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https://issues.apache.org/jira/browse/CASSANDRA-14281?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16392940#comment-16392940
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Michael Burman commented on CASSANDRA-14281:
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No, I mean one update is copied to multiple histograms when on the hot path.
Initialized in this method:
[https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/metrics/TableMetrics.java#L993]
And applied for each update here:
[https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/metrics/TableMetrics.java#L1043]
Instead of doing it only when metrics are requested (which is also more
efficient process).
> Improve LatencyMetrics performance by reducing write path processing
> --------------------------------------------------------------------
>
> Key: CASSANDRA-14281
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14281
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Michael Burman
> Assignee: Michael Burman
> Priority: Major
>
> Currently for each write/read/rangequery/CAS touching the CFS we write a
> latency metric which takes a lot of processing time (up to 66% of the total
> processing time if the update was empty).
> The way latencies are recorded is to use both a dropwizard "Timer" as well as
> "Counter". Latter is used for totalLatency and the previous is decaying
> metric for rates and certain percentile metrics. We then replicate all of
> these CFS writes to the KeyspaceMetrics and globalWriteLatencies.
> Instead of doing this on the write phase we should merge the metrics when
> they're read. This is much less common occurrence and thus we save a lot of
> CPU time in total. This also speeds up the write path.
> Currently, the DecayingEstimatedHistogramReservoir acquires a lock for each
> update operation, which causes a contention if there are more than one thread
> updating the histogram. This impacts scalability when using larger machines.
> We should make it lock-free as much as possible and also avoid a single
> CAS-update from blocking all the concurrent threads from making an update.
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