }
 
 
-/**
- * A singleton object that can be used to fetch serializer objects based on the serializer
- * class name. If a previous instance of the serializer object has been created, the get
- * method returns that instead of creating a new one.
- */
-object Serializer {
-
-  private val serializers = new ConcurrentHashMap[String, Serializer]
-  private var _default: Serializer = _
-
-  def default = _default
-
-  def setDefault(clsName: String): Serializer = {
-    _default = get(clsName)
-    _default
-  }
-
-  def get(clsName: String): Serializer = {
-    if (clsName == null) {
-      default
-    } else {
-      var serializer = serializers.get(clsName)
-      if (serializer != null) {
-        // If the serializer has been created previously, reuse that.
-        serializer
-      } else this.synchronized {
-        // Otherwise, create a new one. But make sure no other thread has attempted
-        // to create another new one at the same time.
-        serializer = serializers.get(clsName)
-        if (serializer == null) {
-          val clsLoader = Thread.currentThread.getContextClassLoader
-          serializer = Class.forName(clsName, true, clsLoader).newInstance().asInstanceOf[Serializer]
-          serializers.put(clsName, serializer)
-        }
-        serializer
-      }
-    }
-  }
-}
-
-
 /**
  * An instance of a serializer, for use by one thread at a time.
  */

core/src/main/scala/spark/serializer/SerializerManager.scala

@@ -0,0 +1,45 @@
+package spark.serializer
+
+import java.util.concurrent.ConcurrentHashMap
+
+
+/**
+ * A service that returns a serializer object given the serializer's class name. If a previous
+ * instance of the serializer object has been created, the get method returns that instead of
+ * creating a new one.
+ */
+private[spark] class SerializerManager {
+
+  private val serializers = new ConcurrentHashMap[String, Serializer]
+  private var _default: Serializer = _
+
+  def default = _default
+
+  def setDefault(clsName: String): Serializer = {
+    _default = get(clsName)
+    _default
+  }
+
+  def get(clsName: String): Serializer = {
+    if (clsName == null) {
+      default
+    } else {
+      var serializer = serializers.get(clsName)
+      if (serializer != null) {
+        // If the serializer has been created previously, reuse that.
+        serializer
+      } else this.synchronized {
+        // Otherwise, create a new one. But make sure no other thread has attempted
+        // to create another new one at the same time.
+        serializer = serializers.get(clsName)
+        if (serializer == null) {
+          val clsLoader = Thread.currentThread.getContextClassLoader
+          serializer =
+            Class.forName(clsName, true, clsLoader).newInstance().asInstanceOf[Serializer]
+          serializers.put(clsName, serializer)
+        }
+        serializer
+      }
+    }
+  }
+}

core/src/main/scala/spark/storage/DiskStore.scala

@@ -2,6 +2,7 @@ package spark.storage
 
 import java.io.{File, FileOutputStream, OutputStream, RandomAccessFile}
 import java.nio.ByteBuffer
+import java.nio.channels.FileChannel
 import java.nio.channels.FileChannel.MapMode
 import java.util.{Random, Date}
 import java.text.SimpleDateFormat
@@ -26,14 +27,16 @@ private class DiskStore(blockManager: BlockManager, rootDirs: String)
 
     private val f: File = createFile(blockId /*, allowAppendExisting */)
 
-    private var repositionableStream: FastBufferedOutputStream = null
+    // The file channel, used for repositioning / truncating the file.
+    private var channel: FileChannel = null
     private var bs: OutputStream = null
     private var objOut: SerializationStream = null
-    private var validLength = 0L
+    private var lastValidPosition = 0L
 
