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

Avi Kivity
2010-06-02T02:51:40Z

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?

--
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