From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54679) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btS8p-00079Z-8w for qemu-devel@nongnu.org; Mon, 10 Oct 2016 00:24:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1btS8k-0002sG-U1 for qemu-devel@nongnu.org; Mon, 10 Oct 2016 00:24:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43176) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btS8k-0002s7-Kb for qemu-devel@nongnu.org; Mon, 10 Oct 2016 00:24:38 -0400 Date: Mon, 10 Oct 2016 07:24:36 +0300 From: "Michael S. Tsirkin" Message-ID: <20161010071739-mutt-send-email-mst@kernel.org> References: <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> <20161010033744.GW1597@yliu-dev.sh.intel.com> <20161010064113-mutt-send-email-mst@kernel.org> <20161010035910.GY1597@yliu-dev.sh.intel.com> <8F6C2BD409508844A0EFC19955BE09414E7BC050@SHSMSX103.ccr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8F6C2BD409508844A0EFC19955BE09414E7BC050@SHSMSX103.ccr.corp.intel.com> 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: "Wang, Zhihong" Cc: Yuanhan Liu , Maxime Coquelin , Stephen Hemminger , "dev@dpdk.org" , "qemu-devel@nongnu.org" On Mon, Oct 10, 2016 at 04:16:19AM +0000, Wang, Zhihong wrote: > > > > -----Original Message----- > > From: Yuanhan Liu [mailto:yuanhan.liu@linux.intel.com] > > Sent: Monday, October 10, 2016 11:59 AM > > To: Michael S. Tsirkin > > Cc: Maxime Coquelin ; Stephen Hemminger > > ; dev@dpdk.org; qemu- > > devel@nongnu.org; Wang, Zhihong > > Subject: Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature > > > > On Mon, Oct 10, 2016 at 06:46:44AM +0300, Michael S. Tsirkin wrote: > > > On Mon, Oct 10, 2016 at 11:37:44AM +0800, Yuanhan Liu wrote: > > > > 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. > > > > > > for memset 0? gcc 5.3.1 on fedora happily inlines it. > > > > Good to know! > > > > > > 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 > > > > > > I'd say that's weird. what's your config? any chance you > > > are using an old compiler? > > > > Not really, it's gcc 5.3.1. Maybe Zhihong could explain more. IIRC, > > he said the memset is not well optimized for Ivybridge server. > > The dst is remote in that case. It's fine on Haswell but has complication > in Ivy Bridge which (wasn't supposed to but) causes serious frontend issue. > > I don't think gcc inlined it there. I'm using fc24 gcc 6.1.1. I just wrote some test code and compiled with gcc -O2, it did get inlined. It's probably this: uint16_t head_size = vq->hw->vtnet_hdr_size; ... memset(hdr, 0, head_size); IOW head_size is not known to compiler. Try sticking a bool there instead of vtnet_hdr_size, and memset(hdr, 0, bigheader ? 10 : 12); > > > > --yliu