Using x86 pause instr in SpinPause
Peter Levart
peter.levart at marand.si
Wed Aug 29 04:29:54 PDT 2012
Here's an interesting explanation about the impact of pause instruction in spin-wait loops:
http://software.intel.com/en-us/articles/long-duration-spin-wait-loops-on-hyper-threading-technology-enabled-intel-processors/
4 years later: It may be that newer CPUs are more clever now.
Regards, Peter
On Tuesday, August 28, 2012 11:36:23 AM Eric Caspole wrote:
> Hi everybody,
> I have made a webrev making a one-line change to remove use of PAUSE
> in linux x64. This will bring linux into sync with windows where
> SpinPause is just "return 0" as Dan indicates below.
>
> http://cr.openjdk.java.net/~ecaspole/nopause/webrev.00/webrev/
>
> We find that it is better not to use PAUSE in this kind of spin
> routine. Apparently someone discovered that on windows x64 years ago.
> Thanks,
> Eric
>
> On Aug 16, 2012, at 5:07 PM, Daniel D. Daugherty wrote:
> > On Win64, SpinPause() has been "return 0" since mid-2005. Way back
> >
> > when Win64 code was in os_win32_amd64.cpp:
> > SCCS/s.os_win32_amd64.cpp:
> >
> > D 1.9.1.1 05/07/04 03:20:45 dice 12 10 00025/00000/00334
> > MRs:
> > COMMENTS:
> > 5030359 -- back-end synchonization improvements - adaptive
> >
> > spinning, etc
> >
> > When the i486 and amd64 cpu dirs were merged back in 2007, the code
> >
> > became like it is below (#ifdef'ed):
> > D 1.32 07/09/17 09:11:33 sgoldman 37 35 00264/00008/00218
> > MRs:
> > COMMENTS:
> > 5108146 Merge i486 and amd64 cpu directories.
> > Macro-ized register names. Inserted amd64 specific code.
> >
> > Looks like on Linux-X64, the code has used the PAUSE instruction
> >
> > since mid-2005:
> > D 1.3 05/07/04 03:14:09 dice 4 3 00031/00000/00353
> > MRs:
> > COMMENTS:
> > 5030359 -- back-end synchonization improvements - adaptive
> >
> > spinning, etc
> >
> > We'll have to see if Dave Dice remember why he implemented
> > it this way...
> >
> > Dan
> >
> > On 8/16/12 12:01 PM, Eric Caspole wrote:
> >> Hi everybody,
> >> Does anybody know the reason why SpinPause is simply "return 0" on
> >> Win64 but uses PAUSE on Linux in a .s file?
> >> We would like to remove PAUSE from linux too.
> >>
> >> Thanks,
> >> Eric
> >>
> >>
> >> ./src/os_cpu/windows_x86/vm/os_windows_x86.cpp
> >>
> >> 548 extern "C" int SpinPause () {
> >> 549 #ifdef AMD64
> >> 550 return 0 ;
> >> 551 #else
> >> 552 // pause == rep:nop
> >> 553 // On systems that don't support pause a rep:nop
> >> 554 // is executed as a nop. The rep: prefix is ignored.
> >> 555 _asm {
> >> 556 pause ;
> >> 557 };
> >> 558 return 1 ;
> >> 559 #endif // AMD64
> >> 560 }
> >>
> >> src/os_cpu/linux_x86/vm/linux_x86_64.s
> >>
> >> 63 .globl SpinPause
> >> 64 .align 16
> >> 65 .type SpinPause, at function
> >> 66 SpinPause:
> >> 67 rep
> >> 68 nop
> >> 69 movq $1, %rax
> >> 70 ret
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