Aleksandar Milivojevic wrote:
> On the sidenote, since i586 is minimal supported platform, I don't know
> why packages are still built as i386, and not as i586.
*Very* frequently asked question in some quarters. The short answer is
that it doesn't appear to be worth it.
There were very few instructions added to the instruction set between
the 386 and the 686, and apparently most of those that were added
aren't the sort of instructions that noticably accelerate programs.
The exceptions, the instructions that do help performance, give better
control of spinlocks and multi-threaded code. That does help, especially
on multi-processor kit, but the code that uses those instructions tends
to be abstracted into glibc, which *is* compiled for the different
processors. [1]
In the meantime, some surprisingly recent kit (VIA CPUs, for example)
don't include all the 686 support.
The Fedora development crew have repeatedly said that they would be very
interested to see benchmarks that prove a particular program would
benefit from different compile settings. [2] But they do expect these
benchmarks to be rigourous: same kit, the same location on the drive (if
relevant), system rebooted between runs, just different compile flags.
As you say, these days the programs are *optimised* for Pentium 4s:
apparently this works well for most other modern x86 CPUs.
James.
[1] Remember that all these programs are cross-platform: they can't rely
on running on an Intel-compatible processor, so the Right Thing is
usually to push this sort of thing into an OS library.
[2] That includes things like changing -O2 (compile for speed, at least
on small program segments) to -Os (compile for size, which will use
caches better). Stuff that really can change performance levels.
--
James Wilkinson | "The US Air Force is removing harmful "greenhouse"
Exeter Devon UK | gases from the cooling systems of intercontinental
E-mail address: james | ballistic missiles. This will minimise damage to the
@westexe.demon.co.uk | ozone layer in the event of a nuclear holocaust."
| -- The Guardian.
