Notes on the Fedora Extras version of ATLAS by Quentin Spencer updated: September 12, 2005 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 Fedora Extras build system has multiple computers for building i386 packages, and it is not possible to know the hardware specifications of the computer that will be handling a given build request when it is submitted. The Debian project was probably the first to distribute precompiled ATLAS libraries. Camm Maguire, the creator of the Debian ATLAS package, has found a way of dealing with both of these challenges. The system essentially consists of "recorded builds" where ATLAS is compiled on particular hardware such that the resulting binary has reasonable performance on a range of hardware. The build parameters from the resulting optimization are stored in the source package and "played back" when a binary package is built. This has the additional advantage that builds using this method can be completed in minutes rather than the hours required to complete the standard compile-time empirical tests. A significant amount of research has been done to find the best compile parameters for the Debian packages, so the most sensible approach to making a package for Fedora is to make use of that research by applying the Debian patches directly to the source and to emulate the same build process. The spec file for this package does exactly that. 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/atlas rather than /usr/lib). 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_custom_atlas" to a value of 1. This can be done from the command line as in the following example: rpmbuild -D "enable_custom_atlas 1" --rebuild atlas-3.6.0-1.src.rpm This command will create an atlas package called atlas-custom, which installs the optimized atlas libraries in /usr/lib/custom and the blas-compatible libraries in /usr/lib/atlas/custom. It should also be noted here that because custom compilation enables all compile-time empirical tests instead of simply replaying a previous build, the compilation time will be considerably longer, and can require several hours to complete, depending on the hardware. The 3.6.0 release of ATLAS will not compile shared libraries for the i386 architecture extensions (sse, sse2, 3dnow) when using the gcc 4.0.x compiler included in Fedora Core 4. For this reason, the package currently provides only base i386 shared libraries, but provides static libraries for the extensions. It appears that the problem will not be resolved until the 3.8.0 release of ATLAS, and there is currently no indication of how soon that can be expected.