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https://issues.apache.org/jira/browse/CASSANDRA-4681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458362#comment-13458362
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Jonathan Ellis commented on CASSANDRA-4681:
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The "fix" here defeats the purpose of the SlabAllocator, and you will pay a
penalty in STW GC for non-uniform value sizes. So I'm unenthusiastic about
adopting this for general use.
Instead you might try preallocating a queue of buffers or regions to reduce
init time.
(Note that we never allocate unneeded bytebuffers, that is the point of the
UNINITIALIZED dance instead of just each thread creating the BB and CASing to
see who wins.)
> SlabAllocator spends a lot of time in Thread.yield
> --------------------------------------------------
>
> Key: CASSANDRA-4681
> URL: https://issues.apache.org/jira/browse/CASSANDRA-4681
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Affects Versions: 1.1.5
> Environment: OEL Linux
> Reporter: Oleg Kibirev
> Attachments: SlabAllocator.java
>
>
> When profiling high volume inserts into Cassandra running on a host with fast
> SSD and CPU, Thread.yield() invoked by SlabAllocator appeared as the top item
> in CPU samples. The fix is to return a regular byte buffer if current slab is
> being initialized by another thread. So instead of:
> if (oldOffset == UNINITIALIZED)
> {
> // The region doesn't have its data allocated yet.
> // Since we found this in currentRegion, we know that
> whoever
> // CAS-ed it there is allocating it right now. So
> spin-loop
> // shouldn't spin long!
> Thread.yield();
> continue;
> }
> do:
> if (oldOffset == UNINITIALIZED)
> return ByteBuffer.allocate(size);
> I achieved 4x speed up in my (admittedly specialized) benchmark by using an
> optimized version of SlabAllocator attached. Since this code is in the
> critical path, even doing excessive atomic instructions or allocating
> unneeded extra ByteBuffer instances has a measurable effect on performance
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