[gmx-users] ic vs gcc : strange results

Erik Lindahl lindahl at csb.stanford.edu
Thu Feb 19 13:40:02 CET 2004


If you get such a big difference it is almost certainly because you are 
using the 64-bit mode, without SSE assembly loops. We have 64-bit 
loops, and they will appear soon, but in the meantime you will get 
better performance by adding flags to compile in 32-bit mode.

(Or perhaps that's what the intel compiler is doing...)

Anyway, check your logfile and see if the simulation is using SSE loops.



On Feb 19, 2004, at 1:30 PM, Michel Cuendet wrote:

> It does indeed improve the speed of about 10 - 15% on my system. As I 
> said, I am running on an Opteron 240. I used  CFLAGS  =  "-O3 -ip 
> -unroll".
> Cheers.
> Lieven Buts wrote:
>> On Thursday 19 February 2004 13:02, Michel Cuendet wrote:
>>>  However I recompiled only mdrun with icc, whereas grompp is gcc. 
>>> Could
>>> this create problems ?
>> Please note that compiling mdrun with icc is unlikely to provide any 
>> benefit, since Gromacs uses assembler inner loops on Intel, which 
>> should beat even the code generated by icc.
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