Cannot spawn task on interactive thread after starting Julia 1.9.0-beta4 with `-t1,1`

[simsurace]

The CLI help for -t states:

-t, --threads {auto|N[,auto|M]}
       Enable N[+M] threads; N threads are assigned to the `default`
       threadpool, and if M is specified, M threads are assigned to the
       `interactive` threadpool...

So I would expect to be able to spawn a task on the interactive thread when starting Julia with -t1,1 (one default and one interactive thread). But this fails on Julia 1.9.0-beta4:

$ julia +beta -q -t1,1

julia> fetch(Threads.@spawn :interactive Threads.threadpool())
:default

It works when starting Julia with -t2,1 (two default and one interactive thread):

$ julia +beta -q -t2,1

julia> fetch(Threads.@spawn :interactive Threads.threadpool())
:interactive

I think this is a bug.

julia> versioninfo()
Julia Version 1.9.0-beta4
Commit b75ddb787ff (2023-02-07 21:53 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × Intel(R) Core(TM) i7-9700K CPU @ 3.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-14.0.6 (ORCJIT, skylake)
  Threads: 2 on 8 virtual cores

[simsurace]

I did some digging and I believe there is a bug in

julia/src/partr.c

Lines 71 to 77 in 45b7e7a

	JL_DLLEXPORT int jl_set_task_threadpoolid(jl_task_t *task, int8_t tpid) JL_NOTSAFEPOINT
	{
	if (tpid < 0 || tpid >= jl_n_threadpools)
	return 0;
	task->threadpoolid = tpid;
	return 1;
	}

Setting the threadpool does not work, but setting the thread id works:

$ julia +beta -q -t1,1

julia> test(set_threadpoolid!)
:default

julia> test(set_threadid!)
:interactive

where

set_threadpoolid!(task) = ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), task, Int8(1))
set_threadid!(task) = ccall(:jl_set_task_tid, Cint, (Any, Int16), task, Int16(1))

function test(f)
    task = @task Threads.threadpool()
    task.sticky = false
    f(task)
    schedule(task)
    return fetch(task)
end

This is despite the fact that

julia> ccall(:jl_threadpoolid, Int8, (Int16,), 0)
0

julia> ccall(:jl_threadpoolid, Int8, (Int16,), 1)
1

[simsurace]

@jpsamaroo You might be able to help.

[jpsamaroo]

@kpamnany

[kpamnany]

This is a bug introduced by #46609, here. Previously, nthreads() returned the total number of threads in all the threadpools, so the if check would fail. Now, threadpoolsize() returns the number of threads in the default thread pool only, which is 1 for the breaking case.

Looks like nthreads() was deprecated (and changed to mean nthreads(:default) which seems like a bad change) which is presumably why it was replaced with threadpoolsize(), but it probably should have been replaced with maxthreadid()?

Cc: @vtjnash and @vchuravy

[simsurace]

See related discussions #48589 and #48643

[simsurace]

I do not know how best to fix this, but I would think it is important to

• Put this on the 1.9 milestone
• Add a test workflow to the CI to detect this type of bug

I could contribute the second item, but I'm unfamiliar with this project's CI pipelines. Where are they defined? Where would such a workflow go (e.g. starting Julia with different thread configurations and checking for correct behavior)?

[kpamnany]

Thanks for the good report @simsurace.

[simsurace]

My pleasure