Processor optimisation (Re: Ubuntu & Debian packages)

[Riku Nurkka]

On Thu, 2004-09-30 at 04:05, Matt Zimmerman wrote:
> Ubuntu packages are built using the i486 instruction set, with instructions
> chosen based on timing information optimised for the Pentium 4. This
> produces a result which provides benefits for modern processors, without
> gratuitously breaking compatibility with older ones.

Wouldn't i686 be even more compatible than Pentium 4? 

> Note that my network gateway is an i486-class embedded system 
> They are not as uncommon as you might think, when you consider 
> that many Linux systems are not desktops.

Yea, I know. But I've understood that Ubuntu is mainly a GNOME-based
_desktop_ system, not really meant for servers or gateways. Other
distributions are better for gateways. Shouldn't Ubuntu try find its own
market and stay there - and leave the difficult mission of trying to
fulfill all possible other OS needs to Debian proper...?

> Note that optimising for i586 seems to produce binaries which run more
> slowly on i686 processors than when optimised for i386.  There is definitely
> a lot of folklore around compiler optimisations which is not exactly in line
> with reality. :-)

Good point. (I wouln't mind having an i686 or Athlon XP optimized
Debian, though... ;-)

> We do intend to explore the possibility of creating binaries using
> instructions which are not available on i486, and perform controlled
> experiments to determine whether the performance difference is measurable,
> but this was not a goal for the Warty release, given other, more immediate
> priorities.  We'll talk more about it sometime after the release.

Nice. When you get to that, why not also consider other possible means
of making the system and software run faster. I mean things like
prelinking, Con Kolivas performance patches etc. 

Just some ideas. Anyway, I think that it could be useful for Ubuntu to
try to differentiate itself more from "your average Debian desktop". Not
too much though, because it is also useful to keep the Debian
compatibilty as close to 100% as possible.

Riku