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https://issues.apache.org/jira/browse/CASSANDRA-20250?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18035338#comment-18035338
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Dmitry Konstantinov edited comment on CASSANDRA-20250 at 11/4/25 1:41 PM:
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Hi [~burmanm] thank you for the update. Do you have a profile to see the
details for your case?
We have had a discussion here about doing some caching to reduce read cost but
it was about reducing cost of thread-local aggregator logic for metrics like
timers/counter/meters. You are probably talking about other cases when we have
some gauges with a custom calculation logic, like: CASSANDRA-13338 and
CASSANDRA-18111 . For these cases I have not seen an agreement about a common
solution as of now but probably introducing some common caching mechanism for
them should be a good option (for example, something similar to
com.codahale.metrics.CachedGauge)
was (Author: dnk):
Hi [~burmanm] thank you for the update. Do you have a profile to see the
details for your case?
We have had a discussion here about doing some caching to reduce read cost but
it was about reducing cost of thread-local aggregator logic for metrics like
timers/counter/meters. You are probably talking about other cases when we have
some gauges with a custom calculation logic, like: CASSANDRA-13338 and
CASSANDRA-18111 . For these cases I have not see an agreement about a common
solution as of now but probably introducing some common caching mechanism for
them would be a good option (for example, something similar to
com.codahale.metrics.CachedGauge)
> Optimize Counter, Meter and Histogram metrics using thread local counters
> -------------------------------------------------------------------------
>
> 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
> Fix For: 5.1
>
> Attachments: 5.1_profile_cpu.html,
> 5.1_profile_cpu_without_metrics.html, 5.1_tl4_profile_cpu.html,
> CASSANDRA-20250_ci_summary.html, CASSANDRA-20250_results_details.tar.xz,
> Histogram_AtomicLong.png, async_profiler_cpu_profiles.zip,
> cas_reverse_graph_metrics.png, cpu_profile_insert.html,
> image-2025-02-18-23-22-19-983.png, jmh-result.json, vmstat.log,
> vmstat_without_metrics.log
>
> Time Spent: 11h 50m
> Remaining Estimate: 0h
>
> 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)
> LongAdder is used by Dropwizard Counter/Meter and Histogram metrics for
> counting purposes. It has severe memory overhead + while has a better scaling
> than AtomicLong we still have to pay some cost for the concurrent operations.
> Additionally, in case of Meter - we have a non-optimal behaviour when we
> count the same things several times.
> The idea (suggested by [~benedict]) is to switch to thread-local counters
> which we can store in a common thread-local array to reduce memory overhead.
> In this way we can avoid concurrent update overheads/contentions and to
> reduce memory footprint as well.
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