From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bruce Richardson Subject: Re: [PATCH v2] ring: check for zero objects mc dequeue / mp enqueue Date: Tue, 22 Mar 2016 10:13:08 +0000 Message-ID: <20160322101307.GA19268@bricha3-MOBL3> References: <1458229783-15547-1-git-send-email-l@nofutznetworks.com> <20160318101823.GC4848@bricha3-MOBL3> <56EBD806.8010707@6wind.com> <17186869.jQBbCLbaVI@xps13> <20160318141632.GC12932@bricha3-MOBL3> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Cc: Mauricio =?iso-8859-1?Q?V=E1squez?= , Thomas Monjalon , "dev@dpdk.org" , Olivier Matz , Lazaros Koromilas To: "Xie, Huawei" Return-path: Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by dpdk.org (Postfix) with ESMTP id E16E22C1A for ; Tue, 22 Mar 2016 11:13:11 +0100 (CET) Content-Disposition: inline In-Reply-To: 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 Mon, Mar 21, 2016 at 05:47:44PM +0000, Xie, Huawei wrote: > On 3/18/2016 10:17 PM, Bruce Richardson wrote: > > On Fri, Mar 18, 2016 at 01:47:29PM +0100, Mauricio V=E1squez wrote: > >> Hi, > >> > >> > >> On Fri, Mar 18, 2016 at 11:35 AM, Thomas Monjalon >>> wrote: > >>> 2016-03-18 11:27, Olivier Matz: > >>>> On 03/18/2016 11:18 AM, Bruce Richardson wrote: > >>>>>>> + /* Avoid the unnecessary cmpset operation below, which = is > >>> also > >>>>>>> + * potentially harmful when n equals 0. */ > >>>>>>> + if (n =3D=3D 0) > >>>>>>> > >>>>>> What about using unlikely here? > >>>>>> > >>>>> Unless there is a measurable performance increase by adding in > >>> likely/unlikely > >>>>> I'd suggest avoiding it's use. In general, likely/unlikely should= only > >>> be used > >>>>> for things like catestrophic errors because the penalty for takin= g the > >>> unlikely > >>>>> leg of the code can be quite severe. For normal stuff, where the = code > >>> nearly > >>>>> always goes one way in the branch but occasionally goes the other= , the > >>> hardware > >>>>> branch predictors generally do a good enough job. > >>>> Do you mean using likely/unlikely could be worst than not using it > >>>> in this case? > >>>> > >>>> To me, using unlikely here is not a bad idea: it shows to the comp= iler > >>>> and to the reader of the code that is case is not the usual case. > >>> It would be nice to have a guideline section about likely/unlikely = in > >>> doc/guides/contributing/design.rst > >>> > >>> Bruce gave a talk at Dublin about this kind of things. > >>> I'm sure he could contribute more design guidelines ;) > >>> > >> There is a small explanation in the section "Branch Prediction" of > >> doc/guides/contributing/coding_style.rst, but I do not know if that = is > >> enough to understand when to use them. > >> > >> I've made a fast check and there are many PMDs that use them to chec= k if > >> number of packets is zero in the transmission function. > > Yeah, and I wonder how many of those are actually necessary too :-) > > > > It's not a big deal either way, I just think the patch is fine as-is = without > > the extra macros. >=20 > IMO we use likely/unlikely in two cases, catastrophic errors and the > code nearly always goes one way, i.e, preferred/favored fast path. > Likely/unlikely helps not only for branch predication but also for cach= e For branch prediction, anything after the first time through the code pat= h the prediction will be based on what happened before rather than any stat= ic hints in the code. > usage. The code generated for the likely path will directly follow the > branch instruction. To me, it is reasonable enough to add unlikely for = n > =3D=3D 0, which we don't expect to happen. > I remember with/without likely, compiler could generate three kind of > instructions. Didn't deep dive into it. >=20 > > > > /Bruce > > >=20