From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: <bruce.richardson@intel.com> Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by dpdk.org (Postfix) with ESMTP id F2ABD5A53 for <dev@dpdk.org>; Mon, 15 Jun 2015 16:46:39 +0200 (CEST) Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga101.fm.intel.com with ESMTP; 15 Jun 2015 07:46:38 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.13,618,1427785200"; d="scan'208";a="743721690" Received: from bricha3-mobl3.ger.corp.intel.com ([10.243.20.22]) by fmsmga002.fm.intel.com with SMTP; 15 Jun 2015 07:46:29 -0700 Received: by (sSMTP sendmail emulation); Mon, 15 Jun 2015 15:46:28 +0025 Date: Mon, 15 Jun 2015 15:46:28 +0100 From: Bruce Richardson <bruce.richardson@intel.com> To: Olivier MATZ <olivier.matz@6wind.com> Message-ID: <20150615144628.GB580@bricha3-MOBL3> References: <87110795-201A-4A1E-A4CC-A778AA7C8218@cisco.com> <557ED116.7040508@6wind.com> <20150615134409.GA7500@bricha3-MOBL3> <2601191342CEEE43887BDE71AB97725836A0A838@irsmsx105.ger.corp.intel.com> <557EDB91.9010503@6wind.com> <20150615141258.GA580@bricha3-MOBL3> <557EE1A0.609@6wind.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <557EE1A0.609@6wind.com> Organization: Intel Shannon Ltd. User-Agent: Mutt/1.5.23 (2014-03-12) Cc: "dev@dpdk.org" <dev@dpdk.org>, "Damjan Marion \(damarion\)" <damarion@cisco.com> Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline 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, 15 Jun 2015 14:46:40 -0000 On Mon, Jun 15, 2015 at 04:30:56PM +0200, Olivier MATZ wrote: > > > On 06/15/2015 04:12 PM, Bruce Richardson wrote: > > On Mon, Jun 15, 2015 at 04:05:05PM +0200, Olivier MATZ wrote: > >> Hi, > >> > >> On 06/15/2015 03:54 PM, Ananyev, Konstantin wrote: > >>> > >>> > >>>> -----Original Message----- > >>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson > >>>> Sent: Monday, June 15, 2015 2:44 PM > >>>> To: Olivier MATZ > >>>> Cc: dev@dpdk.org; Damjan Marion (damarion) > >>>> Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline > >>>> > >>>> On Mon, Jun 15, 2015 at 03:20:22PM +0200, Olivier MATZ wrote: > >>>>> Hi Damjan, > >>>>> > >>>>> On 06/10/2015 11:47 PM, Damjan Marion (damarion) wrote: > >>>>>> > >>>>>> Hi, > >>>>>> > >>>>>> We noticed 7% performance improvement by simply moving rte_mbuf.next field to the 1st cache line. > >>>>>> > >>>>>> Currently, it falls under /* second cache line - fields only used in slow path or on TX */ > >>>>>> but it is actually used at several places in rx fast path. (e.g.: i40e_rx_alloc_bufs() is setting that field to NULL). > >>>>>> > >>>>>> Is there anything we can do here (stop using next field, or move it to 1st cache line)? > >>>>> > >>>>> Agree, this is also something I noticed, see: > >>>>> http://dpdk.org/ml/archives/dev/2015-February/014400.html > >>>>> > >>>>> I did not have the time to do performance testing, but it's something > >>>>> I'd like to do as soon as I can. I don't see any obvious reason not to > >>>>> do it. > >>>>> > >>>>> It seems we currently just have enough room to do it (8 bytes are > >>>>> remaining in the first cache line when compiled in 64 bits). > >>>> > >>>> This, to me, is the obvious reason not to do it! It prevents us from taking in > >>>> any other offload fields in the RX fast-path into the mbuf. > >>>> > >>>> That being said, I can see why we might want to look to move it - but from the > >>>> work done in the ixgbe driver, I'd be hopeful we can get as much performance with > >>>> it on the second cache line for most cases, through judicious use of prefetching, > >>>> or otherwise. > >>>> > >>>> It took a lot of work and investigation to get free space in the mbuf - especially > >>>> in cache line 0, and I'd like to avoid just filling the cache line up again as > >>>> long as we possibly can! > >>> > >>> Yep, agree with Bruce here. > >>> Plus, with packet_type going to be 4B and vlan_tci_outer, > >>> we just don't have 8 free bytes at the first cache line any more. > >> > >> I don't understand why m->next would not be a better candidate than > >> rx offload fields to be in the first cache line. For instance, m->next > >> is mandatory and must be initialized when allocating a mbuf (to be > >> compared with m->seqn for instance, which is also in the first cache > >> line). So if we want to do some room in the first cache line, I > >> think we can. > >> > >> To me, the only reason for not doing it now is because we don't > >> have a full performance test report (several use-cases, drivers, ...) > >> that shows it's better. > >> > > Because the "next" field is not mandatory to be set on initialization. It can > > instead be set only when needed, and cleared on free if it is used. > > > > The next pointers always start out as NULL when the mbuf pool is created. The > > only time it is set to non-NULL is when we have chained mbufs. If we never have > > any chained mbufs, we never need to touch the next field, or even read it - since > > we have the num-segments count in the first cache line. If we do have a multi-segment > > mbuf, it's likely to be a big packet, so we have more processing time available > > and we can then take the hit of setting the next pointer. Whenever we go to > > free that mbuf for that packet, the code to do the freeing obviously needs to > > read the next pointer so as to free all the buffers in the chain, and so it can > > also reset the next pointer to NULL when doing so. > > > > In this way, we can ensure that the next pointer on cache line 1 is not a problem > > in our fast path. > > This is a good idea, but looking at the drivers, it seems that today > they all set m->next to NULL in the rx function. What you are suggesting > is to remove all of them, and document somewhere that all mbufs in a > pool are supposed to have their m->next set to NULL, correct? Yes. However, this restriction should not be visible to the applications, as it's generally necessary to call pktmbuf_free (or a function based off it) to free an mbuf - whether in user code or a library. > > I think what you are describing could also apply to reference counter > (set to 1 by default), right? Yes, except that on free, the refcnt is actually set to zero, as we need to do so because of the atomic nature of a refcnt e.g. it's 2 and then two threads decrement it simultaneously. In a case like this, it doesn't matter whether its set to 1 again on free or on allocation, as it's on cacheline zero. /Bruce > > > Olivier > > > > > > /Bruce > >