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https://issues.apache.org/jira/browse/HADOOP-14989?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Erik Krogen updated HADOOP-14989:
---------------------------------
Description:
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.
was:
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 each sink configured (including JMX)
only receives a portion of the average information.
{{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 only a single sink ever
snapshots, this would result in the expected behavior that the value is the
average over the reporting period. However, if multiple sinks 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:
* From an operator perspective, ensure only one sink is configured. This is not
realistic given that the JMX cache exhibits the same behavior.
* 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.
Open to suggestions & input here.
> 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
> Assignee: 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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