In R how to control multi-threading in BLAS parallel matrix product

Question

I have a question regarding the use of BLAS parallelized matrix product in R (being the default matrix product at least since R-3.4, maybe earlier).

The default behavior (at least on my machine) is now for the matrix product (c.f. example below) to use all the cores available on the machine, which can be a problem.

Do you know how to control the number of cores used for standard matrix product in R?

Thanks in advance

Example:

n=10000
p=1000
q=5000
A = matrix(runif(n*p),nrow=n, ncol=p)
B = matrix(runif(p*q),nrow=p, ncol=q)
C = A %*% B # multi-threaded matrix product

Session info:

> sessionInfo()
R version 3.4.1 (2017-06-30)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.3 LTS

Matrix products: default
BLAS: /usr/lib/openblas-base/libblas.so.3
LAPACK: /usr/lib/libopenblasp-r0.2.18.so

locale:
 [1] LC_CTYPE=fr_FR.utf8       LC_NUMERIC=C             
 [3] LC_TIME=fr_FR.utf8        LC_COLLATE=fr_FR.utf8    
 [5] LC_MONETARY=fr_FR.utf8    LC_MESSAGES=fr_FR.utf8   
 [7] LC_PAPER=fr_FR.utf8       LC_NAME=C                
 [9] LC_ADDRESS=C              LC_TELEPHONE=C           
[11] LC_MEASUREMENT=fr_FR.utf8 LC_IDENTIFICATION=C      

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] compiler_3.4.1

Answer 1

The package RhpcBLASctl does just that.

From its DESCRIPTION:

Control the number of threads on 'BLAS' (Aka 'GotoBLAS', 'ACML' and 'MKL'). and possible to control the number of threads in 'OpenMP'. get a number of logical cores and physical cores if feasible.

We mention it in the CRAN Task View on HPC.