-O3 vs. -O2 (was Re: [ale] sharing an experience....debian....)

Thu Feb 12 04:02:22 EST 2004

Well, the line in make.conf, after changing it for the machine in
question, reads

CFLAGS="-O3 -march=pentium -funroll-loops -pipe"

so, there's -O3 *and* -funroll-loops.  The bit about the larger binaries
not being helped very much by cache is worrisome because we're talking
about 16KB L1 and and L2 on the mobo (dunno how big) on the P/120 that's
cranking away in the basement right now.

- Jeff

On Wed, 2004-02-11 at 22:58, Matthew Macumber wrote:
> -O3 will turn on loop unrolling, which can dramatically increase the size of 
> the binaries. This can cause some code to not fit in the CPU cache. Depending 
> on the architecture, a cache miss can cause execution delay of up to 10 
> cycles. So, this can be the cause of the diminished performance.
> 
> -Matthew
> 
> On February 11, 2004 10:44 pm, Jeff Hubbs wrote:
> > There's another reason why I ask.  I frequently install Gentoo on slow
> > machines because a) I want to wring every last bit of performance I can
> > out of the hardware available and b) I'm not under time pressure to do
> > so :)
> >
> > But, according to this, in doing so and accepting the -O3 optimization
> > suggested, I may only be hurting myself, ESPECIALLY if after building
> > gcc, glibc, etc. in the bootstrap phase with -O3 and subsequently
> > compiling everything else with that gcc, it might be taking m hours
> > (days?) longer than necessary.
> >
> > I'd run tests. but I'd better do it on a CPU from this century or it'll
> > take so long I'll forget what I'm testing.
> >
> > - Jeff
> >
> > On Wed, 2004-02-11 at 22:37, Jeff Hubbs wrote:
> > > Can someone explain to me why the -O3 optimization would make programs
> > > run *slower*?  In asking, I'm not trying to cast aspersions or challenge
> > > the assertion, I'm really only just trying to ask how this is so.
> > >
> > > - Jeff
> > >
> > > On Wed, 2004-02-11 at 19:23, James Sumners wrote:
> > > > Stupid exmh didn't send this with the correct From: address earlier
> > > > today.
> > > >
> > > > >From the November 04, 2003 Debian news
> > > >
> > > > (http://lists.debian.org/debian-news/debian-news-2003/msg00056.html):
> > > >
> > > >  --- Begin Quote ---
> > > >  Debian faster than Gentoo? Matt Garman [2]wondered why his C++
> > > >  program ran dramatically slower when compiled on a Gentoo machine than
> > > >  when compiled with Debian Sid. He later [3]reported that recompiling
> > > >  the Gentoo C++ libraries with less aggressive optimization flags (-O2
> > > >  instead of -O3) eliminated the speed difference. Matt also [4]added
> > > >  that Debian and other distributions are conservative, but set up by
> > > >  very experienced people.
> > > >
> > > >   2. http://article.gmane.org/gmane.linux.gentoo.user/50924
> > > >   3. http://article.gmane.org/gmane.linux.gentoo.user/50973
> > > >   4. http://article.gmane.org/gmane.linux.gentoo.user/50953
> > > >  --- End Quote ---
> > > >
> > > >  Basically, Red Hat (Fedora) packages are typically compiled in a
> > > > haphazard fashion in relation to the other packages in the
> > > > distribution. The packages in Debian go through an arduous testing
> > > > process and are built according to rather strict rules.
> > > >
> > > > James Sumners
> > > >
> > > > On Wed, 11 Feb 2004 13:12:30 -0500
> > > >
> > > > Keith Morris - IQ <keith at iqtv.com> wrote:
> > > > > Hi all, just wanted to share a quick experience and get your opinions
> > > > > if you are willing.
> > > > >
> > > > > I have been using linux off and on since Redhat 5.1 and exclusively
> > > > > for the last 3 years at home.  I have pretty much been a redhat man
> > > > > with no complaints.  I use Fedora Core 1 on my 2 main machines at
> > > > > home (both 1.6GHz).  Well, recently I got a PIII 866 to play with and
> > > > > installed FC1 on it and the speed was really quite sluggish.  Played
> > > > > with a few more distros which I wasn't really happy with.  Well, I've
> > > > > always been afraid of Debian with it's infamous hell-installer, but
> > > > > with it's new Beta installer, I decided to try it.
> > > > >
> > > > > After a pretty painless net-install, I was totally shocked with the
> > > > > speed of Debian.  Easily twice as fast as Fedora on the 866 using the
> > > > > *SAME* software (Gnome 2.4, KDE 3.1.5, OpenOffice, Evolution, XFT
> > > > > support, antialiasing, etc.) I still have not gotten the sound
> > > > > working, but believe that I will be able to (with a few posts to the
> > > > > ALE list :)) The only main difference is that I formatted / with
> > > > > reiserfs instead of ext3 with FC1.
> > > > >
> > > > > I was wondering if anyone could guess why debian would be sooooo much
> > > > > faster with basically the same "Desktop" type of configuration? Do
> > > > > many of you use Debian?
> > > > >
> > > > > I will probably stick with Debian on this machine to learn about the
> > > > > different ins-and-outs.  I absolutely *love* apt-get for debian.  I
> > > > > have used it with rpms on redhat, but it seems better with
> > > > > debs...dunno...
> > > > >
> > > > > Anyway, thanks for letting me share.
> > > > >
> > > > > Keith Morris
> > > > > Creative Director
> > > > > Design / Effects
> > > > > IQ television group
> > > > > http://www.iqtv.com
> > > > >
> > > > >
> > > > > _______________________________________________
> > > > > Ale mailing list
> > > > > Ale at ale.org
> > > > > http://www.ale.org/mailman/listinfo/ale
> 
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://www.ale.org/mailman/listinfo/ale
-- 
Jeff Hubbs <hbbs at comcast.net>