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

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Message-ID: <4C0366E0.6040203@redhat.com>
Date:	Mon, 31 May 2010 09:36:00 +0200
From:	Jes Sorensen <Jes.Sorensen@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 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
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