Notes on the Fedora version of ATLAS

by Quentin Spencer

updated: October 4, 2005

updated by Deji Akingunola

October 15, 2008

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 i386 systems, 4 ATLAS subpackages are built for 3Dnow, SSE1, 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.

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.