     override def open(): DiskBlockObjectWriter = {
-      repositionableStream = new FastBufferedOutputStream(new FileOutputStream(f))
-      bs = blockManager.wrapForCompression(blockId, repositionableStream)
+      val fos = new FileOutputStream(f, true)
+      channel = fos.getChannel()
+      bs = blockManager.wrapForCompression(blockId, new FastBufferedOutputStream(fos))
       objOut = serializer.newInstance().serializeStream(bs)
       this
     }
@@ -41,9 +44,9 @@ private class DiskStore(blockManager: BlockManager, rootDirs: String)
     override def close() {
       objOut.close()
       bs.close()
-      objOut = null
+      channel = null
       bs = null
-      repositionableStream = null
+      objOut = null
       // Invoke the close callback handler.
       super.close()
     }
@@ -54,25 +57,23 @@ private class DiskStore(blockManager: BlockManager, rootDirs: String)
     // Return the number of bytes written for this commit.
     override def commit(): Long = {
       bs.flush()
-      validLength = repositionableStream.position()
-      validLength
+      val prevPos = lastValidPosition
+      lastValidPosition = channel.position()
+      lastValidPosition - prevPos
     }
 
     override def revertPartialWrites() {
-      // Flush the outstanding writes and delete the file.
-      objOut.close()
-      bs.close()
-      objOut = null
-      bs = null
-      repositionableStream = null
-      f.delete()
+      // Discard current writes. We do this by flushing the outstanding writes and
+      // truncate the file to the last valid position.
+      bs.flush()
+      channel.truncate(lastValidPosition)
     }
 
     override def write(value: Any) {
       objOut.writeObject(value)
     }
 
-    override def size(): Long = validLength
+    override def size(): Long = lastValidPosition
   }
 
   val MAX_DIR_CREATION_ATTEMPTS: Int = 10
@@ -86,7 +87,8 @@ private class DiskStore(blockManager: BlockManager, rootDirs: String)
 
   addShutdownHook()
 
-  def getBlockWriter(blockId: String, serializer: Serializer, bufferSize: Int): BlockObjectWriter = {
+  def getBlockWriter(blockId: String, serializer: Serializer, bufferSize: Int)
+    : BlockObjectWriter = {
     new DiskBlockObjectWriter(blockId, serializer, bufferSize)
   }
 

core/src/main/scala/spark/storage/ShuffleBlockManager.scala

@@ -7,27 +7,31 @@ private[spark]
 class ShuffleWriterGroup(val id: Int, val writers: Array[BlockObjectWriter])
 
 
+private[spark]
+trait ShuffleBlocks {
+  def acquireWriters(mapId: Int): ShuffleWriterGroup
+  def releaseWriters(group: ShuffleWriterGroup)
+}
+
+
 private[spark]
 class ShuffleBlockManager(blockManager: BlockManager) {
 
-  def forShuffle(shuffleId: Int, numBuckets: Int, serializer: Serializer): Shuffle = {
-    new Shuffle(shuffleId, numBuckets, serializer)
-  }
-
-  class Shuffle(shuffleId: Int, numBuckets: Int, serializer: Serializer) {
-
-    // Get a group of writers for a map task.
-    def acquireWriters(mapId: Int): ShuffleWriterGroup = {
-      val bufferSize = System.getProperty("spark.shuffle.file.buffer.kb", "100").toInt * 1024
-      val writers = Array.tabulate[BlockObjectWriter](numBuckets) { bucketId =>
-        val blockId = ShuffleBlockManager.blockId(shuffleId, bucketId, mapId)
-        blockManager.getDiskBlockWriter(blockId, serializer, bufferSize).open()
+  def forShuffle(shuffleId: Int, numBuckets: Int, serializer: Serializer): ShuffleBlocks = {
+    new ShuffleBlocks {
+      // Get a group of writers for a map task.
+      override def acquireWriters(mapId: Int): ShuffleWriterGroup = {
+        val bufferSize = System.getProperty("spark.shuffle.file.buffer.kb", "100").toInt * 1024
+        val writers = Array.tabulate[BlockObjectWriter](numBuckets) { bucketId =>
+          val blockId = ShuffleBlockManager.blockId(shuffleId, bucketId, mapId)
+          blockManager.getDiskBlockWriter(blockId, serializer, bufferSize).open()
+        }
+        new ShuffleWriterGroup(mapId, writers)
       }
-      new ShuffleWriterGroup(mapId, writers)
-    }
 
-    def releaseWriters(group: ShuffleWriterGroup) = {
-      // Nothing really to release here.
+      override def releaseWriters(group: ShuffleWriterGroup) = {
+        // Nothing really to release here.
+      }
     }
   }
 }