Setting MKL thread number

[hakkelt]

I'm not sure if I understood the docs correctly, but I assumed that BLAS.set_num_threads(n) == mkl_set_num_threads(n) and BLAS.get_num_threads() == mkl_get_max_threads() . It seems, however, that it might not be the case. At least, if I'm calling mkl_set_num_threads first, BLAS.get_num_threads shows the correct value, but calling BLAS.set_num_threads does not affect the result of mkl_get_max_threads. Moreover, if I'm calling BLAS.set_num_threads first, then subsequent calls to mkl_set_num_threads would not change the value returned by BLAS.get_num_threads... I also tried MKL_DYNAMIC=TRUE, but there was no difference. Can someone clarify what's going on?

$ MKL_DYNAMIC=FALSE julia --project

julia> using MKL, MKL.MKL_jll

julia> using LinearAlgebra

julia> get_max_threads() = ccall((:mkl_get_max_threads, libmkl_rt), Int32, ());

julia> set_max_threads(n) = ccall((:mkl_set_num_threads, libmkl_rt), Cvoid, (Ptr{Int32},), Ref(Int32(n)));

julia> mkl_get_dynamic() = ccall((:mkl_get_dynamic, libmkl_rt), Int32, ());

julia> mkl_get_dynamic()
0

julia> BLAS.get_num_threads()
96

julia> get_max_threads()
96

julia> set_max_threads(55)

julia> get_max_threads()
55

julia> BLAS.get_num_threads()
55

julia> BLAS.set_num_threads(60)

julia> BLAS.get_num_threads()
60

julia> get_max_threads()
55

$ MKL_DYNAMIC=FALSE julia --project

julia> using MKL, MKL.MKL_jll

julia> using LinearAlgebra

julia> get_max_threads() = ccall((:mkl_get_max_threads, libmkl_rt), Int32, ());

julia> set_max_threads(n) = ccall((:mkl_set_num_threads, libmkl_rt), Cvoid, (Ptr{Int32},), Ref(Int32(n)));

julia> mkl_get_dynamic() = ccall((:mkl_get_dynamic, libmkl_rt), Int32, ());

julia> mkl_get_dynamic()
0

julia> BLAS.get_num_threads()
96

julia> BLAS.set_num_threads(55)

julia> BLAS.get_num_threads()
55

julia> get_max_threads()
96

julia> set_max_threads(60)

julia> get_max_threads()
60

julia> BLAS.get_num_threads()
55

[mkrainiuk]

Hi @hakkelt, in terms of oneMKL logic BLAS.set_num_threads(n) == mkl_set_num_threads(n) and BLAS.get_num_threads() == mkl_get_max_threads() are not always correct.
As far as I know BLAS.set_num_threads uses mkl_domain_set_num_threads and BLAS.get_num_threads() uses mkl_domain_get_max_threads. In case of domain specific function mkl_domain_set_num_threads was not used your assumption is correct: BLAS should use number of thread defined by mkl_set_num_thread, but if mkl_domain_set_num_threads was defined BLAS should use domain specific number of threads instead of the number specified by more general mkl_set_num_threads function. Hope it clarifies the oneMKL behavior.