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Date: Mon, 04 Oct 2010 09:01:14 -0500
From: Anthony Liguori <aliguori@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] Re: [PATCH] virtio: Use ioeventfd for virtqueue
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To: Avi Kivity <avi@redhat.com>
Cc: Steve Dobbelstein <steved@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Khoa Huynh <khoa@us.ibm.com>, sri@linux.vnet.ibm.com

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