linux-kernel - Re: [PATCH] vhost-net: utilize PUBLISH_USED

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Message-Id: <201005201348.53702.rusty@rustcorp.com.au>
Date:	Thu, 20 May 2010 13:48:52 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, davem@...emloft.net,
	Juan Quintela <quintela@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arnd Bergmann <arnd@...db.de>, kvm@...r.kernel.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, alex.williamson@...hat.com,
	amit.shah@...hat.com
Subject: Re: [PATCH] vhost-net: utilize PUBLISH_USED_IDX feature

On Thu, 20 May 2010 07:57:18 am Michael S. Tsirkin wrote:
> On Wed, May 19, 2010 at 08:04:51PM +0300, Avi Kivity wrote:
> > On 05/18/2010 04:19 AM, Michael S. Tsirkin wrote:
> >> With PUBLISH_USED_IDX, guest tells us which used entries
> >> it has consumed. This can be used to reduce the number
> >> of interrupts: after we write a used entry, if the guest has not yet
> >> consumed the previous entry, or if the guest has already consumed the
> >> new entry, we do not need to interrupt.
> >> This imporves bandwidth by 30% under some workflows.
> >>
> >> Signed-off-by: Michael S. Tsirkin<mst@...hat.com>
> >> ---
> >>
> >> Rusty, Dave, this patch depends on the patch
> >> "virtio: put last seen used index into ring itself"
> >> which is currently destined at Rusty's tree.
> >> Rusty, if you are taking that one for 2.6.35, please
> >> take this one as well.
> >> Dave, any objections?
> >>    
> >
> > I object: I think the index should have its own cacheline,
> 
> The issue here is that host/guest do not know each
> other's cache line size. I guess we could just put it
> at offset 128 or something like that ... Rusty?

I was assuming you'd put it at the end of the padding.

I think it's a silly optimization, but Avi obviously feels strongly about
it and I respect his opinion.

Please resubmit...
Thanks,
Rusty.
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