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https://issues.apache.org/jira/browse/CASSANDRA-20250?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17926397#comment-17926397
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Benedict Elliott Smith edited comment on CASSANDRA-20250 at 2/12/25 2:39 PM:
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This looks great. I think a global EWMA is correct, but I think we should
consider updating it in a background thread. I think it's fine for this to be
updated e.g. minutely, and we can update all EWMA together in a batch. We can
store the moving average in another array that is shared between all moving
averages in the same way as the Meter, making it also more efficient to update.
The reduced frequency would slightly change the shape of the decay curve, but I
don't think it should matter particularly. We could anyway do it e.g. every 5
or 15s just fine I expect, or simply make it configurable and let users decide
if they care (they probably don't).
This has the added advantage that we don't consult the clock on every update,
which should further reduce the cost.
was (Author: benedict):
This looks great. I think a global EWMA is correct, but I think we should
consider updating it in a background thread. I think it's fine for this to be
updated e.g. minutely, and we can update all EWMA together in a batch. We can
store the moving average in another array that is shared between all moving
averages in the same way as the Meter, making it also more efficient to update.
The reduced frequency would slightly change the shape of the decay curve, but I
don't think it should matter particularly. We could anyway do it e.g. every 5
or 15s just fine I expect, or simply make it configurable and let users decide
if they care (they probably don't).
> Provide the ability to disable specific metrics collection
> ----------------------------------------------------------
>
> Key: CASSANDRA-20250
> URL: https://issues.apache.org/jira/browse/CASSANDRA-20250
> Project: Apache Cassandra
> Issue Type: New Feature
> Components: Observability/Metrics
> Reporter: Dmitry Konstantinov
> Assignee: Dmitry Konstantinov
> Priority: Normal
> Attachments: 5.1_profile_cpu.html,
> 5.1_profile_cpu_without_metrics.html, async_profiler_cpu_profiles.zip,
> cpu_profile_insert.html, jmh-result.json, vmstat.log,
> vmstat_without_metrics.log
>
>
> Cassandra has a lot of metrics collected, many of them are collected per
> table, so their instance number is multiplied by number of tables. From one
> side it gives a better observability, from another side metrics are not for
> free, there is an overhead associated with them:
> 1) CPU overhead: in case of simple CPU bound load: I already see like 5.5% of
> total CPU spent for metrics in cpu framegraphs for read load and 11% for
> write load.
> Example: [^cpu_profile_insert.html] (search by "codahale" pattern). The
> framegraph is captured using Async profiler build:
> async-profiler-3.0-29ee888-linux-x64
> 2) memory overhead: we spend memory for entities used to aggregate metrics
> such as LongAdders and reservoirs + for MBeans (String concatenation within
> object names is a major cause of it, for each table+metric name combination a
> new String is created)
>
> The idea of this ticket is to allow an operator to configure a list of
> disabled metrics in cassandra.yaml, like:
> {code:java}
> disabled_metrics:
> - metric_a
> - metric_b
> {code}
> From implementation point of view I see two possible approaches (which can be
> combined):
> # Generic: when a metric is registering if it is listed in disabled_metrics
> we do not publish it via JMX and provide a noop implementation of metric
> object (such as histogram) for it.
> Logging analogy: log level check within log method
> # Specialized: for some metrics the process of value calculation is not for
> free and introduces an overhead as well, in such cases it would be useful to
> check within specific logic using an API (like: isMetricEnabled) do we need
> to do it. Example of such metric:
> ClientRequestSizeMetrics.recordRowAndColumnCountMetrics
> Logging analogy: an explicit 'if (isDebugEnabled())' condition used when a
> message parameter is expensive.
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