Blake Eggleston commented on CASSANDRA-15389:

Yes that's much better. I pulled in your commit, did the same with apply, split 
up apply, and removed the Long.MIN_VALUE stop sentinel

> 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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