From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yuanhan Liu Subject: Re: [PATCH v3] examples/vhost: fix perf regression Date: Thu, 21 Jul 2016 09:34:26 +0800 Message-ID: <20160721013426.GA28708@yliu-dev.sh.intel.com> References: <1468936391-138371-1-git-send-email-jianfeng.tan@intel.com> <1469061765-50814-1-git-send-email-jianfeng.tan@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: dev@dpdk.org, zhihong.wang@intel.com, qian.q.xu@intel.com To: Jianfeng Tan Return-path: Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by dpdk.org (Postfix) with ESMTP id D56E42BA2 for ; Thu, 21 Jul 2016 03:30:23 +0200 (CEST) Content-Disposition: inline In-Reply-To: <1469061765-50814-1-git-send-email-jianfeng.tan@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 Thu, Jul 21, 2016 at 12:42:45AM +0000, Jianfeng Tan wrote: > We find significant perfermance drop introduced by below commit, > when vhost example is started with --mergeable 0 and inside vm, > kernel virtio-net driver is used to do ip based forwarding. > > The commit, 859b480d5afd ("vhost: add guest offload setting"), adds > support for VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6, > in vhost lib. But inside vhost example, the way to disable tso only > excludes the direction from virtio to vhost, but not the opposite > direction. When mergeable is disabled, it triggers big_packets path > of virtio-net driver to prepare to receive possible big packets with > size of 64K. Because mergeable is off, for each entry of avail ring, > virtio driver uses 19 desc chained together, with one desc pointing > to header, other 18 desc pointing to 4K-sized pages. But QEMU only > creates 256 desc entries for each vq, which results in that only 13 > packets can be received. VM kernel can quickly handle those packets > and go to sleep (HLT). > > As QEMU has no option to set the desc entries of a vq, so here, > we disable VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6 > with VIRTIO_NET_F_HOST_TSO4 and VIRTIO_NET_F_HOST_TSO6 when we > disable tso of vhost example, to avoid VM kernel virtio driver > go into big_packets path. > > Fixes: 9fd72e3cbd29 ("examples/vhost: add virtio offload") > > Reported-by: Qian Xu > Signed-off-by: Jianfeng Tan > --- > v3: reword commit log. Yes, much better. One minor nit: you forgot to carry the Tested-by from Qian. Acked-by: Yuanhan Liu Thanks. --yliu