From: "Mauricio Vásquez" <mauricio.vasquezbernal@studenti.polito.it>
To: Thomas Monjalon <thomas.monjalon@6wind.com>
Cc: dev@dpdk.org, Bruce Richardson <bruce.richardson@intel.com>,
Olivier Matz <olivier.matz@6wind.com>,
Lazaros Koromilas <l@nofutznetworks.com>
Subject: Re: [dpdk-dev] [PATCH v2] ring: check for zero objects mc dequeue / mp enqueue
Date: Fri, 18 Mar 2016 13:47:29 +0100 [thread overview]
Message-ID: <CAPwdgqj0XcC2H=BvpPXApF7g6AH9JsNpsNV+3tcTatsC39XPNw@mail.gmail.com> (raw)
In-Reply-To: <17186869.jQBbCLbaVI@xps13>
Hi,
On Fri, Mar 18, 2016 at 11:35 AM, Thomas Monjalon <thomas.monjalon@6wind.com
> 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 == 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 taking 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 compiler
> > 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 check if
number of packets is zero in the transmission function.
next prev parent reply other threads:[~2016-03-18 12:47 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-17 15:49 Lazaros Koromilas
2016-03-17 16:09 ` Mauricio Vásquez
2016-03-18 10:18 ` Bruce Richardson
2016-03-18 10:27 ` Olivier Matz
2016-03-18 10:35 ` Bruce Richardson
2016-03-18 10:35 ` Thomas Monjalon
2016-03-18 12:47 ` Mauricio Vásquez [this message]
2016-03-18 14:16 ` Bruce Richardson
2016-03-21 17:47 ` Xie, Huawei
2016-03-22 10:13 ` Bruce Richardson
2016-03-22 14:38 ` Xie, Huawei
2016-03-21 12:23 ` Olivier Matz
2016-03-22 16:49 ` Thomas Monjalon
2016-03-25 11:15 ` Olivier Matz
2016-03-28 15:48 ` Lazaros Koromilas
2016-03-29 8:54 ` Bruce Richardson
2016-03-29 15:29 ` Olivier MATZ
2016-03-29 16:04 ` Bruce Richardson
2016-03-29 17:35 ` Lazaros Koromilas
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