From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: <maxime.coquelin@redhat.com> Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id 68FF32931 for <dev@dpdk.org>; Tue, 23 Aug 2016 16:14:47 +0200 (CEST) Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D48194E4CD; Tue, 23 Aug 2016 14:14:46 +0000 (UTC) Received: from [10.36.4.245] (vpn1-4-245.ams2.redhat.com [10.36.4.245]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id u7NEEiJO004694 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Tue, 23 Aug 2016 10:14:46 -0400 To: Yuanhan Liu <yuanhan.liu@linux.intel.com>, dev@dpdk.org References: <1471939839-29778-1-git-send-email-yuanhan.liu@linux.intel.com> <1471939839-29778-7-git-send-email-yuanhan.liu@linux.intel.com> From: Maxime Coquelin <maxime.coquelin@redhat.com> Message-ID: <373f1d24-f5a5-eed1-e342-87fc6c7cc3cb@redhat.com> Date: Tue, 23 Aug 2016 16:14:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: <1471939839-29778-7-git-send-email-yuanhan.liu@linux.intel.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.68 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 23 Aug 2016 14:14:46 +0000 (UTC) Subject: Re: [dpdk-dev] [PATCH 6/6] examples/vhost: add an option to enable Tx zero copy 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: Tue, 23 Aug 2016 14:14:47 -0000 On 08/23/2016 10:10 AM, Yuanhan Liu wrote: > Add an option, --tx-zero-copy, to enable Tx zero copy. > > One thing worth noting while using Tx zero copy is the nb_tx_desc has > to be small enough so that the eth driver will hit the mbuf free > threshold easily and thus free mbuf more frequently. > > The reason behind that is, when Tx zero copy is enabled, guest Tx used > vring will be updated only when corresponding mbuf is freed. If mbuf is > not freed frequently, the guest Tx vring could be starved. > > Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com> > --- > examples/vhost/main.c | 19 ++++++++++++++++++- > 1 file changed, 18 insertions(+), 1 deletion(-) > > diff --git a/examples/vhost/main.c b/examples/vhost/main.c > index 9974f0b..e3437ad 100644 > --- a/examples/vhost/main.c > +++ b/examples/vhost/main.c > @@ -130,6 +130,7 @@ static uint32_t enable_tx_csum; > static uint32_t enable_tso; > > static int client_mode; > +static int tx_zero_copy; > > /* Specify timeout (in useconds) between retries on RX. */ > static uint32_t burst_rx_delay_time = BURST_RX_WAIT_US; > @@ -297,6 +298,17 @@ port_init(uint8_t port) > > rx_ring_size = RTE_TEST_RX_DESC_DEFAULT; > tx_ring_size = RTE_TEST_TX_DESC_DEFAULT; > + > + /* > + * When Tx zero copy is enabled, guest Tx used vring will be updated > + * only when corresponding mbuf is freed. Thus, the nb_tx_desc > + * (tx_ring_size here) must be small enough so that the driver will > + * hit the free threshold easily and free mbufs timely. Otherwise, > + * guest Tx vring would be starved. > + */ > + if (tx_zero_copy) > + tx_ring_size = 64; I have a concern about more complex applications, where the mbufs might not be consumed sequentially. If one mbuf gets stuck for a while, whereas all others are consumed, we would face starvation. For example, the packet is to be routed to a VM, which is paused, and the routing thread keeps retrying to enqueue the packet for a while. Anyway, this feature is optional and off by default, so having the feature applied is not a blocker. Thanks! Maxime