Dmitry Konstantinov created CASSANDRA-21524:
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Summary: 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
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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