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

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Date:	Wed, 02 Jun 2010 22:39:02 +0200
From:	Jes Sorensen <Jes.Sorensen@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	"Michael S. Tsirkin" <mst@...hat.com>, qemu-devel@...gnu.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [Qemu-devel] Re: [PATCHv2-RFC 0/2] virtio: put last seen used
 index	into ring itself

On 05/31/10 09:46, Rusty Russell wrote:
> On Thu, 27 May 2010 05:20:35 am Michael S. Tsirkin wrote:
>> Here's a rewrite of the original patch with a new layout.
>> I haven't tested it yet so no idea how this performs, but
>> I think this addresses the cache bounce issue raised by Avi.
>> Posting for early flames/comments.
> 
> Sorry, not without some evidence that it'll actually reduce cacheline
> bouncing.  I *think* it will, but it's not obvious: the host may keep
> looking at avail_idx as we're updating last_seen.  Or does qemu always
> look at both together anyway?
> 
> Can someone convince me this is a win?
> Rusty.

Hi Rusty,

I ran some tests using the vring index publish patch with virtio_blk.
The numbers are based on running IOZone on a ramdisk passed to the guest
via virtio. While I didn't see any throughput improvements, I saw a
20-30% reduction in the VMExit count for the full run. This was measured
grabbing the VMExit count prior and after the run and calculating the
difference.

I have the numbers in a PDF, so I will email it to you in private as I
don't like sending PDFs to the mailing list. However if anybody else
wants the numbers feel free to ping me off list and I'll forward them.

Cheers,
Jes

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