[jira] [Commented] (CASSANDRA-15389) Minimize BTree iterator allocations

["Blake Eggleston (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-15389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16987244#comment-16987244 ] 

Blake Eggleston commented on CASSANDRA-15389:
---------------------------------------------

Just pushed up some changes addressing most of your comments.

*Rows#collectStats:* the overflow checks aren't actually doing anything, since the longs are being shifted/masked to 32 bits. Force of habit when casing longs to ints :). Addressed the other comments
*SerializationHeader:* fixed. Now using a single rewindable iterator, and added a check to LeafBTreeSearchIterator to check the current position before doing a binary search. 
*BTreeRow* I had a bug in my {{hasComplexDeletion}} re-implementation that made reverse iteration / stop condition seem unnecessary. We actually do need both for hasComplexDeletion to work properly. Otherwise we’d only detect complex deletion if it’s on the final complex column.

> 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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