[Rcpp-devel] RcppArmadillo BLAS/LAPACK force number of threads to 1

[Dirk Eddelbuettel]

Hi Scott,


On 12 August 2014 at 08:43, Scott Ritchie wrote:
| Hi Everyone,
| 
| I'm having trouble with restricting the Armadillo function "svd_econ" to run on
| a single thread.
| 
| I am able to restrict it if I `export OPENBLAS_NUM_THREADS=1` before opening an
| R session, but not from within R (`Sys.setenv(OPENBLAS_NUM_THREADS=1)` has no
| effect).
| 
| I'm wondering if theirs an option native to armadillo/RcppArmadillo that I'm
| missing to enable me to:
| 
|   • Restrict the number of threads used to 1
|   • In a library agnostic manner (i.e. if the user has a different LAPACK or
|     BLAS library installed).
| 
| or if I can "trick" Rcpp into thinking the machine only has 1 core?

I fear you may be misstating / misdiagnosing the problem.

I am fairly certain that there is not a single line in Rcpp or
(Rcpp)Armadillo which sets thread counts for your LAPACK / BLAS.

Rather, and this is a point that I tried to make in the 'Benchmarking GPUs
and CPUs on Debian-based systems' package (gcbd on CRAN etc) and its vignette
is that __BLAS and LAPACK sit behind a standardized interface__ and can be
installed / swapped via plug and play. (Which in turn enables the
benchmarking ...)

If you're on a Debian-based system, just remove openblas and use atlas. Case
closed.  Atlas will not multithread (unless something changed recently).

There is also a helper function or package somewhere, and a post by Claudia
Beleites on StackOverflow, which show the explicit function call to reset
OpenBLAS in its count.  But if everything else fails -- just use a different
BLAS.  I sometimes do that too when I use multicore / parallel to launch
multiple R jobs and each one of those should not spawn additional linear
algebra threads.

Hope this helps,  Dirk


 
| Regards,
| 
| Scott Ritchie
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http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org