On Saturday, 12 August 2017 at 23:30, Richard W.M. Jones wrote:
> On Sat, Aug 12, 2017 at 12:40:18PM +0200, Kevin Kofler wrote:
> > Richard W.M. Jones wrote:
> > > I have no particular affinity for Atlas.  But if we're going to
> > > replace it, is OpenBLAS a complete drop-in replacement for Atlas that
> > > requires no or at least very minimal changes?  In what ways is it better
> > > or worse?
> > 
> > Proper support for runtime CPU feature detection on x86 architectures 
> > (x86_64 and i686). ATLAS expects to be tuned to every single machine, and 
> > distro packages can only be built to the lowest common denominator. 
> > (Anything else can only be in atlas-* subpackages that have to be manually 
> > installed.) OpenBLAS can also do that, but it also has a mode (used in 
> > packaging) where it will check for the available vector instruction sets 
> > (MMX, SSE*, AVX*) and pick the highest one available on the machine that is 
> > implemented for the called function. E.g., it can use SSE3 and newer on 
> > x86_64 when available, without breaking the SSE2-only x86_64 baseline. 
> > (Please note that this is only supported on x86 at this time. For ARM, it 
> > is 
> > like ATLAS, you can only compile for the baseline.) This can make a big 
> > difference in distro packages.
> > 
> > There might also be other performance benefits. OpenBLAS is derived from 
> > GotoBLAS (which was put under a BSD license when Prof. Goto left TACC in 
> > 2010, so that the community can continue development, which is exactly what 
> > OpenBLAS is doing). GotoBLAS has, since its proprietary times, had a 
> > reputation of being a really fast implementation.
> Sounds all good.  Are source-level changes needed to dependent
> packages and if so are they simple to make?

Yes, you don't need to add -L%{_libdir}/atlas to LDFLAGS anymore and you
link with -lopenblas.

See scalapack, arpack or elpa packages for example. In fact, I had to
switch elpa to openblas to match scalapack. Otherwise I got test

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