linux-kernel - Re: [PATCH] use unfair spinlock when running on hypervisor.

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Message-ID: <4C05C722.1010804@redhat.com>
Date:	Wed, 02 Jun 2010 05:51:14 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, hpa@...or.com, mingo@...e.hu, npiggin@...e.de,
	tglx@...utronix.de, mtosatti@...hat.com
Subject: Re: [PATCH] use unfair spinlock when running on hypervisor.

On 06/01/2010 08:27 PM, Andi Kleen wrote:
> On Tue, Jun 01, 2010 at 07:52:28PM +0300, Avi Kivity wrote:
>    
>> We are running everything on NUMA (since all modern machines are now NUMA).
>>   At what scale do the issues become observable?
>>      
> On Intel platforms it's visible starting with 4 sockets.
>    

Can you recommend a benchmark that shows bad behaviour?  I'll run it 
with ticket spinlocks and Gleb's patch.  I have a 4-way Nehalem-EX, 
presumably the huge number of threads will magnify the problem even more 
there.

>>>> I understand that reason and do not propose to get back to old spinlock
>>>> on physical HW! But with virtualization performance hit is unbearable.
>>>>
>>>>          
>>> Extreme unfairness can be unbearable too.
>>>
>>>        
>> Well, the question is what happens first.  In our experience, vcpu
>> overcommit is a lot more painful.  People will never see the NUMA
>> unfairness issue if they can't use kvm due to the vcpu overcommit problem.
>>      
> You really have to address both, if you don't fix them both
> users will eventually into one of them and be unhappy.
>    

That's definitely the long term plan.  I consider Gleb's patch the first 
step.

Do  you have any idea how we can tackle both problems?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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