[gmx-developers] Opteron

[Erik Lindahl]

Hi,

> I actually got better performance by disabling the x86 assembly and 
> using g77. Let
> me redo the test as it is significant. I have been told that the 
> Opteron emulates an x87
> FPU so if the assembly routines use x87 only then performance will not 
> be great. Again
> I need to confirm this with AMD.

That must be an error - either in the compile (no SSE support found if 
you use 64-bit mode) or at runtime in the linux kernel.

The assembly code uses SSE instructions (not x87), and they will be 
twice as fast as compiled x87 code.

There is at least one other report from a user on the mailing list who 
got a significant speedup when he compiled in 32-bit mode and enabled 
SSE.

My guess would be that you are running in 64-bit mode, and probably 
just comparing C vs. Fortran!

Check the gromacs logfile - it should say that it is checking for SSE 
and report the result.

Cheers,

Erik
>
> The machines are being designated for test drives and devel work. If 
> they get heavily
> loaded then I will add more hardware. If you crash either then we have 
> learned something important, so ..... go ahead make my day.
>

Great. I don't think it will crash, but in one way it would be "good" 
since it should be impossible, and thus AMD will probably be very keen 
on fixing it ASAP :-)

Cheers,

Erik