Linear Algebra Libraries

ATLAS, BLIS and OpenBLAS provide BLAS and a subset of LAPACK. Both BLIS and OpenBLAS provide several flavors: a sequential version, a threaded one, and another with OpenMP support (all of them with or without support for 64-bit integers).

Due to implementation differences, it is important that all components of a particular software stack link to the same BLAS/LAPACK implementation. Also, users may want to choose a particular implementation that works best for them at run time. This guideline gives a structure that can enforce the first while allowing the second, as well as providing a transparent fallback mechanism to Netlib’s reference implementation for those symbols not included in the selected backend via FlexiBLAS.

FlexiBLAS is a framework that wraps both BLAS and LAPACK APIs in a single library. BLAS/LAPACK consumers must link against FlexiBLAS, and this wrapper is able to redirect calls to a selected optimized backend with negligible overhead. It also provides transparent fallback to Netlib’s reference implementation if a certain symbol is not present in the selected backend. These are the main features:

First, only FlexiBLAS’s development package MUST be listed in BuildRequires:

which brings all the necessary development files, both for the 32-bit (the most common) and the 64-bit integer interface.

The packager MUST specify flexiblas or flexiblas64 (for the 32-bit or 64-bit interface respectively) as both the BLAS and LAPACK library names where applicable, and packages using pkg-config will automatically obtain the proper flags for the headers and libraries. Similarly, CMake-based projects using FindBLAS will automatically detect and configure the proper flags for FlexiBLAS (since CMake v3.19), and no further action will be required from the packager.

Unfortunately, many upstream projects present heterogeneous ways of accessing these APIs. In a best-case scenario, the building framework may define specific options to explicitly set the BLAS and/or LAPACK libraries. More commonly, the packager MUST ensure that %{_includedir}/flexiblas and %{_libdir}/flexiblas (or %{_includedir}/flexiblas64 and %{_libdir}/flexiblas64) are injected as header and library locations in the proper flags and configuration files, and/or -lflexiblas (or -lflexiblas64) is provided to the linker. In rare occasions, hardcoded paths in source files MUST be modified, and patches MAY be required. The packager SHOULD work with upstream to standardize the way in which these libraries are detected and configured.

Optimized BLAS/LAPACK backends are much faster than Netlib’s reference implementation, but in return results may vary a little. Consequently, tests that are too tight (with too small tolerances) may fail. In these cases, the packager SHOULD enable the reference implementation in the %check section as follows:

or, alternatively, via FLEXIBLAS64 for builds using 64-bit integers.

A package compiled against FlexiBLAS pulls out the corresponding flexiblas-netlib(64) subpackage, which in turn requires the default optimized backend (i.e., flexiblas-openblas-openmp(64)). This is set via the "default=IMPLEMENTATION-NAME" key (by default, default=openblas-openmp), present in the main configuration file shipped in the main subpackages, %{_sysconfdir}/flexiblasrc and %{_sysconfdir}/flexiblas64rc.

To allow system-level selection of other BLAS/LAPACK implementations, more backends must be installed in the first place (e.g., flexiblas-atlas, flexiblas-blis-serial…​), and then they can be swapped system-wide via the flexiblas CLI tool, or just by modifying the "default" key in the configuration file by hand.

Persistent user-level selection of system-provided BLAS/LAPACK implementations can be done via the CLI tool:

provided the sub-package for IMPLEMENTATION-NAME is installed.

Non-persistent user-level selection can be triggered via an environment variable:

User-level selection of user-owned BLAS/LAPACK libraries can be achieved just by changing IMPLEMENTATION-NAME with a path to any custom BLAS/LAPACK-compatible library in the examples above.