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


From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [PATCH] virtio: Use ioeventfd for virtqueue notify
Date: Mon, 04 Oct 2010 09:01:14 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.12) Gecko/20100915 Lightning/1.0b1 Thunderbird/3.0.8

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




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