From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yuanhan Liu Subject: Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature Date: Wed, 12 Oct 2016 11:21:38 +0800 Message-ID: <20161012032138.GH16751@yliu-dev.sh.intel.com> References: <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> <20161010073721-mutt-send-email-mst@kernel.org> <20161011065749.GO1597@yliu-dev.sh.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Wang, Zhihong" , Maxime Coquelin , Stephen Hemminger , "dev@dpdk.org" , "qemu-devel@nongnu.org" To: "Michael S. Tsirkin" , Thomas Monjalon Return-path: Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by dpdk.org (Postfix) with ESMTP id 6D6336CB5 for ; Wed, 12 Oct 2016 05:20:49 +0200 (CEST) Content-Disposition: inline In-Reply-To: <20161011065749.GO1597@yliu-dev.sh.intel.com> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Tue, Oct 11, 2016 at 02:57:49PM +0800, Yuanhan Liu wrote: > > > > > > There was an example: the vhost enqueue optmization patchset from > > > > > > Zhihong [0] uses memset, and it introduces more than 15% drop (IIRC) Though it doesn't matter now, but I have verified it yesterday (with and wihtout memset), the drop could be up to 30+%. This is to let you know that it could behaviour badly if memset is not inlined. > > > > > > 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. > > > > > > So try something like this then: > > Yes, I saw memset is inlined when this diff is applied. I have another concern though: It's a trick could let gcc do the inline, I am not quite sure whether that's ture with other compilers (i.e. clang, icc, or even, older gcc). For this case, I think I still prefer some trick like *(struct ..*) = {0, } Or even, we may could introduce rte_memset(). IIRC, that has been proposed somehow before? --yliu