Re: [Qemu-devel] slow virtio network with vhost=on and multiple cores

[Peter Lieven]

Jan Kiszka wrote:
> On 2012-11-06 12:24, Dietmar Maurer wrote:
>>> On 2012-11-06 10:46, Dietmar Maurer wrote:
>>>>>> This obviously breaks vhost when using multiple cores.
>>>>>
>>>>> With "obviously" you mean you already have a clue why?
>>>>>
>>>>> I'll try to reproduce.
>>>>
>>>> No, sorry - just meant the performance regression is obvious (factor
>>>> 20 to
>>> 40).
>>>>
>>>
>>> OK. Did you try to bisect over qemu-kvm as well?
>>
>> No (I thought that is the same code base?)
>
> Already answered, though accidentally in private only: it is the same
> code base now, but qemu-kvm has a different history and may contain
> versions that didn't suffer from the issue.
>
> Meanwhile I quickly tried to reproduce but didn't succeed so far
> (>10GBit between host and guest with vhost=on and 2 guest cores).
> However, I finally realized that we are talking about a pretty special
> host kernel which I don't have around. I guess this is better dealt with
> by Red Hat folks. Specifically, they should know what features that
> kernel exposes and what it lacks.

Hi Jan,

I see the same issue also with an Ubuntu 12.04 (3.2.0) host kernel.
The performance drawback can also only be seen if the guest kernel
is rather old (e.g. 2.6.32).

Maybe the old guest kernel misses a feature the new injection routines
need and the failure path is very slow?

Peter

>
> Jan
>
>