[gmx-users] Re: SSE on AMD (fwd)

Erik Lindahl lindahl at csb.stanford.edu
Thu Oct 9 23:13:00 CEST 2003


Gromacs checks the cpu flags at startup (it would be impossible to do 
every step, since the CPUID instruction is very slow, and the code also 
generates exceptions in some cases...).

> So, do you figure out what may happen to a job started on AMD cpu (with
> gmx hacked to use 3Dnow) when it migrates to intel cpu?  And 
> vice-versa?

If you try to execute a 3DNow instruction on an Intel chip you will get 
"illegal instruction error" and the program crashes immediately.

If you start on a pentium, it depends. pre-AthlonXP cpus don't support 
SSE and will give the same error. No 32-bit Athlons support SSE2, so 
that will also crash, but we only use SSE2 in double precision.

I have no idea if mosix does something smart to handle subtle CPU 



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