RFR(S): 8147844: new method j.l.Runtime.onSpinWait() and the corresponding x86 hotspot instrinsic
ivan at azulsystems.com
Tue Jan 26 10:59:58 UTC 2016
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:
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.
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  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.
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