[CASSANDRA-15213] DecayingEstimatedHistogramReservoir Inefficiencies - ASF JIRA

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Bug Category:
Degradation - Performance Bug/Regression
Severity:
Low
Complexity:
Normal
Discovered By:
Adhoc Test
Platform:

All
Impacts:

None
Since Version:

4.0-alpha
Source Control Link:

adc3cdde2ae2a78f7d2bb66da47a07545d3e06cf
Test and Documentation Plan:

Hide

Existing microbenchmarks, new cluster benchmarks, quick theories tests

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Existing microbenchmarks, new cluster benchmarks, quick theories tests

LongAdder introduced to trunk consumes 9MiB of heap without user schemas, and this will grow significantly under contention and user schemas with many tables. This is because LongAdder is a very heavy class designed for single contended values.
- This can likely be improved significantly, without significant loss of performance in the contended case, by simply increasing the size of our primitive backing array and providing multiple buckets, with each thread picking a bucket to increment, or simply multiple backing arrays. Probably a better way still to do this would be to introduce some competition detection to the update, much like LongAdder utilises, that increases the number of backing arrays under competition.
- To save memory this approach could partition the space into chunks that are likely to be updated together, so that we do not need to duplicate the entire array under competition.
Similarly, binary search is costly and a measurable cost as a share of the new networking work (without filtering it was > 10% of the CPU used overall). We can compute an approximation floor(log2 n / log2 1.2) extremely cheaply, to save the random memory access costs.

relates to

CASSANDRA-14281 Improve LatencyMetrics performance by reducing write path processing