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Date: Mon, 4 Oct 2010 18:25:02 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] Re: [PATCH] virtio: Use ioeventfd for virtqueue
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To: Anthony Liguori <aliguori@linux.vnet.ibm.com>
Cc: Steve Dobbelstein <steved@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, kvm@vger.kernel.org, qemu-devel@nongnu.org, Khoa Huynh <khoa@us.ibm.com>, Avi Kivity <avi@redhat.com>, sri@linux.vnet.ibm.com

On Mon, Oct 04, 2010 at 11:20:19AM -0500, Anthony Liguori wrote:
> On 10/04/2010 11:12 AM, Michael S. Tsirkin wrote:
> >On Mon, Oct 04, 2010 at 09:01:14AM -0500, Anthony Liguori wrote:
> >>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 think you don't even need to try that hard.
> >Just comment this line:
> >//   proxy->pci_dev.msix_mask_notifier = virtio_pci_mask_notifier;
> >this is what switches to irqfd when msi vector is unmasked.
> 
> That drops to userspace though for all irqs, no?

Exactly.

> Or did you mean that commenting that line out improves performance
> demonstrating the overhead of irqfd?
> 
> Regards,
> 
> Anthony Liguori

Haven't tried this, but possibly.

> >>  I suspect it could give vhost a nice boost.
> >>
> >>Regards,
> >>
> >>Anthony Liguori