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From "Oleg Kibirev (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4681) SlabAllocator spends a lot of time in Thread.yield
Date Tue, 18 Sep 2012 21:17:07 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458180#comment-13458180
] 

Oleg Kibirev commented on CASSANDRA-4681:
-----------------------------------------

The benchmark exercised two column families. I think the key is that I am using a large number
of very small column names/values and insert concurrently from dozens of threads (due to characteristics
of the real world use case). The system used has 24 CPU cores and an SSD raid. So basically
there is a very high rate of allocations and large number of concurrent threads hitting the
same SlabAllocator. 
                
> 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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