On Sunday 13 July 2003 11:33, John Summerfield wrote:
> Is that all there is? Are there beneficial new instructions available in
> 64-bit mode? Are there improved optimisations?
>
> Reportedly the Opteron (64-bit) is much quicker than the Xeons (32-bit).
> I know it ain't necessarily so that this has implications for the
> mainframe, but it might.
The Opteron has a few features that make it work a lot faster in 64 bit
mode than in 32 bit mode, for example 16 general purpose registers instead
of just 8 registers for 32 bit. In addition, the Opteron is already
very fast in 32 bit mode, compared with a Xeon or Athlon MP.
For 64 bit s390 systems there are only a few extra features and the
performance win is typically eaten up by using larger binaries.
Sometimes it's a bit faster, sometimes a bit slower.
The new z990 machines have some new instruction that make running in
64 bit mode a bit faster again, but you have to rebuild your
applications from source with a patched gcc-3.3 in order to use
that.
There are a few reasons to use 64 bit systems:
- You need more than 2 GB of physical ram in one guest/partition or
per process. This is not possible with 31 bit systems.
- You need to do a lot of 64 bit integer arithmetics. This is very
rare and if it applies to your case, you most likely already know
about this.
- You want to use a proprietary application that is only available
for 64 bit. Although there are ways to mix 64 and 32 bit applications
on a 64 bit kernel, usually you want to avoid that. Similarly,
if you want to use a 32 bit application, avoid running it on a 64
bit distro.
- You are stuck with SLES 7 and cannot use a newer distribution. On
SLES 7, the 31 bit kernel is still at 2.4.7, while the 64 bit
kernel is at least at 2.4.17. That is rather old as well, but
a lot of things have happened between those two.
- You build your main application from source
If none of those reasons apply, using a 31 bit distribution can
decrease the memory requirements per guest.
Arnd <><
