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https://issues.apache.org/jira/browse/HDFS-10663?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16920888#comment-16920888
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hemanthboyina commented on HDFS-10663:
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hi [~shahrs87] can i take up this Jira ?
> Comparison of two System.nanoTime methods return values are against standard
> java recommendations.
> --------------------------------------------------------------------------------------------------
>
> 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
> Priority: Major
>
> 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 restarted and upgraded to new software.
> While debugging, I came across this heartbeat check code where the comparison
> of two System.nanoTime is against the java's 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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