[jira] [Updated] (HDFS-10663) Comparison of two System.nanoTime methods return values are against standard java recoemmendations.

["Rushabh S Shah (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HDFS-10663?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Rushabh S Shah updated HDFS-10663:
----------------------------------
    Summary: Comparison of two System.nanoTime methods return values are against standard java recoemmendations.  (was: Comparison of two System.nanoTimes methods return values are against standard java recoemmendations.)

> Comparison of two System.nanoTime 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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