Michael Burman commented on CASSANDRA-14281:

Updated the patch to include merging two reservoir snapshots (with certain 
preassumptions given the use-case inside Cassandra I was intending the feature 
to be used for)


For contention reductions (I'm not going to reduce them more right now) 
aggregating would allow using ThreadLocal instances of LatencyMetrics, but at 
that point we should also get rid of the CAS operations. 

> Reduce contention on DecayingEstimatedHistogramReservoir
> --------------------------------------------------------
>                 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, 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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