[HEADSUP] Atlas changed libraries

Przemek Klosowski przemek.klosowski at nist.gov
Mon Sep 23 20:36:28 UTC 2013


On 09/23/2013 08:46 AM, Susi Lehtola wrote:
> On Mon, 23 Sep 2013 14:34:50 +0200
> Kevin Kofler <kevin.kofler at chello.at> wrote:
>> Matthew Miller wrote:
>>> Actually, what ATLAS upstream intends is for the program to be
>>> recompiled on every installation (or boot, even). I think we used
>>> to have packages that did that; this is a compromise.
>> Yes, ATLAS upstream has always been smoking deep crack. It looks like
>> OpenBLAS is the much better option, now that it is available. (ATLAS
>> used to be the best available as Free Software back when GotoBLAS was
>> still proprietary, but now that the Goto code has been freed, ATLAS
>> is not looking so great anymore.)
> Well, it's not too hard to understand why ATLAS does things the way it
> does. It's already in the acronym: Automatically Tuned Linear Algebra
> Software. You generate a library that is optimal for your processor. In
> comparison, GotoBLAS (OpenBLAS) has been hand-tuned in assembly for
> every supported CPU.
>
> On the other hand, I'm not sure why they don't just take their tool and
> pregenerate lists of optimal parameters for every available CPU. That
> way you could compile everything in the same package and do runtime CPU
> detection. Currently binary distributions have to do some hackaround to
> generate a reasonably efficient one-size-fits-all library.
They can't cover every combination of CPU microarchitecture/cache sizes 
and main memory configuration and speed. It can't even be a 
per-motherboard configuration, because one could put in different speed 
DIMMs. and in principle that could change the optimal ATLAS setup. 
Recompiling it at every boot is a little extreme, though, albeit in 
principle someone could change the memory between boots.


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