[gmx-users] Opteron-Bug

[Erik Lindahl]

Hi Thomas,

Just to confirm what David said - yes, there was a hardware issue with  
all recent AMD processors, including Opterons.

I have worked around this in the code starting with 3.2.1, but since  
I'm partly under NDA with AMD (which they still haven't released me  
from) unfortunately I cannot comment on exactly what the bug is. If I  
understand it correct they are still trying to find a software fix.

I cannot prevent you from running a diff on different versions of  
x86_sse.s though :-)

Compilers really don't use the parallel SSE instructions at all, they  
just work on one element in all cases I've seen. And, apart from  
Gromacs the only SSE-tuned programs I've seen are simple things like  
video encoding and BLAS that always go through memory in a nice linear  
way. Those won't be affected by the it.

I wouldn't worry about going with Opterons because of this - all CPUs  
have errata, including Intel ones...

Cheers,

Erik


On Mar 19, 2004, at 8:42 PM, David wrote:

> On Thu, 2004-03-18 at 18:49, Thomas Veith wrote:
>> Hi folks,
>>
>> Here we are going to get a cluster which is budgeted for approx. Euro  
>> 150.000,--.
>>
>> To get most bang for our bucks, we thought of going the  
>> AMD-Opteron/AMD64 way; but recently I read here by browsing on your  
>> ML that there exists an Opteron Bug, but I couldn't find any more  
>> info to this.
>>
>> So my question is: Is there a hardware bug in (SSE/2, anywhere)  
>> Opteron, is it fixed, was it related only to gromacs or could this  
>> affect any hand-coded assembly?
>>
> Yes there seems to be a hardware bug  in all SSE capable Athlons, but  
> if
> I am informed correctly gromacs 3.2.1 has a workaround. The bug was in
> an obscure instruction relating to using unaligned memory. It is very
> unlikely that this affects other code, as compilers tend to avoid such
> instructions for performance reasons.
>
> I think that Opterons are a good choice (I have some myself and posted
> benchmarks last fall).
>
>> Thanx in advance for your replies :-)
>>
>> Best regards,
>> Thomas
>>
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> --  
> David.
> _______________________________________________________________________ 
> _
> David van der Spoel, PhD, Assist. Prof., Molecular Biophysics group,
> Dept. of Cell and Molecular Biology, Uppsala University.
> Husargatan 3, Box 596,  	75124 Uppsala, Sweden
> phone:	46 18 471 4205		fax: 46 18 511 755
> spoel at xray.bmc.uu.se	spoel at gromacs.org   http://xray.bmc.uu.se/~spoel
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