Re: confused about performance
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.
--
Len Sorensen
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