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From "Rushabh S Shah (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-10663) Comparison of two System.nanoTimes methods return values are against standard java recoemmendations.
Date Wed, 20 Jul 2016 18:20:20 GMT
Rushabh S Shah created HDFS-10663:
-------------------------------------

             Summary: Comparison of two System.nanoTimes methods return values are against
standard java recoemmendations.
                 Key: HDFS-10663
                 URL: https://issues.apache.org/jira/browse/HDFS-10663
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: namenode
            Reporter: Rushabh S Shah
            Assignee: Rushabh S Shah


I was chasing a bug where the namenode didn't declare a datanode dead even when the last contact
time was 2.5 hours before.
Before I could debug, the datanode was re-imaged (all the logs were deleted) and the namenode
was upgraded to new software.
While debugging, I came across this heartbeat check code where the comparison of two System.nanoTime
is against the java recommended way.
Here is the hadoop code:
{code:title=DatanodeManager.java|borderStyle=solid}

  /** Is the datanode dead? */
  boolean isDatanodeDead(DatanodeDescriptor node) {
    return (node.getLastUpdateMonotonic() <
            (monotonicNow() - heartbeatExpireInterval));
  }
{code}

The montonicNow() is calculated as:
{code:title=Time.java|borderStyle=solid}
  public static long monotonicNow() {
    final long NANOSECONDS_PER_MILLISECOND = 1000000;

    return System.nanoTime() / NANOSECONDS_PER_MILLISECOND;
  }
{code}

As per javadoc of System.nanoTime, it is clearly stated that we should subtract two nano time
output 
{noformat}
To compare two nanoTime values

 long t0 = System.nanoTime();
 ...
 long t1 = System.nanoTime();
one should use t1 - t0 < 0, not t1 < t0, because of the possibility of numerical overflow.
{noformat}




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