performance impact of JNI GetCritical*

[Michael Bien]

Thanks for the answer Clemens,

comments inline...

On 03/18/2010 06:20 PM, Clemens Eisserer wrote:
> Hi Michael,
>
>    
>> How optimized are the Get/ReleasePrimitiveArrayCritical JNI functions?
>> Do they disable GC or do they only pin the array at the specific address?
>>      
> For moving gc's they actually stop the whole GC process as long as all
> arrays are released by ReleasePrimitiveArrayCritical.
> So basically the cost is some thread-synchronization (to make the gc
> know on all threads not to run now), plus the impact on the GC. So for
> small arrays theres quite high overhead.
> CMS itself is non-moving (old gen) so basically there could be some
> optimizations, but no idea wether this is actually done.
>
>    
>> lets say I have a short java array of length 12. Should I copy it to a
>> direct NIO buffer and use the buffer as vehicle, use Get/ReleaseCritical
>> or something else?
>>      
> For very short arrays this is quite likely beneficial (at least if
> you're on JDK7, at least older JDK6 releases do get/releasecritical +
> memcpy in jni code).
>
> Just to be curious, whats your actual usecase?
>    
the usecase was to optimize the generated code of JOGL 2 / JOAL / JOCL 
etc. for some corner cases.
I feared that putting load on GetCritical* could cause GC-dependent 
problems. Thats why I am already using ThreadLocal direct NIO buffers in 
the high-level API of JOCL to prevent this situation.

- michael


> - Clemens
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>    

-- 
Michael Bien
http://michael-bien.com/

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