[R-pkg-devel] Setting OpenMP threads (globally) for an R package

[Viechtbauer, Wolfgang (SP)]

Hi Evan,

Check omp_set_num_threads() from the RhpcBLASctl package. I know from experience that it works for limiting the number of threads for BLAS inside a running R session with blas_set_num_threads(1) (instead of setting OPENBLAS_NUM_THREADS=1 before running R). I assume it should work the same for omp_set_num_threads().

Best,
Wolfgang

>-----Original Message-----
>From: R-package-devel [mailto:r-package-devel-bounces using r-project.org] On Behalf Of
>Evan Biederstedt
>Sent: Thursday, 17 March, 2022 14:52
>To: R Package Development
>Subject: [R-pkg-devel] Setting OpenMP threads (globally) for an R package
>
>Hi R-package-devel
>
>I'm developing an R package which uses `parallel::mclapply` and several
>other library dependencies which possibly rely upon OpenMP. Unfortunately,
>some functions explode the amount of memory used.
>
>I've noticed that if I set `export OMP_NUM_THREADS=1` before starting R,
>the memory is far more manageable.
>
>My question is, if there a way for me to achieve this behavior within the R
>package itself?
>
>My initial try was to use `R/zzz.R` and an `.onLoad()` function to load
>these global variables upon loading the library.
>
>```
>.onLoad <- function(libname, pkgname){
>  Sys.setenv(OMP_NUM_THREADS=1)
>}
>```
>
>But this doesn't work. The memory still explodes. In fact, I'm worried that
>this cannot be done within an R package itself, as R has already started,
>e.g.  https://stackoverflow.com/a/27320691/5269850
>
>Is there a recommended approach for this problem when writing R packages?
>
>Package here: https://github.com/kharchenkolab/numbat
>
>Related question on SO:
>https://stackoverflow.com/questions/71507979/set-openmp-threads-for-all-
>dependencies-in-r-package
>
>Any help appreciated. Thanks, Evan