Extra thread when using IJulia

[carstenbauer]

In the REPL (with JULIA_NUM_THREADS=6)

Threads.nthreads(:default) # gives 6
count(i -> threadpool(i) == :default, 1:Threads.maxthreadid()) # gives 6

as expected. However, in Jupyter (IJulia) I get

Threads.nthreads(:default) # gives 6
count(i -> threadpool(i) == :default, 1:Threads.maxthreadid()) # gives 7 ?!?!

I assume that the extra thread is the heartbeat thread. But in any case, this seems inconsistent. Upstream issue?

[stevengj]

Yup.

I'm not sure if there is a way to prevent Julia from claiming ownership of this thread? See also JuliaLang/julia#47196.

Or maybe Threads.nthreads should not count foreign threads.