Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17617#discussion_r119275510
  
    --- Diff: core/src/main/scala/org/apache/spark/deploy/SparkHadoopUtil.scala 
---
    @@ -143,14 +144,18 @@ class SparkHadoopUtil extends Logging {
        * Returns a function that can be called to find Hadoop FileSystem bytes 
read. If
        * getFSBytesReadOnThreadCallback is called from thread r at time t, the 
returned callback will
        * return the bytes read on r since t.
    -   *
    -   * @return None if the required method can't be found.
        */
       private[spark] def getFSBytesReadOnThreadCallback(): () => Long = {
    -    val threadStats = 
FileSystem.getAllStatistics.asScala.map(_.getThreadStatistics)
    -    val f = () => threadStats.map(_.getBytesRead).sum
    -    val baselineBytesRead = f()
    -    () => f() - baselineBytesRead
    +    val f = () => 
FileSystem.getAllStatistics.asScala.map(_.getThreadStatistics.getBytesRead).sum
    +    val baseline = (Thread.currentThread().getId, f())
    +    val bytesReadMap = new ConcurrentHashMap[Long, Long]()
    +
    +    () => {
    --- End diff --
    
    I think it's better to create an anonymous `Function0` instance and treat 
`bytesReadMap` as a member variable and document the multi-thread semantic for 
the `apply` method.


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