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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HADOOP-14989) metrics2 JMX cache refresh result in inconsistent Mutable(Stat|Rate) values
Date Sat, 02 Jun 2018 17:39:00 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-14989?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Steve Loughran updated HADOOP-14989:
    Target Version/s: 2.7.8  (was: 2.7.7)

> metrics2 JMX cache refresh result in inconsistent Mutable(Stat|Rate) values
> ---------------------------------------------------------------------------
>                 Key: HADOOP-14989
>                 URL: https://issues.apache.org/jira/browse/HADOOP-14989
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: metrics
>    Affects Versions: 2.6.5
>            Reporter: Erik Krogen
>            Priority: Critical
>         Attachments: HADOOP-14989.test.patch
> While doing some digging in the metrics2 system recently, we noticed that the way {{MutableStat}}
values are collected (and thus {{MutableRate}}, since it is based off of {{MutableStat}})
mean that every time the value is snapshotted, all previous information is lost. So every
time a JMX cache refresh occurs, it resets the {{MutableStat}}, meaning that all configured
metrics sinks do not consider the previous statistics in their emitted values. The same behavior
is true if you configured multiple sink periods.
> {{MutableStat}}, to compute its average value, maintains a total value since last snapshot,
as well as operation count since last snapshot. Upon snapshotting, the average is calculated
as (total / opCount) and placed into a gauge metric, and total / operation count are cleared.
So the average value represents the average since the last snapshot. If we have only a single
sink period ever snapshotting, this would result in the expected behavior that the value is
the average over the reporting period. However, if multiple sink periods are configured, or
if the JMX cache is refreshed, this is another snapshot operation. So, for example, if you
have a FileSink configured at a 60 second interval and your JMX cache refreshes itself 1 second
before the FileSink period fires, the values emitted to your FileSink only represent averages
_over the last one second_.
> A few ways to solve this issue:
> * Make {{MutableRate}} manage its own average refresh, similar to {{MutableQuantiles}},
which has a refresh thread and saves a snapshot of the last quantile values that it will serve
up until the next refresh. Given how many {{MutableRate}} metrics there are, a thread per
metric is not really feasible, but could be done on e.g. a per-source basis. This has some
downsides: if multiple sinks are configured with different periods, what is the right refresh
period for the {{MutableRate}}? 
> * Make {{MutableRate}} emit two counters, one for total and one for operation count,
rather than an average gauge and an operation count counter. The average could then be calculated
downstream from this information. This is cumbersome for operators and not backwards compatible.
To improve on both of those downsides, we could have it keep the current behavior but _additionally_
emit the total as a counter. The snapshotted average is probably sufficient in the common
case (we've been using it for years), and when more guaranteed accuracy is required, the average
could be derived from the total and operation count.
> The two above suggestions will fix this for both JMX and multiple sink periods, but may
be overkill. Multiple sink periods are probably not necessary though we should at least document
the behavior.
> Open to suggestions & input here.

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