From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by dpdk.org (Postfix) with ESMTP id 2144291E8 for ; Thu, 8 Sep 2016 09:56:40 +0200 (CEST) Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by orsmga102.jf.intel.com with ESMTP; 08 Sep 2016 00:56:40 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.30,298,1470726000"; d="scan'208";a="1036942022" Received: from yliu-dev.sh.intel.com (HELO yliu-dev) ([10.239.67.162]) by fmsmga001.fm.intel.com with ESMTP; 08 Sep 2016 00:56:39 -0700 Date: Thu, 8 Sep 2016 15:57:09 +0800 From: Yuanhan Liu To: Maxime Coquelin Cc: souvikdey33 , stephen@networkplumber.org, huawei.xie@intel.com, dev@dpdk.org Message-ID: <20160908075709.GN23158@yliu-dev.sh.intel.com> References: <20160829230240.20164-1-sodey@sonusnet.com> <20160907032547.GG23158@yliu-dev.sh.intel.com> <20160908073029.GM23158@yliu-dev.sh.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Subject: Re: [dpdk-dev] [PATCH v2] add mtu set in virtio X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 08 Sep 2016 07:56:41 -0000 On Thu, Sep 08, 2016 at 09:50:34AM +0200, Maxime Coquelin wrote: > > > On 09/08/2016 09:30 AM, Yuanhan Liu wrote: > >On Wed, Sep 07, 2016 at 11:16:47AM +0200, Maxime Coquelin wrote: > >> > >> > >>On 09/07/2016 05:25 AM, Yuanhan Liu wrote: > >>>On Tue, Aug 30, 2016 at 09:57:39AM +0200, Maxime Coquelin wrote: > >>>>Hi Souvik, > >>>> > >>>>On 08/30/2016 01:02 AM, souvikdey33 wrote: > >>>>>Signed-off-by: Souvik Dey > >>>>> > >>>>>Fixes: 1fb8e8896ca8 ("Signed-off-by: Souvik Dey ") > >>>>>Reviewed-by: Stephen Hemminger > >>>>> > >>>>>Virtio interfaces should also support setting of mtu, as in case of cloud > >>>>>it is expected to have the consistent mtu across the infrastructure that > >>>>>the dhcp server sends and not hardcoded to 1500(default). > >>>>>--- > >>>>>drivers/net/virtio/virtio_ethdev.c | 12 ++++++++++++ > >>>>>1 file changed, 12 insertions(+) > >>>> > >>>>FYI, there are some on-going changes in the VIRTIO specification > >>>>so that the VHOST interface exposes its MTU to its VIRTIO peer. > >>>>It may also be used as an alternative of what you patch achieves. > >>>> > >>>>I am working on its implementation in Qemu/DPDK, our goal being to > >>>>reduce performance drops for small packets with Rx mergeable buffers > >>>>feature enabled. > >>> > >>>Mind to educate me a bit on how that works? > >> > >>Of course. > >> > >>Basically, this is a way to advise the MTU we want in the guest. > >>In the guest, if GRO is not enabled: > >> - In case of Kernel virtio-net, it could be used to > >>size the SKBs at the expected MTU. If possible, we could disable Rx > >>mergeable buffers. > >> - In case of virtio PMD, if the MTU advised by host is lower than the > >>pre-allocated mbuf size for the receive queue, then we should not need > >>mergeable buffers. > > > >Thanks for the explanation! > > > >I see. So, the point is to avoid using mergeable buffers while it is > >enabled. > > > >>Does that sound reasonnable? > > > >Yeah, maybe. Just don't know how well it may work in real life. Have > >you got any rought data so far? > > The PoC is not done yet, only Qemu part is implemented. > But what we noticed is that for small packets, we have a 50% > degradation when rx mergeable buffers are on when running PVP > use-case. > > Main part of the degradation is due an additional cache-miss in > virtio-pmd receive path, because we fetch the header to get the number > of buffer. > > When sending only small packets and removing this access, we recover > 25% of the degradation. > > The 25% remaining part may be reduced significantly with Zhihong series. > > Hope it answer your questions. Yes, it does and thanks for the info. --yliu