Notes on the Fedora version of ATLAS

by Quentin Spencer
updated: October 4, 2005

updated by Deji Akingunola
October 15, 2008

updated by Deji Akingunola
June 15, 2011

Because ATLAS relies on compile-time optimizations to obtain improved
performance over BLAS and LAPACK, the resulting binaries are closely
tied to the hardware on which they are compiled, and can likely result
in very poor performance on other hardware.  For this reason,
including a package like ATLAS in Fedora requires some compromises.
Firstly, a binary ATLAS package must perform reasonably well on the
entire range of hardware on which it could potentially be installed.
Optimizing ATLAS for the most modern hardware can result in
significant performance penalties for users using the same package on
older hardware.  Second, building from the same source package must
result in identical binaries for any computer of a particular
architecture.  This is because the binaries installed on a user's
computer are built on a computer in the Fedora Extras build system,
which will have hardware different from the end user's hardware, and
quite possibly different from other available hardware in the build
system.

As of version 3.8.2 (in Fedora), ATLAS builds uses achitectural defaults,
are partial results of past searches when the compiler and architecture
are known, to discover the appropriate kernels used to build all the required
libraries. They make install time quicker and also ensure that good results are
obtained, since they typically represent several searches and/or user
intervention into the usual search so that maximum performance is found.
The result is a set of libraries that will not
necessarily achieve optimal performance on any given hardware but
should still offer significant performance gains over the reference
BLAS and LAPACK libraries on most hardware.  The binary package
includes the atlas libraries as well as binary-compatible blas and
lapack libraries that should work as a drop-in replacement for the
standard ones (they are installed in /usr/lib{64}/atlas* rather than
/usr/lib{64}).

For 32bit x86 systems, the default atlas package on was built using Pentium Pro
architectural defaults using just x87 math optimization. In addition to the
base 32bit build, 4 ATLAS subpackages are built for 3Dnow, SSE, SSE2, and SSE3
ix86 extensions, using architectural defaults obtained from Athlon K7, PIII,
Pentium 4 with SSE2 extension and PENTIUM 4 with SSE3 extensions respectively.

On 64bit x86 systems the default atlas package on was built with SSE2 optimization using architetural default made for AMD's HAMMER processor, and an additional
SSE3-enabled subpackage was built also using architetural default made for AMD's HAMMER processor.
  
This packaging allows multiple installation of different atlas sub-packages
at the same time. The alternatives system (read 'man alternatives' for usage)
is used in the -devel subpackages to select the appropriate location for the 
architectural dependent header files.
 
This package is designed to build RPMs that are identical regardless
of where they are compiled and that provide reasonable performance on
a wide range of hardware. For users who want optimal performance on
particular hardware, custom RPMs can be built from the source package
by setting the RPM macro "enable_native_atlas" to a value of 1. This
can be done from the command line as in the following example:

rpmbuild -D "enable_native_atlas 1" --rebuild atlas-3.8.3-1.src.rpm

This will cause the ATLAS build to use the achitectural default most
appropriate for the system on which the package is to be built.