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From: Ingo Molnar <mingo@elte.hu>
To: Pekka Enberg <penberg@kernel.org>
Cc: Jesper Juhl <jj@chaosbits.net>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Daniel Lezcano <daniel.lezcano@free.fr>,
	Eric Paris <eparis@redhat.com>,
	Roman Zippel <zippel@linux-m68k.org>,
	linux-kbuild@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: PATCH][RFC][resend] CC_OPTIMIZE_FOR_SIZE should default to N
Date: Tue, 22 Mar 2011 11:27:41 +0100	[thread overview]
Message-ID: <20110322102741.GA4448@elte.hu> (raw)
In-Reply-To: <AANLkTinmAJsXy=iJ5Fq-+CLTAGXTtLGhULRUf=Q1rkY1@mail.gmail.com>


* Pekka Enberg <penberg@kernel.org> wrote:

> Hi Jesper,
> 
> On Mon, Mar 21, 2011 at 10:08 PM, Jesper Juhl <jj@chaosbits.net> wrote:
> > I believe that the majority of systems we are built on want a -O2 compiled
> > kernel. Optimizing for size (-Os) is mainly benneficial for embedded
> > systems and systems with very small CPU caches (correct me if I'm wrong).
> 
> Please take a look at commit 0910b44 ("Expose "Optimize for size"
> option for everybody") for the reasoning behind defaulting to -Os.

If that situation has changed - if GCC has regressed in this area then a commit 
changing the default IMHO gains a lot of credibility if it is backed by careful 
measurements using perf stat --repeat or similar tools.

See the hard numbers in this upstream commit for example:

  ea7145477a46: x86: Separate out entry text section

there we were able to prove the positive effects of a pretty subtle change to 
the layout of the instruction cache, with a measurement noise in the 0.1% 
range.

Thanks,

	Ingo

  parent reply	other threads:[~2011-03-22 10:27 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-03-21 20:08 Jesper Juhl
2011-03-22  2:52 ` Steven Rostedt
2011-03-22  8:21 ` Pekka Enberg
2011-03-22  8:25   ` Jesper Juhl
2011-03-22 10:27   ` Ingo Molnar [this message]
2011-03-22 16:59     ` Linus Torvalds
2011-03-23 17:45       ` Andi Kleen
2011-03-23 21:14       ` Ingo Molnar

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