From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: <bruce.richardson@intel.com> Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by dpdk.org (Postfix) with ESMTP id 5A4125588 for <dev@dpdk.org>; Fri, 18 Mar 2016 11:04:02 +0100 (CET) Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga104.fm.intel.com with ESMTP; 18 Mar 2016 03:04:01 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.24,354,1455004800"; d="scan'208";a="68891688" Received: from bricha3-mobl3.ger.corp.intel.com ([10.237.221.44]) by fmsmga004.fm.intel.com with SMTP; 18 Mar 2016 03:03:59 -0700 Received: by (sSMTP sendmail emulation); Fri, 18 Mar 2016 10:03:58 +0025 Date: Fri, 18 Mar 2016 10:03:58 +0000 From: Bruce Richardson <bruce.richardson@intel.com> To: Jianbo Liu <jianbo.liu@linaro.org> Cc: "Lu, Wenzhuo" <wenzhuo.lu@intel.com>, "Zhang, Helin" <helin.zhang@intel.com>, "Ananyev, Konstantin" <konstantin.ananyev@intel.com>, "dev@dpdk.org" <dev@dpdk.org> Message-ID: <20160318100358.GA4848@bricha3-MOBL3> References: <1457965558-15331-1-git-send-email-jianbo.liu@linaro.org> <6A0DE07E22DDAD4C9103DF62FEBC09090343BBF2@shsmsx102.ccr.corp.intel.com> <CAP4Qi39rHsZR3qMBnRJgcoWGVjS8eDYcvfPoLxCZX1KBJm2eqQ@mail.gmail.com> <20160316111454.GB24668@bricha3-MOBL3> <CAP4Qi3-XTE4wEmPCbrFi7KPAr0vFW3ttn86yr3KsC7=qsir6+Q@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <CAP4Qi3-XTE4wEmPCbrFi7KPAr0vFW3ttn86yr3KsC7=qsir6+Q@mail.gmail.com> Organization: Intel Shannon Ltd. User-Agent: Mutt/1.5.23 (2014-03-12) Subject: Re: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the tail of rx hwring 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: Fri, 18 Mar 2016 10:04:02 -0000 On Thu, Mar 17, 2016 at 10:20:01AM +0800, Jianbo Liu wrote: > On 16 March 2016 at 19:14, Bruce Richardson <bruce.richardson@intel.com> wrote: > > On Wed, Mar 16, 2016 at 03:51:53PM +0800, Jianbo Liu wrote: > >> Hi Wenzhuo, > >> > >> On 16 March 2016 at 14:06, Lu, Wenzhuo <wenzhuo.lu@intel.com> wrote: > >> > HI Jianbo, > >> > > >> > > >> >> -----Original Message----- > >> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jianbo Liu > >> >> Sent: Monday, March 14, 2016 10:26 PM > >> >> To: Zhang, Helin; Ananyev, Konstantin; dev@dpdk.org > >> >> Cc: Jianbo Liu > >> >> Subject: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the > >> >> tail of rx hwring > >> >> > >> >> When checking rx ring queue, it's possible that loop will break at the tail while > >> >> there are packets still in the queue header. > >> > Would you like to give more details about in what scenario this issue will be hit? Thanks. > >> > > >> > >> vPMD will place extra RTE_IXGBE_DESCS_PER_LOOP - 1 number of empty > >> descriptiors at the end of hwring to avoid overflow when do checking > >> on rx side. > >> > >> For the loop in _recv_raw_pkts_vec(), we check 4 descriptors each > >> time. If all 4 DD are set, and all 4 packets are received.That's OK in > >> the middle. > >> But if come to the end of hwring, and less than 4 descriptors left, we > >> still need to check 4 descriptors at the same time, so the extra empty > >> descriptors are checked with them. > >> This time, the number of received packets is apparently less than 4, > >> and we break out of the loop because of the condition "var != > >> RTE_IXGBE_DESCS_PER_LOOP". > >> So the problem arises. It is possible that there could be more packets > >> at the hwring beginning that still waiting for being received. > >> I think this fix can avoid this situation, and at least reduce the > >> latency for the packets in the header. > >> > > Packets are always received in order from the NIC, so no packets ever get left > > behind or skipped on an RX burst call. > > > > /Bruce > > > > I knew packets are received in order, and no packets will be skipped, > but some will be left behind as I explained above. > vPMD will not received nb_pkts required by one RX burst call, and > those at the beginning of hwring are still waiting to be received till > the next call. > > Thanks! > Jianbo HI Jianbo, ok, I understand now. I'm not sure that this is a significant problem though, since we are working in polling mode. Is there a performance impact to your change, because I don't think that we can reduce performance just to fix this? Regards, /Bruce