74 lines
3.9 KiB
Plaintext
74 lines
3.9 KiB
Plaintext
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Notes on the Fedora Extras version of ATLAS
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by Quentin Spencer
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updated: September 12, 2005
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Because ATLAS relies on compile-time optimizations to obtain improved
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performance over BLAS and LAPACK, the resulting binaries are closely
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tied to the hardware on which they are compiled, and can likely result
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in very poor performance on other hardware. For this reason,
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including a package like ATLAS in Fedora requires some compromises.
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Firstly, a binary ATLAS package must perform reasonably well on the
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entire range of hardware on which it could potentially be installed.
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Optimizing ATLAS for the most modern hardware can result in
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significant performance penalties for users using the same package on
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older hardware. Second, building from the same source package must
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result in identical binaries for any computer of a particular
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architecture. This is because the Fedora Extras build system has
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multiple computers for building i386 packages, and it is not possible
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to know the hardware specifications of the computer that will be
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handling a given build request when it is submitted.
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The Debian project was probably the first to distribute precompiled
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ATLAS libraries. Camm Maguire, the creator of the Debian ATLAS
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package, has found a way of dealing with both of these challenges.
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The system essentially consists of "recorded builds" where ATLAS is
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compiled on particular hardware such that the resulting binary has
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reasonable performance on a range of hardware. The build parameters
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from the resulting optimization are stored in the source package and
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"played back" when a binary package is built. This has the additional
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advantage that builds using this method can be completed in minutes
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rather than the hours required to complete the standard compile-time
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empirical tests.
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A significant amount of research has been done to find the best
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compile parameters for the Debian packages, so the most sensible
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approach to making a package for Fedora is to make use of that
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research by applying the Debian patches directly to the source and to
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emulate the same build process. The spec file for this package does
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exactly that. The result is a set of libraries that will not
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necessarily achieve optimal performance on any given hardware but
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should still offer significant performance gains over the reference
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BLAS and LAPACK libraries on most hardware. The binary package
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includes the atlas libraries as well as binary-compatible blas and
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lapack libraries that should work as a drop-in replacement for the
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standard ones (they are installed in /usr/lib/atlas rather than
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/usr/lib).
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This package is designed to build RPMs that are identical regardless
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of where they are compiled and that provide reasonable performance on
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a wide range of hardware. For users who want optimal performance on
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particular hardware, custom RPMs can be built from the source package
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by setting the RPM macro "enable_custom_atlas" to a value of 1. This
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can be done from the command line as in the following example:
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rpmbuild -D "enable_custom_atlas 1" --rebuild atlas-3.6.0-1.src.rpm
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This command will create an atlas package called atlas-custom, which
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installs the optimized atlas libraries in /usr/lib/custom and the
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blas-compatible libraries in /usr/lib/atlas/custom. It should also be
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noted here that because custom compilation enables all compile-time
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empirical tests instead of simply replaying a previous build, the
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compilation time will be considerably longer, and can require several
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hours to complete, depending on the hardware.
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The 3.6.0 release of ATLAS will not compile shared libraries for the
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i386 architecture extensions (sse, sse2, 3dnow) when using the gcc
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4.0.x compiler included in Fedora Core 4. For this reason, the
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package currently provides only base i386 shared libraries, but
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provides static libraries for the extensions. It appears that the
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problem will not be resolved until the 3.8.0 release of ATLAS, and
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there is currently no indication of how soon that can be expected.
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