From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jes Sorensen Subject: Re: [Qemu-devel] Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself Date: Wed, 02 Jun 2010 22:39:02 +0200 Message-ID: <4C06C166.3050201@redhat.com> References: <201005311716.43573.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "Michael S. Tsirkin" , qemu-devel@nongnu.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org To: Rusty Russell Return-path: In-Reply-To: <201005311716.43573.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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