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

[Dirk Eddelbuettel]

On 11 August 2014 at 19:23, Yixuan Qiu wrote:
| Good point by Dirk.
| 
| And why not just adding a line "export OPENBLAS_NUM_THREADS=1" to your .bashrc
| file, or compiling OpenBlas again by setting NUM_THREADS = 1 in the
| Makefile.rule?

Unless you insist on it you should not need to recompile OpenBLAS.  

You should have a choice of distributions or system which does this right.

Dirk
 
| 
| Best,
| Yixuan
| 
| 
| 2014-08-11 18:55 GMT-04:00 Dirk Eddelbuettel <edd at debian.org>:
| 
| 
|     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
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| 
| 
| 
| --
| Yixuan Qiu <yixuan.qiu at cos.name>
| Department of Statistics,
| Purdue University

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org