RFR(S): 8147844: new method j.l.Runtime.onSpinWait() and the corresponding x86 hotspot instrinsic

Ivan Krylov ivan at azulsystems.com
Wed Jan 27 22:37:55 UTC 2016

On 28/01/2016 01:05, Vladimir Ivanov wrote:
> Ivan,
> There's no need in yet another flag (-XX:±UseOnSpinWaitIntrinsic). 
> -XX:DisableIntrinsic=_onSpinWait should do the same.
Good suggestion. I will accommodate that.



> Best regards,
> Vladimir Ivanov
> On 1/26/16 1:59 PM, Ivan Krylov wrote:
>> Hello,
>> Some of you may have a seen a few e-mails on the core-libs alias about a
>> proposed “spin wait hint”. The JEP is forming up nicely at
>> https://bugs.openjdk.java.net/browse/JDK-8147832. There seems to be a
>> consensus on the API side. It is now in a draft state and I hope this
>> JEP will get targeted for java 9 shortly.  The upcoming API changes can
>> be seen at the webrev:
>> http://cr.openjdk.java.net/~ikrylov/8147844.jdk.00/
>> At this time I would like to ask for a review of the hs-comp changes.
>> The plan is push changes into class libraries and hotspot synchronously
>> but that may happen after the JEP gets targeted.
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8147844
>> Webrev: http://cr.openjdk.java.net/~ikrylov/8147844.hs.00/
>> The idea of the fix is pretty simple: hotspot replaces a call to
>> java.lang.Runtime.onSpinWait() with an intrinsic that is effectively a
>> 'pause' instruction on x86.  This intrinsic is guarded by the
>> -XX:±UseOnSpinWaitIntrinsic flag. For non-x86 platforms there is a
>> verification code that makes sure the flag is off, VM will just execute
>> at empty method java.lang.Runtime.onSpinWait() – effectively a no-op.
>> According the [1] the 'pause' instruction is functional since SSE2, but
>> even on CPUs prior to SSE2 the  'pause' instruction is a no-op and hence
>> harmless, there seems to be no need to add guarding code for older
>> generations of Intel CPUs.
>> The proposed patch includes a simple regression test that simply makes
>> sure that method java.lang.Runtime.onSpinWait() gets intrinsified.
>> There are several other producer-consumer-like performance tests ready
>> that the authors of this JEP would be happy to make available under
>> JEP-230 but I am uncertain about the process.
>> Thanks,
>> Ivan
>> [1]  -
>> https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops 

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