linux-kernel - Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself

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Message-ID: <20100531132950.GC1704@redhat.com>
Date:	Mon, 31 May 2010 16:29:50 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jes Sorensen <Jes.Sorensen@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring
	itself

On Mon, May 31, 2010 at 09:36:00AM +0200, Jes Sorensen wrote:
> On 05/30/10 13:22, Michael S. Tsirkin wrote:
> > On Fri, May 28, 2010 at 11:56:54AM +0200, Jes Sorensen wrote:
> >> It looks pretty good to me, however one thing I have been thinking of
> >> while reading through it:
> >>
> >> Rather than storing a pointer within the ring struct, pointing into a
> >> position within the same struct. How about storing a byte offset instead
> >> and using a cast to get to the pointer position? That would avoid the
> >> pointer dereference, which is less effective cache wise and harder for
> >> the CPU to predict.
> >>
> >> Not sure whether it really matters performance wise, just a thought.
> > 
> > I think this won't work: when PUBLUSH_USED_IDX is negotiated,
> > the pointer is to within the ring.
> 
> Hmmm shame, it would be a nice optimization.
> 
> Maybe it's time to introduce the v2 ring format, rather than having
> adding more kludges to the existing one?
> 
> Cheers,
> Jes

There has been discussion about a ring format that does not
use indexes at all. My guess is that would be a good point
for v2 ring format. But making that a product
and tuning might take a while. So definitely something to
keep in mind but I would not want that to block this optimization.

-- 
MST
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