[jira] [Updated] (CASSANDRA-21656) large mutations over-allocate on the commit log path

["Jon Haddad (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-21656?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Jon Haddad updated CASSANDRA-21656:
-----------------------------------
     Bug Category: Parent values: Degradation(12984)
       Complexity: Normal
      Component/s: Local/Commit Log
    Discovered By: Adhoc Test
         Severity: Normal

> large mutations over-allocate on the commit log path
> ----------------------------------------------------
>
>                 Key: CASSANDRA-21656
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-21656
>             Project: Apache Cassandra
>          Issue Type: Bug
>          Components: Local/Commit Log
>            Reporter: Jon Haddad
>            Assignee: Jon Haddad
>            Priority: Normal
>
> Writing a large mutation to the commit log allocates far more memory than the mutation itself, and it does so on every write. The allocation is native, not heap, and it grows with mutation size. Mutations under about 1 MiB are unaffected.
> Measured on a 4 MiB mutation, 2400 writes, single thread, memory-mapped segments, {{cassandra-6.0}} at {{{}38a7a783{}}}, async-profiler {{{}-e nativemem{}}}:
> ||Measure||Value||
> |native memory per write|10.42 MB|
> |heap bytes per write|958.7|
> |writes/sec, median of 5|850.5|
> The commit log buffers the serialized mutation before copying it into the segment, and the buffer is rebuilt from scratch on each large write. The serialized size is already computed earlier in the same call and then discarded, so it may be possible to size the segment slot from it and serialize once, directly. Whether that size is exact enough to be trusted for the slot is the open question.
> I have a branch (wip) that removes the intermediate buffer. The serialized size is already computed earlier in the same call, so the segment slot is sized from it and the mutation is serialized straight into it.
> Here are some stats:
>  
> ||Measure||Before||After||Change||
> |Native memory per write, 4 MiB mutation|10.42 MB|19.0 KB|-99.8%|
> |Native memory total, 2400 writes|23.30 GiB|43.6 MiB|-99.8%|
> |Heap bytes per write, 4 MiB mutation|958.7|547.3|-42.9%|
> |Heap bytes per write, 200 B mutation|161.7|161.6|not resolved|
> The branch is a little early, but it's my hope that this will help reduce the pain people experience when writing large objects.



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