From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=43862 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OEeJG-0007NT-MW for qemu-devel@nongnu.org; Wed, 19 May 2010 04:08:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OEeIq-0007Mw-ED for qemu-devel@nongnu.org; Wed, 19 May 2010 04:07:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:16025) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OEeIq-0007Ll-0V for qemu-devel@nongnu.org; Wed, 19 May 2010 04:06:56 -0400 Message-ID: <4BF39C12.7090407@redhat.com> Date: Wed, 19 May 2010 11:06:42 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself References: <20100505205814.GA7090@redhat.com> <201005071253.53393.rusty@rustcorp.com.au> <4BE9AF9A.8080005@redhat.com> <201005191709.16401.rusty@rustcorp.com.au> In-Reply-To: <201005191709.16401.rusty@rustcorp.com.au> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Rusty Russell Cc: qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, "Michael S. Tsirkin" On 05/19/2010 10:39 AM, Rusty Russell wrote: > > I think we're talking about the last 2 entries of the avail ring. That means > the worst case is 1 false bounce every time around the ring. It's low, but why introduce an inefficiency when you can avoid doing it for the same effort? > I think that's > why we're debating it instead of measuring it :) > Measure before optimize is good for code but not for protocols. Protocols have to be robust against future changes. Virtio is warty enough already, we can't keep doing local optimizations. > Note that this is a exclusive->shared->exclusive bounce only, too. > A bounce is a bounce. Virtio is already way too bouncy due to the indirection between the avail/used rings and the descriptor pool. A device with out of order completion (like virtio-blk) will quickly randomize the unused descriptor indexes, so every descriptor fetch will require a bounce. In contrast, if the rings hold the descriptors themselves instead of pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for every descriptor, amortized. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.