R_DATATABLE_NUM_THREADS env var alone ignored

[jangorecki]

Shouldn't we default num_proc_percent to 100% when num_threads is provided?

R_DATATABLE_NUM_THREADS=2 Rscript -e 'data.table::getDTthreads(T)'
#  omp_get_num_procs()            2
#  R_DATATABLE_NUM_PROCS_PERCENT  unset (default 50)
#  R_DATATABLE_NUM_THREADS        2
#  omp_get_thread_limit()         2147483647
#  omp_get_max_threads()          2
#  OMP_THREAD_LIMIT               unset
#  OMP_NUM_THREADS                unset
#  RestoreAfterFork               true
#  data.table is using 1 threads. See ?setDTthreads.
#[1] 1

data.table/src/openmp-utils.c

Line 44 in d3bd408

ans = imax(ans*perc/100, 1);

[MichaelChirico]

Hmm I think it's slightly different. Actually what's happening is R_DATATABLE_NUM_THREADS is too high so it's ignored:

ans = imin(ans, getIntEnv("R_DATATABLE_NUM_THREADS", INT_MAX));

the min here is imax(ans*perc/100, 1), i.e. 1.

For me I have omp_get_num_procs()=8 so your code runs as expected:

R_DATATABLE_NUM_THREADS=2 Rscript -e 'data.table::getDTthreads(T)'
#   omp_get_num_procs()            8
#   R_DATATABLE_NUM_PROCS_PERCENT  unset (default 50)
#   R_DATATABLE_NUM_THREADS        2
#   omp_get_thread_limit()         2147483647
#   omp_get_max_threads()          8
#   OMP_THREAD_LIMIT               unset
#   OMP_NUM_THREADS                unset
#   RestoreAfterFork               true
#   data.table is using 2 threads. See ?setDTthreads.
# [1] 2

[MichaelChirico]

I guess your point is if NUM_THREADS is set and NUM_PROCS_PERCENT is not, NUM_THREADS the default of NUM_PROCS_PERCENT becomes 100...

[jangorecki]

try using 5

[jangorecki]

maybe that is actually good, so it is more flexible to cap threads to min of two provided. Then we just need to mention that in manual.

[mattdowle]

On the principle of the first-thought-best, let's go with Jan's first thought and change it. When you use setDTthreads() at the R prompt it works at you expect, so fixing the environment variable is more like making its behaviour consistent with setDTthreads.