''serialize V for String value is not implemented" error occurs when executing several spark examples.(It seems that these examples have string key)

[Y-Ymb]

Description

Spark exmaples which probably have string types input failed on current Sparkle.

Environment

current HerlettPackard/sparkle and replacing HewlettPackard/sparkle/alps with hvolos/alps

Reproducible example

Using CountVectorizerExample under spark repository

./bin/spark-submit \
  --class org.apache.spark.examples.ml.CountVectorizerExample \
  --master "spark://$(hostname):7077" \
  --deploy-mode client \
  --executor-memory 16G \
  --conf "spark.executor.extraJavaOptions=-XX:MaxDirectMemorySize=16G" \
  ./examples/target/scala-2.11/jars/spark-examples_2.11-2.0.0.jar \

Steps to reproduce

1. Build sparkle and start spark process
2. Move spark directory (cd $SPARK_HOME) and executing example command

Result message(a part of driver stack trace)

Driver stacktrace:
	at org.apache.spark.scheduler.DAGScheduler.org$apache$spark$scheduler$DAGScheduler$$failJobAndIndependentStages(DAGScheduler.scala:1450)
	at org.apache.spark.scheduler.DAGScheduler$$anonfun$abortStage$1.apply(DAGScheduler.scala:1438)
	at org.apache.spark.scheduler.DAGScheduler$$anonfun$abortStage$1.apply(DAGScheduler.scala:1437)
	at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
	at org.apache.spark.scheduler.DAGScheduler.abortStage(DAGScheduler.scala:1437)
	at org.apache.spark.scheduler.DAGScheduler$$anonfun$handleTaskSetFailed$1.apply(DAGScheduler.scala:811)
	at org.apache.spark.scheduler.DAGScheduler$$anonfun$handleTaskSetFailed$1.apply(DAGScheduler.scala:811)
	at scala.Option.foreach(Option.scala:257) 
[...]
Caused by: java.lang.RuntimeException: serialize V for String value is not implemented
	at com.hp.hpl.firesteel.shuffle.MapSHMShuffleStore.serializeVString(MapSHMShuffleStore.java:360)
	at com.hp.hpl.firesteel.shuffle.MapSHMShuffleStore.serializeKVPair(MapSHMShuffleStore.java:386)
	at org.apache.spark.shuffle.shm.ShmShuffleWriter.write(ShmShuffleWriter.scala:205)
	at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:79)
	at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:47)
	at org.apache.spark.scheduler.Task.run(Task.scala:85)
	at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:274)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)