performance impact of JNI GetCritical*

[charlie hunt]

Y. Srinivas Ramakrishna wrote:
> On 03/18/10 11:49, Y. Srinivas Ramakrishna wrote:
>
>   
>>>>> 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).
>>>>         
>> Right; this is a fact that I learnt much to my delight recently.
>> I requested that this improvement be backported to JDK 6, but
>> I do not know if/when that might happen.
>>     
>
> Let me clarify/correct my comment a bit.
>
> What I meant to say was that NIO performance/behaviour improved a lot
> in JDK 7 because it stopped using JNI CS, using unsafe instead.
> (This also had the pleasant side-effect of working around a particularly
> egregious CMS bug related to JNI CS recently discussed on this alias,
> and still unfortunately not fixed, which is what initially brought this
> change to my attention.)
>
> As regards Michael's original question on whether NIO or JNI CS
> is better suited for short arrays, at the risk of stating the obvious,
> I think the answer is that how much better NIO is (in JDK 7) would likely
> depend on the frequency of copying and size of these arrays: If JNI CS
> are very short-lived and infrequent and the arrays are as small as 12 bytes,
> the difference would likely be negligible. As frequency of use (and degree
> of concurrent use) and/or array size increases, NIO would likely become
> increasingly better than JNI CS (in JDK 7 at least). At least that's my hunch.
>
> -- ramki
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>   

Fwiw, the enhancement Ramki mentioned of using unsafe in NIO is being 
integrated into JDK 6u20, (a future release since JDK 6u18 is the most 
current release).

charlie ...
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