Re: [Qemu-devel] Re: [PATCH] virtio: Use ioeventfd for virtqueue notify

Anthony Liguori
2010-10-04T14:01:47Z

On 10/04/2010 03:04 AM, Avi Kivity wrote:
> On 10/04/2010 03:18 AM, Anthony Liguori wrote:
>> On 10/03/2010 09:28 AM, Michael S. Tsirkin wrote:
>>>
>>>> This is using eventfd as well.
>>> Sorry, I meant irqfd.
>>
>> I've tried using irqfd in userspace. It hurts performance quite a
>> bit compared to doing an ioctl so I would suspect this too.
>>
>> A last_used_idx or similar mechanism should help performance quite a
>> bit on top of ioeventfd too.
>>
>
> Any idea why? While irqfd does quite a bit of extra locking, it
> shouldn't be that bad.

Not really. It was somewhat counter intuitive.

A worthwhile experiment might be to do some layering violations and have
vhost do an irq injection via an ioctl and see what the performance
delta is. I suspect it could give vhost a nice boost.

Regards,

Anthony Liguori