From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42356) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XboDT-0004ag-LM for qemu-devel@nongnu.org; Wed, 08 Oct 2014 06:11:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XboDL-0003WC-Sx for qemu-devel@nongnu.org; Wed, 08 Oct 2014 06:11:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58719) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XboDL-0003W2-LX for qemu-devel@nongnu.org; Wed, 08 Oct 2014 06:11:23 -0400 Date: Wed, 8 Oct 2014 13:14:43 +0300 From: "Michael S. Tsirkin" Message-ID: <20141008101443.GA4291@redhat.com> References: <542A6B70.7090607@huawei.com> <20140930093356.GA3673@redhat.com> <5434EB0B.8010800@cloudius-systems.com> <20141008091547.GB3872@redhat.com> <54350919.8050401@cloudius-systems.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54350919.8050401@cloudius-systems.com> Subject: Re: [Qemu-devel] [QA-virtio]:Why vring size is limited to 1024? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Jason Wang , qemu-devel@nongnu.org, liuyongan@huawei.com, qinchuanyu@huawei.com, "Zhangjie (HZ)" , akong@redhat.com On Wed, Oct 08, 2014 at 12:51:21PM +0300, Avi Kivity wrote: > > On 10/08/2014 12:15 PM, Michael S. Tsirkin wrote: > >On Wed, Oct 08, 2014 at 10:43:07AM +0300, Avi Kivity wrote: > >>On 09/30/2014 12:33 PM, Michael S. Tsirkin wrote: > >>>a single descriptor might use all of > >>>the virtqueue. In this case we wont to be able to pass the > >>>descriptor directly to linux as a single iov, since > >>> > >>You could separate maximum request scatter/gather list size from the > >>virtqueue size. They are totally unrelated - even now you can have a larger > >>request by using indirect descriptors. > >We could add a feature to have a smaller or larger S/G length limit. > >Is this something useful? > > > > Having a larger ring size is useful, esp. with zero-copy transmit, and you > would need the sglist length limit in order to not require linearization on > linux hosts. So the limit is not useful in itself, only indirectly. > > Google cloud engine exposes virtio ring sizes of 16384. OK this sounds useful, I'll queue this up for consideration. Thanks! > Even more useful is getting rid of the desc array and instead passing descs > inline in avail and used. You expect this to improve performance? Quite possibly but this will have to be demonstrated. -- MST