[gmx-users] intel compiler + amd64: help needed.

Erik Lindahl lindahl at sbc.su.se
Fri Jun 2 11:17:13 CEST 2006

> Erik: I'm sure about the flags. Not using the ones for SSE3,  
> despite my processor having those. I'll just try to activate those  
> as soon as I get a compiling procedure. By now, I even using the -- 
> disable-x86-64-sse flag for configure, just to be sure. ;) Also, I  
> tried the "CC_FOR_BUILD=gcc" environment, but despite it has gone  
> throw the invsqrt compilation, I looked in and it was compiling  
> everything with gcc, and nothing with icc. :(

You so do NOT want to do disable SSE in Gromacs. To get the extra 2%  
performance from Intel icc you just killed the assembly loops which  
will reduce your performance by roughly 50%.

The Gromacs SSE code is completely portable, and checks if the  
instructions are supported at runtime.

> Yang: Yes, I'm using the intel compiler with emt64. I reinstalled  
> the software step by step specifically to be sure about this point.  
> And I understand that gromacs has an assembly core, and that this  
> is basically the "heart" of the program speed. But, even thow,  
> 5-10% means something, and every nano-second counts. Specially when  
> you have a big delay to deal with, and that this 10% mean  
> 0.25ns. ;)  Even then, after I successfully compile it we will be  
> able to say for sure how much if extra performance (if it yelds  
> any) does the intel compiler gives on AMD64 machines. Moreover, as  
> I said in a long time ago message: I want to use QM/MM with gromacs 
> +cpmd, and at this point I really dislike the idea of compiling one  
> thing with intel (cpmd) and the other with gcc (gromacs). Since I  
> can't make cpmd work on gcc, I'm trying to make gromacs work on  
> intel. ;)

We have already tried the intel compiler multiple times (both on ia32  
and x86-64 platforms), and it gets you roughly 2% extra performance  
in Gromacs.

A really nice thing with icc is that the object format is completely  
gcc-compatible, so there shouldn't be any problems mixing.



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