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https://issues.apache.org/jira/browse/CASSANDRA-15215?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17437244#comment-17437244
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Benjamin Lerer commented on CASSANDRA-15215:
--------------------------------------------
This ticket might be a good place to also improve the performance of the
[computeUnsignedVIntSize|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/utils/vint/VIntCoding.java#L279]
method which relies on a division operation.
[~blambov] mentioned to me that:
{code}
return 9 - ((magnitude - 1) / 7);
{code}
could be replaced by
{code}
return (639 - magnitude * 9) >> 6;
{code}
which produce the same result and avoid the division high cost in term of CPU
time.
> VIntCoding should read and write more efficiently
> -------------------------------------------------
>
> Key: CASSANDRA-15215
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15215
> Project: Cassandra
> Issue Type: Bug
> Components: Local/Compaction, Local/SSTable
> Reporter: Benedict Elliott Smith
> Assignee: Aleksandr Sorokoumov
> Priority: Normal
> Fix For: 3.0.x, 3.11.x, 4.x
>
>
> Most vints occupy significantly fewer than 8 bytes, and most buffers have >=
> 8 bytes spare, in which case we can construct the relevant bytes in a
> register and memcpy them to the correct position. Since we read and write a
> lot of vints, this waste is probably measurable, particularly during
> compaction and flush, and can probably be considered a performance bug.
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