From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <yuanhan.liu@linux.intel.com>
Received: from mga04.intel.com (mga04.intel.com [192.55.52.120])
 by dpdk.org (Postfix) with ESMTP id CC64619F5
 for <dev@dpdk.org>; Mon, 10 Oct 2016 05:58:20 +0200 (CEST)
Received: from orsmga003.jf.intel.com ([10.7.209.27])
 by fmsmga104.fm.intel.com with ESMTP; 09 Oct 2016 20:58:20 -0700
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.31,470,1473145200"; d="scan'208";a="888373160"
Received: from yliu-dev.sh.intel.com (HELO yliu-dev) ([10.239.67.162])
 by orsmga003.jf.intel.com with ESMTP; 09 Oct 2016 20:58:18 -0700
Date: Mon, 10 Oct 2016 11:59:10 +0800
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>,
 Stephen Hemminger <stephen@networkplumber.org>, dev@dpdk.org,
 qemu-devel@nongnu.org, "Wang, Zhihong" <zhihong.wang@intel.com>
Message-ID: <20161010035910.GY1597@yliu-dev.sh.intel.com>
References: <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>
 <bfe463e9-0d1a-c136-4115-7620f9e6ec90@redhat.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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161010064113-mutt-send-email-mst@kernel.org>
User-Agent: Mutt/1.5.23 (2014-03-12)
Subject: Re: [dpdk-dev] [Qemu-devel] [PATCH 1/2] vhost: enable any layout
	feature
X-BeenThere: dev@dpdk.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: patches and discussions about DPDK <dev.dpdk.org>
List-Unsubscribe: <http://dpdk.org/ml/options/dev>,
 <mailto:dev-request@dpdk.org?subject=unsubscribe>
List-Archive: <http://dpdk.org/ml/archives/dev/>
List-Post: <mailto:dev@dpdk.org>
List-Help: <mailto:dev-request@dpdk.org?subject=help>
List-Subscribe: <http://dpdk.org/ml/listinfo/dev>,
 <mailto:dev-request@dpdk.org?subject=subscribe>
X-List-Received-Date: Mon, 10 Oct 2016 03:58:21 -0000

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.

	--yliu