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https://issues.apache.org/jira/browse/CASSANDRA-17523?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Caleb Rackliffe updated CASSANDRA-17523:
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Fix Version/s: 4.x
> Reduce histogram snapshot long[] allocation overhead during speculative read
> and write threshold updates
> --------------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-17523
> URL: https://issues.apache.org/jira/browse/CASSANDRA-17523
> Project: Cassandra
> Issue Type: Improvement
> Components: Consistency/Coordination, Observability/Metrics
> Reporter: Caleb Rackliffe
> Assignee: Caleb Rackliffe
> Priority: Normal
> Fix For: 4.x
>
>
> Every 5 seconds with the default {{read_request_timeout}} (or in the old
> naming scheme {{{}read_request_timeout_in_ms{}}}), a scheduled task updates
> the speculation thresholds (for reads and writes) for all active tables.
> However, there are a few issues with the way we do this:
>
> 1.) Whether or not the {{SpeculativeRetryPolicy}} implementations in use
> actually looks at them, we create latency histogram snapshots to pass to
> {{{}calculateThreshold(){}}}. We could trivially avoid this by having the
> method take an argument of type {{Sampling}} and build the snapshot only when
> necessary.
>
> 2.) The only reason we build the histogram snapshot is to find the new
> threshold value for the percentile based policies.
> {{EstimatedHistogramReservoirSnapshot}} creates copies of both the decaying
> and non-decaying buckets, but we don’t use the non-decaying values at all for
> percentile calculation. Just avoiding the non-decaying values array creation
> would cut allocations in half.
>
> Given even our snapshots aren’t perfectly consistent, it might also be
> possible to calculate a percentile value directly from the reservoir’s
> decaying buckets, although that might be less accurate, as new values could
> be added to the buckets after a count is calculated.
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