Why spark give out this error message?

[sunday2000]

Hi,
   Do u know what this message mean?
  
 org.apache.spark.shuffle.FetchFailedException: Failed to connect to 
localhost/127.0.0.1:50606
        at 
org.apache.spark.storage.ShuffleBlockFetcherIterator.throwFetchFailedException(ShuffleBlockFetcherIterator.scala:323)
        at 
org.apache.spark.storage.ShuffleBlockFetcherIterator.next(ShuffleBlockFetcherIterator.scala:300)
        at 
org.apache.spark.storage.ShuffleBlockFetcherIterator.next(ShuffleBlockFetcherIterator.scala:51)
        at scala.collection.Iterator$$anon$11.next(Iterator.scala:328)
        at scala.collection.Iterator$$anon$13.hasNext(Iterator.scala:371)
        at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:327)
        at 
org.apache.spark.util.CompletionIterator.hasNext(CompletionIterator.scala:32)
        at 
org.apache.spark.InterruptibleIterator.hasNext(InterruptibleIterator.scala:39)
        at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:327)
        at scala.collection.Iterator$class.foreach(Iterator.scala:727)
        at scala.collection.AbstractIterator.foreach(Iterator.scala:1157)
        at 
org.apache.spark.api.python.PythonRDD$.writeIteratorToStream(PythonRDD.scala:452)
        at 
org.apache.spark.api.python.PythonRunner$WriterThread$$anonfun$run$3.apply(PythonRDD.scala:280)
        at org.apache.spark.util.Utils$.logUncaughtExceptions(Utils.scala:1765)
        at 
org.apache.spark.api.python.PythonRunner$WriterThread.run(PythonRDD.scala:239)
Caused by: java.io.IOException: Failed to connect to localhost/127.0.0.1:50606