From: Thomas Monjalon <thomas.monjalon@6wind.com>
To: Bruce Richardson <bruce.richardson@intel.com>
Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH 1/2] ixgbe: add "cold" attribute to setup/teardown fns
Date: Fri, 03 Jul 2015 21:57:26 +0200 [thread overview]
Message-ID: <2644782.PAW8lcSTnr@xps13> (raw)
In-Reply-To: <20150703155642.GB8096@bricha3-MOBL3>
2015-07-03 16:56, Bruce Richardson:
> On Fri, Jul 03, 2015 at 05:45:34PM +0200, Thomas Monjalon wrote:
> > Hi Bruce,
> >
> > 2015-07-03 16:40, Bruce Richardson:
> > > As well as the fast-path functions in the rxtx code, there are also
> > > functions which set up and tear down the descriptor rings. Since these
> > > are not performance critical functions, there is no need to have them
> > > extensively optimized, so we add __attribute__((cold)) to their
> > > definitions. This has the side-effect of making debugging them easier as
> > > the compiler does not optimize them as heavily, so more variables are
> > > accessible by default in gdb.
> >
> > What is the benefit, compared to -O0?
>
> First off, it's per function, rather than having to use -O0 globally. Secondly,
> it doesn't disable optimization, it just tells the compiler that the code is
> not on the hotpath - whether or not the compiler optimizes it is up to the
> compiler itself. From GCC documentation:
> https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
>
> "The cold attribute on functions is used to inform the compiler that the
> function is unlikely to be executed. The function is optimized for size rather
> than speed and on many targets it is placed into a special subsection of the
> text section so all cold functions appear close together, improving code
> locality of non-cold parts of program. The paths leading to calls of cold
> functions within code are marked as unlikely by the branch prediction mechanism.
> It is thus useful to mark functions used to handle unlikely conditions, such as
> perror, as cold to improve optimization of hot functions that do call marked
> functions in rare occasions."
I know it may provide some optimization of the hot path.
I was asking compared to -O0 because you were justifying this change for debug.
In other words, for debugging, -O0 is probably better. So the reason of this
change should be the optimization. And it would be interesting to know if you
have seen some performance improvement.
next prev parent reply other threads:[~2015-07-03 19:58 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-03 15:40 [dpdk-dev] [PATCH 0/2] Fix crash with vpmd and mbuf debug Bruce Richardson
2015-07-03 15:40 ` [dpdk-dev] [PATCH 1/2] ixgbe: add "cold" attribute to setup/teardown fns Bruce Richardson
2015-07-03 15:45 ` Thomas Monjalon
2015-07-03 15:56 ` Bruce Richardson
2015-07-03 19:57 ` Thomas Monjalon [this message]
2015-07-06 9:20 ` Bruce Richardson
2015-07-06 9:26 ` Thomas Monjalon
2015-07-03 23:03 ` Stephen Hemminger
2015-07-03 15:40 ` [dpdk-dev] [PATCH 2/2] ixgbe: check mbuf refcnt when clearing RX/TX ring Bruce Richardson
2015-07-03 15:46 ` Thomas Monjalon
2015-07-03 16:04 ` Bruce Richardson
2015-07-03 19:52 ` Thomas Monjalon
2015-07-20 9:36 ` Ananyev, Konstantin
2015-07-20 9:47 ` Richardson, Bruce
2015-07-06 15:08 ` [dpdk-dev] [PATCH 0/2] Fix crash with vpmd and mbuf debug Thomas Monjalon
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2644782.PAW8lcSTnr@xps13 \
--to=thomas.monjalon@6wind.com \
--cc=bruce.richardson@intel.com \
--cc=dev@dpdk.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).