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https://issues.apache.org/jira/browse/CASSANDRA-15389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16988054#comment-16988054
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Blake Eggleston commented on CASSANDRA-15389:
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I can see how that applies to hasComplex, no problem. What I’m having trouble
seeing is how this would apply to hasComplexDeletion.
Given a set of ComplexColumnData instances, I don’t think there’s any ordering
criteria we can use to search for complex deletion is there? That would prevent
a direct adaptation of `firstComplexIdx` since it's just looking for the first
index of something, and I don’t think we can avoid iteration. That would also
preclude removing the stop condition since there’s no point continuing once
we’ve found a complex deletion.
Knowing the first complex index doesn’t help us without 1) implementing slice
functionality in accumulate, 2) iterating through the complex cells with
BTree#findByIndex, which would be much slower than just walking the tree.
Implementing slice (or findAndAccumulate) would avoid reverse iteration, but I
don’t think that would make the api or implementation easier to understand.
> Minimize BTree iterator allocations
> -----------------------------------
>
> Key: CASSANDRA-15389
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15389
> Project: Cassandra
> Issue Type: Sub-task
> Components: Local/Compaction
> Reporter: Blake Eggleston
> Assignee: Blake Eggleston
> Priority: Normal
> Fix For: 4.0
>
>
> Allocations of BTree iterators contribute a lot amount of garbage to the
> compaction and read paths.
> This patch removes most btree iterator allocations on hot paths by:
> • using Row#apply where appropriate on frequently called methods
> (Row#digest, Row#validateData
> • adding BTree accumulate method. Like the apply method, this method walks
> the btree with a function that takes and returns a long argument, this
> eliminates iterator allocations without adding helper object allocations
> (BTreeRow#hasComplex, BTreeRow#hasInvalidDeletions, BTreeRow#dataSize,
> BTreeRow#unsharedHeapSizeExcludingData, Rows#collectStats,
> UnfilteredSerializer#serializedRowBodySize) as well as eliminating the
> allocation of helper objects in places where apply was used previously^[1]^.
> • Create map of columns in SerializationHeader, this lets us avoid
> allocating a btree search iterator for each row we serialize.
> These optimizations reduce garbage created during compaction by up to 13.5%
>
> [1] the memory test does measure memory allocated by lambdas capturing objects
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