[Rd] Control multi-threading in standard matrix product

[Ghislain Durif]

Dear R Core Team,

I wish to report what can be viewed as a bug or at least a strange 
behavior in R-3.4.1. I ask my question here (as recommended on 
https://www.r-project.org/bugs.html) since I am not member of the R's 
Bugzilla.

When running 'R --vanilla' from the command line, the standard matrix 
product is by default based on BLAS and multi-threaded on all cores 
available on the machine, c.f. following examples:

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


However, the default behavior to use all available cores can be an 
issue, especially on shared computing resources or when the matrix 
product is used in parallelized section of codes (for instance with 
'mclapply' from the 'parallel' package). For instance, the default 
matrix product is single-threaded in R-3.3.2 (I ran a test on my 
machine), this new features will deeply affect the behavior of existing 
R packages that use other multi-threading solutions.

Thanks to this stackoverflow question 
(https://stackoverflow.com/questions/45794290/in-r-how-to-control-multi-threading-in-blas-parallel-matrix-product), 
I now know that it is possible to control the number of BLAS threads 
thanks to the package 'RhpcBLASctl'. However, being able to control the 
number of threads should maybe not require to use an additional package.

In addition, the doc 'matmult' does not mention this point, it points to 
the 'options' doc page and especially the 'matprod' section, in which 
the multi-threading is not discussed.


Here is the results of the 'sessionInfo()' function on my machine for 
R-3.4.1:
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



and for R-3.3.2:
R version 3.3.2 (2016-10-31)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.3 LTS

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


Thanks in advance,
Best regards
||

-- 
Ghislain Durif
--------------------------
Research engineer THOTH TEAM
INRIA Grenoble Alpes (France)


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