Add threads in Julia after initializing

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I know that I can use multiple threads on Julia by initializing a REPL with

julia --threads 4

but is there a way to add threads from within Julia? I can change nthreads by doing this

julia> Threads.nthreads()
1

julia> Threads.nthreads() = 4

julia> Threads.nthreads()
4

but I don't notice any performance increase. I have a feeling that setting the number of threads this way does not work the way I am expecting it to.

julia multithreading multiprocessing
1 Answers

You cannot increase the thread pool once Julia is started.

Perhaps the best option is to use auto with the thread number parameter:

$ julia --threads auto


julia> Threads.nthreads()
8

Another important thing to know is that Julia can call C libraries that have their own independent thread pools. Linear algebra BLAS is the most notable example:

$ julia --threads 2

julia> using LinearAlgebra

julia> BLAS.get_num_threads()
8

julia> Threads.nthreads()
2
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