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https://issues.apache.org/jira/browse/CASSANDRA-21524?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Dmitry Konstantinov updated CASSANDRA-21524:
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Change Category: Performance
Complexity: Normal
Status: Open (was: Triage Needed)
> Row merging logic optimizations
> -------------------------------
>
> Key: CASSANDRA-21524
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21524
> Project: Apache Cassandra
> Issue Type: Improvement
> Components: Local/Other
> Reporter: Dmitry Konstantinov
> Assignee: Dmitry Konstantinov
> Priority: Normal
> Fix For: 6.x, 7.x
>
>
> The ticket is focused on CPU tuning of row merging logic under
> Row$Merger.merge used during reads (and not only).
> * there is no need to add empty interators to columnDataIterators if a row is
> null, also 2 loops over rows can be combined into one
> * MergeIterator logic is generic and can be used in many different use cases,
> for example to merge partitions and merge rows. While it is useful from
> development point of view to avoid duplicated code - it is non-friendly for
> JIT, we have polluted profiles with more that 2 types per call site, so we
> pay for it with frequent metamorphic calls, it is especially costly when we
> iterate over cells. A possible way to avoid this issue is to copy the class
> during a build time to create a separate class with exactly the same code but
> a different name. It allows to avoid development overheads with duplicated
> code to edit and at the same time helps JIT to optimize.
> * When we created a merged row we need to calculated minDeletionTime. If none
> of the merged rows had any deletion or expiring data, neither does the
> result, so we can pass the already-known min local deletion time and avoid
> rescanning the whole btree to recompute it.
> * DeletionTime.deletes check can skip Cell.timestamp invocation (potentially
> megamorphic) if the DeletionTime is LIVE.
> * In most cases merged cells have identical types, so we can add a fast path
> for it to Row.Merger.ColumnDataReducer#useColumnMetadata
> * ColumnDataReducer.versions can be switched from List to array to reduce
> allocations and to avoid extra deferences and checks/conditions.
> * replaceAndSink is large but we can split it to hot and colder parts to
> allow it to inline hot part into reduce method
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