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

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Date:	Mon, 11 May 2009 18:40:49 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Langsdorf, Mark" <mark.langsdorf@....com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Roedel, Joerg" <Joerg.Roedel@....com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD
 SVM

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

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