Re: [PATCH][retry 1] Add support for Pause Filtering to AMD SVM

Avi Kivity
2009-05-11T15:29:13Z

Ingo Molnar wrote:
> * Avi Kivity <avi@redhat.com> wrote:
>
>
>>> I.e. the 3000 cycles value itself could be eliminated as well.
>>> (with just a common-sense max of say 100,000 cycles enforced)
>>>
>> Yeah, though that has a much smaller effect as it's only
>> responsible for a few microseconds of spinning.
>>
>
> 3000 cycles would be 1-2 usecs. Isnt the VM exit+entry cost still in
> that range?
>

It's 3000 executions of rep nop, so you need to account for the entire
spinlock loop body.

The Linux spinlock is

"1:\t"
"cmpl %0, %2\n\t"
"je 2f\n\t"
"rep ; nop\n\t"
"movzwl %1, %2\n\t"
/* don't need lfence here, because loads are in-order */
"jmp 1b\n"

5 instructions, maybe 2-3 cycles, not counting any special rep nop
overhead. Mark, any idea what the spin time is?

VM entry/exit is around 1us on the newer processors.

--
error compiling committee.c: too many arguments to function