[jira] [Commented] (CASSANDRA-19365) invalid EstimatedHistogramReservoirSnapshot::getValue values due to race condition in DecayingEstimatedHistogramReservoir

["Maxim Muzafarov (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-19365?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17880994#comment-17880994 ] 

Maxim Muzafarov commented on CASSANDRA-19365:
---------------------------------------------

In agreement with [~jakubzytka], I will assign the issue to myself to fix the comments and finalize the issue.

> invalid EstimatedHistogramReservoirSnapshot::getValue values due to race condition in DecayingEstimatedHistogramReservoir
> -------------------------------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-19365
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-19365
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Observability/Metrics
>            Reporter: Jakub Zytka
>            Assignee: Jakub Zytka
>            Priority: Normal
>             Fix For: 4.0.x, 4.1.x, 5.0.x, 5.x
>
>          Time Spent: 3.5h
>  Remaining Estimate: 0h
>
> `DecayingEstimatedHistogramReservoir` has a race condition between `update` and `rescaleIfNeeded`.
> A sample which ends up (`update`) in an already scaled decayingBucket (`rescaleIfNeeded`) may still use a non-scaled weight because `decayLandmark` has not been updated yet at the moment of `update`.
>  
> The observed consequence was flooding of the cluster with speculative retries (we happened to hit low-percentile buckets with overweight samples, which drove p99 below true p50 for a long time).
> Please note that despite the manifestation being similar to CASSANDRA-19330, these are two distinct bugs in their own right.
> This bug affects versions 4.0+
> On 3.11 there's locking in DEHR. I did not check earlier versions.



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