From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46199) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btROd-0002rb-Ot for qemu-devel@nongnu.org; Sun, 09 Oct 2016 23:37:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1btROZ-0005Ey-I4 for qemu-devel@nongnu.org; Sun, 09 Oct 2016 23:36:58 -0400 Received: from mga03.intel.com ([134.134.136.65]:39484) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btROZ-0005Ef-BZ for qemu-devel@nongnu.org; Sun, 09 Oct 2016 23:36:55 -0400 Date: Mon, 10 Oct 2016 11:37:44 +0800 From: Yuanhan Liu Message-ID: <20161010033744.GW1597@yliu-dev.sh.intel.com> References: <1474872056-24665-2-git-send-email-yuanhan.liu@linux.intel.com> <20160926221112-mutt-send-email-mst@kernel.org> <20160927031158.GA25823@yliu-dev.sh.intel.com> <20160927224935-mutt-send-email-mst@kernel.org> <20160928022848.GE1597@yliu-dev.sh.intel.com> <20160929205047-mutt-send-email-mst@kernel.org> <2889e609-f750-a4e1-66f8-768bb07a2339@redhat.com> <20160929231252-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160929231252-mutt-send-email-mst@kernel.org> Subject: Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Maxime Coquelin , Stephen Hemminger , dev@dpdk.org, qemu-devel@nongnu.org, "Wang, Zhihong" On Thu, Sep 29, 2016 at 11:21:48PM +0300, Michael S. Tsirkin wrote: > On Thu, Sep 29, 2016 at 10:05:22PM +0200, Maxime Coquelin wrote: > > > > > > On 09/29/2016 07:57 PM, Michael S. Tsirkin wrote: > Yes but two points. > > 1. why is this memset expensive? I don't have the exact answer, but just some rough thoughts: It's an external clib function: there is a call stack and the IP register will bounch back and forth. BTW, It's kind of an overkill to use that for resetting 14 bytes structure. Some trick like *(struct virtio_net_hdr *)hdr = {0, }; Or even hdr->xxx = 0; hdr->yyy = 0; should behaviour better. There was an example: the vhost enqueue optmization patchset from Zhihong [0] uses memset, and it introduces more than 15% drop (IIRC) on my Ivybridge server: it has no such issue on his server though. [0]: http://dpdk.org/ml/archives/dev/2016-August/045272.html --yliu > Is the test completely skipping looking > at the packet otherwise? > > 2. As long as we are doing this, see > Alignment vs. Networking > ======================== > in Documentation/unaligned-memory-access.txt > > > > From the micro-benchmarks results, we can expect +10% compared to > > indirect descriptors, and + 5% compared to using 2 descs in the > > virtqueue. > > Also, it should have the same benefits as indirect descriptors for 0% > > pkt loss (as we can fill 2x more packets in the virtqueue). > > > > What do you think? > > > > Thanks, > > Maxime