Re: confused about performance
On Thu, 2007-06-14 at 16:08 +0200, Leopold Palomo-Avellaneda wrote:
> A Dijous 14 Juny 2007 15:21, Lennart Sorensen va escriure:
> > On Wed, Jun 13, 2007 at 04:40:52PM -0600, Sebastian Kuzminsky wrote:
> > > Hi folks, I just bought a pair of AMD64 systems for a work project,
> > > and I'm confused about the performance I'm getting from them. Both are
> > > identically configured Dell Dimension C521 systems, with Athlon 64 X2
> > > 3800+ CPUs and 1 GB RAM.
> > >
> > > On one I installed using the Etch (4.0r0) i386 netinst CD, then upgraded
> > > to Lenny. This one's running linux-image-2.6.21-1-686.
> > >
> > > On the other I installed using the current (as of 2007-06-13) Lenny d-i
> > > amd64 snapshot netinst CD. This one's running
> > > linux-image-2.6.21-1-amd64.
> > >
> > > The one with the x86 userspace and 686 kernel is faster than the one
> > > with x86_64 userspace and amd64 kernel. The difference is consistently
> > > a few percent in favor of x86 over x86_64.
> > >
> > > My only benchmark is compiling our internal source tree (mostly running
> > > gcc, some g++, flex, bison, etc). We're using gcc-4.1 and g++-4.1.
> > > I've tried it with a cold disk cache and hot disk cache, in both cases
> > > x86 is faster than x86_64.
> > >
> > > I was expecting a win for 64 bit. What's going on here?
> >
> > 64 bit is faster at some things. For things like gcc you may simply be
> > gaining nothing and loosing a few percent due to having to move around 8
> > bytes per pointer rather than 4 bytes per pointer. Certainly on sparc64
> > I believe that is known to cause a slight slow down. On sparc most
> > programs are 32bit I believe, with only a few specific ones that gain
> > from 64bit (like lots of ram) are compiled for 64 bit.
> >
> > Now anything using floating point should gain significantly on 64bit.
> > Of course none of your list of tests do. SSE (the only floating point
> > used on x86_64) is much faster than the old stack based x87
> > instructions.
> >
> > There are also some programs that gain some performance benefit from the
> > extra registers that you get in 64bit mode, but for most programs it
> > probably doesn't really matter. gcc may also not be very good at using
> > those extra registers yet.
> >
> > Of course if you need more than about 3GB ram in your system, 64bit will
> > probably win simply by avoiding the (not insignificant) overhead of PAE
> > (needed to access more than 4GB address space on x86). Also if a
> > program can take advantage of more than 2 or 3GB ram by itself, on 64bit
> > you can use however much ram you have for the application, while on x86
> > you are limited to 3GB ram per application.
>
> really, reading you makes me doubt about the whole port. How many apps do we
> have in the debian pool that can win some kind of performance?
You doubt the port? What, because nobody has more than 3GiB of RAM, or
compiled their own high-performance apps?
--
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/ Jo Shields <jms@osc.ox.ac.uk> \
| Systems Manager, |
\ Oxford Supercomputing Centre /
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