[jira] [Commented] (CASSANDRA-16201) Cassandra 4.0 b2 - OOM / memory pressure due to object array pre-allocations in BatchUpdatesCollector

["Marcus Eriksson (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-16201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17210234#comment-17210234 ] 

Marcus Eriksson commented on CASSANDRA-16201:
---------------------------------------------

[~tsteinmaurer] you sure you don't see this in 3.11? looks like we ignore {{initialCapacity}} in 3.0 here: https://github.com/apache/cassandra/blob/cassandra-3.0/src/java/org/apache/cassandra/utils/btree/BTree.java#L752 but not in 3.11: https://github.com/apache/cassandra/blob/cassandra-3.11/src/java/org/apache/cassandra/utils/btree/BTree.java#L777

> Cassandra 4.0 b2 - OOM / memory pressure due to object array pre-allocations in BatchUpdatesCollector
> -----------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-16201
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-16201
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Thomas Steinmaurer
>            Priority: Normal
>         Attachments: screenshot-1.png
>
>
> In a Cas 2.1 / 3.0 / 3.11 / 4.0b2 comparison test with the same load profile, we see 4.0b2 going OOM from time to time. According to a heap dump, we have multiple NTR threads in a 3-digit MB range.
> This is likely related to object array pre-allocations at the size of {{BatchUpdatesCollector.updatedRows}} per {{BTree}} although there is always only 1 {{BTreeRow}} in the {{BTree}}.
>  !screenshot-1.png! 
> So it seems we have many, many 20K elemnts pre-allocated object arrays resulting in a shallow heap of 80K each, although there is only one element in the array.
> This sort of pre-allocation is causing a lot of memory pressure.



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