[jira] [Commented] (CASSANDRA-16072) Reduce thread contention in CommitLogSegment and HintsBuffer by rewriting CAS loops to atomic adds

["Michael Semb Wever (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-16072?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17301179#comment-17301179 ] 

Michael Semb Wever commented on CASSANDRA-16072:
------------------------------------------------

bq. The patch breaks the CommitLogSegment's closing process (whether capacity becomes full, or endOfBuffer gets re-assigned before buffer becomes null). Investigating this further…

CommitLogSegment depended on the {{compareAndSet}}, to keep {{allocatePosition < endOfBuffer}}. Given that the atomic add approach isn't applicable to this code, I have applied the ConstantBackoffCAS algorithm (see CASSANDRA-15922) here.

Patches
- [3.11|https://github.com/apache/cassandra/compare/cassandra-3.11...thelastpickle:mck/cassandra-3.11_cas_improvements] – [!https://ci-cassandra.apache.org/job/Cassandra-devbranch/482/badge/icon!|https://ci-cassandra.apache.org/blue/organizations/jenkins/Cassandra-devbranch/detail/Cassandra-devbranch/482/pipeline]
- [trunk|https://github.com/apache/cassandra/compare/trunk...thelastpickle:mck/trunk_cas_improvements] – [!https://ci-cassandra.apache.org/job/Cassandra-devbranch/483/badge/icon!|https://ci-cassandra.apache.org/blue/organizations/jenkins/Cassandra-devbranch/detail/Cassandra-devbranch/483/pipeline]

> Reduce thread contention in CommitLogSegment and HintsBuffer by rewriting CAS loops to atomic adds
> --------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-16072
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-16072
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Consistency/Hints, Local/Commit Log
>            Reporter: Michael Semb Wever
>            Assignee: Michael Semb Wever
>            Priority: Normal
>             Fix For: 3.11.x, 4.0.x
>
>
> Follow up to CASSANDRA-15922
> Both CommitLogSegment and HintsBuffer use AtomicIntegers for the current offset when allocating. Like in CASSANDRA\-15922 the loops on {{.compareAndSet(..)}} can be replaced with atomic adds using the {{. getAndAdd(..)}} method.
> In highly contended environments the CAS failures can be high, starving writes in a running Cassandra node. On the same cluster CASSANDRA\-15922 was found, after CASSANDRA\-15922's fix was deployed, there was still problems around commit log flushing and hints. No flamegraph was collected that demonstrated the thread contention as clearly as was found in CASSANDRA\-15922, but the performance fix proposed here hopefully is obvious enough.



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