[Beowulf] Strange Opteron 2350 performance: Gaussian-03

[Joe Landman]

Mikhail Kuzminsky wrote:

> And the question is - may be that OLD binary (w/o recompilation) will 
> run so slow on Opteron 2350 core ?

Greetings Mikhail

   This is possible, depending upon the compiler used.  Though I have to 
admit that I find it odd that it would be the case within the Opteron 
family and not between Opteron and Xeon.

   Intel compilers used to (haven't checked 10.1) switch between fast 
(SSE*) and slow (x87 FP) paths as a function of a processor version 
string.  If this is an old Intel compiler built code, this is possible 
that the code paths may be different, though as noted, I would find that 
surprising if this were the case within the Opteron family.

   Modern PGI compilers (suggested default for Gaussian-03 last I 
checked) have the ability to do this as well, though I don't know how 
they implement it (capability testing hopefully?)

   Out of curiousity, how does streams run on both systems?  Also, it is 
possible, with a larger cache, that you might be running into some odd 
cache effects (tlb/page thrashing).  But DFTs are usually "small" and 
thus "sensitive" to cache size.

   You might be able to instrument the run within a papi wrapper, and 
see if you observe a large number of cache/tlb flushes for some reason.

   On a related note:  are you using a stepping before B3 of 2350?  That 
could impact performance, if you have the patch in place or have the 
tlb/cache turned off in bios (some MB makers created a patch to do this).

Joe


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