Joshua McKenzie commented on CASSANDRA-14281:

Bounced out of Ready to Commit but can't move back to PA. Just a heads up.

> 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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