Using x86 pause instr in SpinPause

[Vitaly Davidovich]

I'm actually curious to know if Eric can explain a bit more why pause is an
issue here, possibly with some benchmark results.

David's point earlier was that he doesn't think there's benefit to it in
the way hotspot spins, but removing pause implies it can actually do harm
rather than simply being unhelpful.

I'm also assuming this is not AMD specific but Intel as well?

Thanks

Sent from my phone
On Aug 29, 2012 8:04 AM, "Peter Levart" <peter.levart at marand.si> wrote:

> 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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