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From "Erik Krogen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11615) FSNamesystemLock metrics can be inaccurate due to millisecond precision
Date Wed, 12 Apr 2017 16:35:41 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11615?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15966175#comment-15966175

Erik Krogen commented on HDFS-11615:

Fair enough, I will change it to nanoseconds. I can put it in the variable names, and I think
that's a good idea - the mix of nano-level and milli-level in the class is a little confusing.

The metric name is harder since {{MutableRate}} is used so you supply a base name and then
the {{MutableRate}} applies the suffix of {{AvgTime}} and {{NumOps}}, so it would look like
{{FSNWriteLockOperationNameNanosAvgTime}} and {{FSNWriteLockOperationNameNanosNumOps}} which
is a little awkward... I could potentially create an additional constructor for {{MutableRate}}
which allows you to specify a time unit. Any thoughts, [~andrew.wang]?

> FSNamesystemLock metrics can be inaccurate due to millisecond precision
> -----------------------------------------------------------------------
>                 Key: HDFS-11615
>                 URL: https://issues.apache.org/jira/browse/HDFS-11615
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs
>    Affects Versions: 2.7.4
>            Reporter: Erik Krogen
>            Assignee: Erik Krogen
>         Attachments: HDFS-11615.000.patch
> Currently the {{FSNamesystemLock}} metrics created in HDFS-10872 track the lock hold
time using {{Timer.monotonicNow()}}, which has millisecond-level precision. However, many
of these operations hold the lock for less than a millisecond, making these metrics inaccurate.
We should instead use {{System.nanoTime()}} for higher accuracy.

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