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https://issues.apache.org/jira/browse/CASSANDRA-21359?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18082701#comment-18082701
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Dmitry Konstantinov commented on CASSANDRA-21359:
-------------------------------------------------
Thank you for the review
6.0 CI run, failed tests:
{code}
Tests / jvm-dtest jdk17 4/16 /
org.apache.cassandra.distributed.test.accord.AccordMigrationTest.testPaxosToAccordCAS-_jdk17_x86_64
Tests / jvm-dtest jdk17 4/16 /
org.apache.cassandra.distributed.test.accord.AccordMigrationTest.testPaxosToAccordSerialRead-_jdk17_x86_64
Tests / dtest-latest jdk17 7/64 /
dtest-latest.bootstrap_test.TestBootstrap.test_consistent_range_movement_false_with_rf1_should_succeed
Tests / dtest-latest jdk17 6/64 /
dtest-latest.bootstrap_test.TestBootstrap.test_consistent_range_movement_true_with_rf1_should_fail
Tests / dtest-latest jdk17 34/64 /
dtest-latest.client_request_metrics_test.TestClientRequestMetrics.test_client_request_metrics
Tests / dtest-latest jdk17 23/64 /
dtest-latest.replace_address_test.TestReplaceAddress.test_replace_with_insufficient_replicas
Tests / dtest-latest jdk17 23/64 /
dtest-latest.replace_address_test.TestReplaceAddress.test_replace_with_insufficient_replicas
Tests / dtest-latest jdk17 23/64 /
dtest-latest.replace_address_test.TestReplaceAddress.test_replace_with_insufficient_replicas
{code}
all are known issues and not related to the change.
> Reduce allocation in row merge logic
> ------------------------------------
>
> Key: CASSANDRA-21359
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21359
> 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-2026-05-09-15-14-10-274.png,
> image-2026-05-09-15-26-52-036.png
>
> Time Spent: 50m
> Remaining Estimate: 0h
>
> background: 10 rows partition read test, CASSANDRA-21354 change applied
> Suggestions (~2% reduction):
> * org.apache.cassandra.utils.MergeIterator.Candidate - has Comparator field
> which is the same for all Candidates under an MergeIterator instance, we can
> move the field to MergeIterator level. Candidate is 4.8% of all allocations
> and we can save 20% (~1% of all allocations) of it for JDK 21 with compressed
> references on.
> {code}
> org.apache.cassandra.utils.MergeIterator$Candidate object internals:
> OFF SZ TYPE DESCRIPTION VALUE
> 0 8 (object header: mark) N/A
> 8 4 (object header: class) N/A
> 12 4 int Candidate.idx N/A
> 16 1 boolean Candidate.equalParent N/A
> 17 3 (alignment/padding gap)
> 20 4 java.util.Iterator Candidate.iter N/A
> 24 4 java.util.Comparator Candidate.comp N/A
> 28 4 java.lang.Object Candidate.item N/A
> 32 4 java.lang.Object Candidate.lowerBound N/A
> 36 4 (object alignment gap)
> Instance size: 40 bytes
> {code}
> without comp we dropped to 32 bytes, so 20% less object size
> {code}
> OFF SZ TYPE DESCRIPTION VALUE
> 0 8 (object header: mark) N/A
> 8 4 (object header: class) N/A
> 12 4 int Candidate.idx N/A
> 16 1 boolean Candidate.equalParent N/A
> 17 3 (alignment/padding gap)
> 20 4 java.util.Iterator Candidate.iter N/A
> 24 4 java.lang.Object Candidate.item N/A
> 28 4 java.lang.Object Candidate.lowerBound N/A
> Instance size: 32 bytes
> {code}
> If compressed refs are disabled then the size: 56 vs 48 bytes
> !image-2026-05-09-15-14-10-274.png|width=400!
> * we can switch from ArrayList to array in
> org.apache.cassandra.db.rows.Row.Merger
> The ArrayList iterator costs 0.72% and BulkIterator.Adapter costs 0.35%. So,
> in total we can save here ~1% of total allocations.
> !image-2026-05-09-15-26-52-036.png|width=400!
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