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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-4681) SlabAllocator spends a lot of time in Thread.yield
Date Fri, 27 Sep 2013 17:25:02 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-4681?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jonathan Ellis updated CASSANDRA-4681:
--------------------------------------

    Attachment: 4691-v3-rebased.txt

Rebased to trunk.

[~enigmacurry], can we test this on at least 16 cores?  Single node cluster is best, no need
to muddy the water w/ network overhead.
                
> 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
>            Assignee: Jonathan Ellis
>            Priority: Minor
>              Labels: performance
>         Attachments: 4681-v3.txt, 4691-v3-rebased.txt, SlabAllocator.java, SlabAllocator.java.list,
slab-list.patch
>
>
> 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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