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

Avi Kivity
2009-05-11T15:40:49Z

Langsdorf, Mark wrote:
>> 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?
>>
>
> If I'm understanding the question right, the contested
> spin locks are being held for 5K to 10K iterations of PAUSE.
> So 10K to 30K cycles if your estimate of the spinlock
> cycle time is correct.
>

My estimate is not very reliable. Can you measure this?

btw, I'd expect you'd get much more significant improvement on Windows.
Booting 16-vcpu Windows 2008 x64 took forever on my dual core host, and
my guess is spinlock contention. Of course you'd need a working yield,
as Ingo said schedule() does very little.

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