[R-sig-Debian] Compiling R-2.11.0 with ATLAS-tuned BLAS and LAPACK

[Dirk Eddelbuettel]

On 2 June 2010 at 17:27, Paul Johnson wrote:
| Question: How does the R executable know which BLAS shared library to use.
| 
| Answer: It uses whatever so name it was told to use at build time, and
| the dynamic linking mechanism of the OS helps it find the file.

The first matching entry wins, similar to how PATH is search, or how R uses
.libPaths()
 
| If you build R with a configure option that tells it to use an
| external BLAS like atlas and you do not have --enable-BLAS-shlib
| specified, then no libRblas.so is created, the R executable will look
| for the specific location of the BLAS library that it found at compile
| time.
| 
| Here's what I see with the deb packages from CRAN
| 
| $ ldd /usr/lib/R/bin/exec/R
|         linux-vdso.so.1 =>  (0x00007fff9c9ff000)
|         libR.so => /usr/lib/libR.so (0x00007fcfe9550000)
|         libc.so.6 => /lib/libc.so.6 (0x00007fcfe91ce000)
|         libblas.so.3gf => /usr/lib/libblas.so.3gf (0x00007fcfe8f32000)
| 
| In Ubuntu 10.04 (lucid), the installation of R from CRAN (r-base,
| r-base-core, etc) causes the installation of the addon packages
| "libblas-dev" and "libblas3gf" and you see above that R is linked
| against it.

Sort of. libblase-dev comes from r-base-dev and is meant for compiling and
linking (eg when installing CRAN packages from source) but not needed just
to run R.
 
| In theory, according to README.Atlas.gz, it should be possible to have
| several BLAS library collections installed at once.

Yes. Just how you can have 'foo' in /bin, /usr/bin, /usr/local/bin, ~/bin,
/opt/foo/bin, ... etc pp

|  From Atlas, we
| could have "base","sse", and "sse2".  The docs say "sse2" is best.

sse, sse2, ... are old names that atlas package of yore used.

These days, on i386/amd64 we onl have atlas-base.

[...]

| Clearly, there is some dynamic linking "magic" going on so that the
| system knows which libatlas3gf.so.to use when R asks for it.  I have
| not seen this before, were 2 identically named so files exist. But
| check the output of
| 
| $ /sbin/ldconfig -p
| 
| 
|   libblas.so.3gf (libc6,x86-64) => /usr/lib/atlas/libblas.so.3gf
|   libblas.so.3gf (libc6,x86-64) => /usr/lib/libblas.so.3gf
| 
| Hm. 2 libraries with the exact same name, the one in the atlas
| directory is found first, so R uses it.  If I remove libatlas3gf-base,
| then, of course, the only one that is found is from libblas3gf.

Same as with PATH. No magic.

| Question: How can one experiment with other versions of BLAS?
| 
| Answer: Either replace the file /usr/lib/libblas.so.3gf with some
| other shared object file, or rebuild R using --enable-BLAS-shlib and
| replace that.

Use LD_CONFIG_PATH or files *.conf in /etc/ld.so.conf.d/.

Besides Goto BLAS, you may also enjoy the (multithreaded-by-default) Intel
MKL Blas you can get via Ubuntu's 'revolution-mkl' package for amd64 and i386.

Hth.

-- 
  Regards, Dirk