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Make OpenBLAS's usage of OpenMP respect openblas_set_num_threads() #5808

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Make OpenBLAS's usage of OpenMP respect openblas_set_num_threads() #5808

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b691cb1
Document openblas_set_num_threads_local(). See #4425
nh2 May 12, 2026
7e4e244
Make OpenBLAS's usage of OpenMP respect openblas_set_num_threads(). F…
nh2 May 12, 2026
d18385b
Refactor: Simplify num_cpu_avail() syntax
nh2 May 12, 2026
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2 changes: 2 additions & 0 deletions cblas.h
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,8 @@ extern "C" {
/*Set the number of threads on runtime.*/
void openblas_set_num_threads(int num_threads);
void goto_set_num_threads(int num_threads);
// "Local" means this number is used when OpenBLAS notices that
// it is already in an OpenMP parallel region (`omp_in_parallel()`).
int openblas_set_num_threads_local(int num_threads);

/*Get the number of threads on runtime.*/
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33 changes: 16 additions & 17 deletions common_thread.h
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Expand Up @@ -138,31 +138,30 @@ typedef struct blas_queue {
extern int blas_server_avail;
extern int blas_omp_number_max;
extern int blas_omp_threads_local;
extern int blas_is_num_threads_set_explicitly;

static __inline int num_cpu_avail(int level) {

#ifdef USE_OPENMP
int openmp_nthreads;
openmp_nthreads=omp_get_max_threads();
if (omp_in_parallel()) openmp_nthreads = blas_omp_threads_local;
#endif
/* If the user explicitly called openblas_set_num_threads(),
respect that setting instead of overriding it with
`omp_get_max_threads()` below (which is to get a default
in case the user hasn't made an explicit choice). */
if (blas_is_num_threads_set_explicitly) {
return blas_cpu_number;
}

#ifndef USE_OPENMP
if (blas_cpu_number == 1
#else
if (openmp_nthreads == 1
#endif
) return 1;
int openmp_nthreads = omp_in_parallel() ? blas_omp_threads_local : omp_get_max_threads();

#ifdef USE_OPENMP
if (openmp_nthreads > blas_omp_number_max){
if (openmp_nthreads > blas_omp_number_max) {
#ifdef DEBUG
fprintf(stderr,"WARNING - more OpenMP threads requested (%d) than available (%d)\n",openmp_nthreads,blas_omp_number_max);
fprintf(stderr, "WARNING - more OpenMP threads requested (%d) than available (%d)\n", openmp_nthreads, blas_omp_number_max);
#endif
openmp_nthreads = blas_omp_number_max;
}
if (blas_cpu_number != openmp_nthreads) {
goto_set_num_threads(openmp_nthreads);
openmp_nthreads = blas_omp_number_max;
}

if (blas_cpu_number != openmp_nthreads) {
goto_set_num_threads(openmp_nthreads); // mutates `blas_cpu_number`
}
#endif

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5 changes: 3 additions & 2 deletions driver/others/blas_server_omp.c
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Expand Up @@ -69,7 +69,8 @@

int blas_server_avail = 0;
int blas_omp_number_max = 0;
int blas_omp_threads_local = 1;
int blas_omp_threads_local = 1; // num threads to use when already inside omp_in_parallel()
int blas_is_num_threads_set_explicitly = 0; // tracks whether the user called openblas_set_num_threads()

extern int openblas_omp_adaptive_env(void);

Expand Down Expand Up @@ -122,7 +123,7 @@ void goto_set_num_threads(int num_threads) {

}
void openblas_set_num_threads(int num_threads) {

blas_is_num_threads_set_explicitly = 1;
goto_set_num_threads(num_threads);
}

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