Using x86 pause instr in SpinPause

[Vitaly Davidovich]

So the VM thread detects contention of some sort and enters a spin-wait;
while it's spinning, is it doing anything at all, like re-reading some
memory? I was assuming, judging by the name, that it simply stalls for some
period of time before re-checking; notifying CPU of this seems legit.
That's wrong though I take it?

Thanks

Sent from my phone
On Aug 16, 2012 7:46 PM, "David Holmes" <david.holmes at oracle.com> wrote:

> On 17/08/2012 9:42 AM, Vitaly Davidovich wrote:
>
>> Why would you not want a pause ins? I thought the point of it was to
>> inform the CPU that you're busy spinning and not doing anything useful
>> so it can make internal adjustments (e.g. scale down frequency on a
>> mobile processor).
>>
>
> That's not really applicable in the contexts in which this is used in the
> VM. Spins are used as a back-off strategy for contended actions - the fact
> they are contended indicate there is active work happening in other threads
> so we are not in an "idle" state.
>
> David
>
>
>> Thanks
>>
>> Sent from my phone
>>
>> On Aug 16, 2012 5:22 PM, "Daniel D. Daugherty"
>> <daniel.daugherty at oracle.com <mailto:daniel.daugherty@**oracle.com<daniel.daugherty at oracle.com>>>
>> 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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