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https://issues.apache.org/jira/browse/CASSANDRA-21354?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18079263#comment-18079263
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Dmitry Konstantinov commented on CASSANDRA-21354:
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A new run after the fix looks ok:
https://pre-ci.cassandra.apache.org/job/cassandra/537/#showFailuresLink
> Avoid serialization and deserialization for coordinator-local single
> partition data read
> ----------------------------------------------------------------------------------------
>
> Key: CASSANDRA-21354
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21354
> Project: Apache Cassandra
> Issue Type: Improvement
> Components: Local/Other
> Reporter: Dmitry Konstantinov
> Assignee: Dmitry Konstantinov
> Priority: Normal
> Fix For: 6.x, 7.x
>
> Attachments: image-2026-05-07-10-10-16-886.png, may3_max_alloc.html,
> may6_noser_alloc.html, read_noser_proto_trunk_ci_summary.htm
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Currently when we execute a local read we fetch data from SSTables and
> Memtables using a merging iterator and write it to a byte buffer. Later when
> we combine a CQL response we deserialize the data back to iterate over them
> as a part of coordinator logic. So, we allocate rows and cells twice here,
> during the read from SSTables/Memtables and during the deserialization by
> coordinator logic if we read data locally (it is a typical scenario because
> usually drivers are sending requests to replicas).
> The idea of optimization: if we do a single partition read of a small number
> of rows we can keep the data in memory and avoid this double row objects
> allocation.
> We should limit amount of such data kept in memory to avoid too much pressure
> on GC due to extended lifetime for these objects and promoting them to an old
> generation.
> So, a system property can be used to limit number of rows we keep in memory
> in this scenario, as well as to disable the logic in case of any issues.
> We cannot get a number of rows in advance, so we do a rows limiting logic in
> the following way: we read first N rows to memory and if we still have
> something we serialize the remaining to a buffer and then concatenate
> iterators for the in-memory rows + deserialized one
> !image-2026-05-07-10-10-16-886.png|width=800!
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