[jira] [Comment Edited] (CASSANDRA-21528) Cache Enum.values() in ClusteringPrefix.Kind deserialization to avoid per-read array allocation

["Stefan Miklosovic (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-21528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18098825#comment-18098825 ] 

Stefan Miklosovic edited comment on CASSANDRA-21528 at 7/24/26 9:30 AM:
------------------------------------------------------------------------

||File:line||Path||Frequency||
  |{{ClusteringPrefix.java:479, 490, 660}}|clustering/bound deserialize|*per clustering prefix* (the ticket itself)|
  |{{io/sstable/ClusteringDescriptor.java:64, 106}}|SSTable cursor read|*per clustering read from disk*|
  |{{io/sstable/SSTableCursorWriter.java:516}}|SSTable write|*per tombstone marker* (flush/compaction)|
  |{{db/ClusteringBoundOrBoundary.java:119}}|range-bound deserialize|*per range-tombstone bound*|


  Recommended fix (covers all 4): add to ClusteringPrefix.Kind:
{code}
  public enum Kind {
      ...
      private static final Kind[] VALUES = values();   // cache; never mutate, never expose
      public static Kind fromOrdinal(int i) { return VALUES[i]; }
  }
{code}


was (Author: smiklosovic):
||File:line||Path||Frequency||
  |{{ClusteringPrefix.java:479, 490, 660}}|clustering/bound deserialize|*per clustering prefix* (the ticket itself)|
  |{{io/sstable/ClusteringDescriptor.java:64, 106}}|SSTable cursor read|*per clustering read from disk*|
  |{{io/sstable/SSTableCursorWriter.java:516}}|SSTable write|*per tombstone marker* (flush/compaction)|
  |{{db/ClusteringBoundOrBoundary.java:119}}|range-bound deserialize|*per range-tombstone bound*|


> Cache Enum.values() in ClusteringPrefix.Kind deserialization to avoid per-read array allocation
> -----------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-21528
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-21528
>             Project: Apache Cassandra
>          Issue Type: Improvement
>          Components: Legacy/Core
>            Reporter: koo
>            Assignee: koo
>            Priority: Normal
>             Fix For: 6.x, 7.x
>
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> *description* 
> When deserializing clusterings and range/slice bounds, the kind is resolved via Kind.values()[in.readByte()]:                
>  -  ClusteringPrefix.java:616 — ClusteringPrefix.Deserializer (streaming path) //  5.0.7
>       
> Enum.values() creates a new copy of its array every time it is called. So each time we read a clustering or a bound, a new Kind[] array is created just to look up one value by index, and then thrown away. This runs on the hot  deserialization path, so it creates a lot of short-lived garbage for no real benefit.
>     
> Impact (measured, JFR allocation profiling on a read/Paxos-LWT-heavy workload):           
>   - Total heap allocation observed: ~170 GB (120s)
>   - ClusteringPrefix$Kind[]: ~5.22 GB (≈3.1%) of that total 
> This garbage is short-lived, so GC pause impact under a modern collector is minimal and CPU cost is negligible; this is purely an allocation-pressure / GC-throughput improvement.                                                                 



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