make thread 1 interactive when there is an interactive pool, so it can run the event loop

base/task.jl

@@ -253,7 +253,7 @@ istaskfailed(t::Task) = (load_state_acquire(t) === task_state_failed)
 Threads.threadid(t::Task) = Int(ccall(:jl_get_task_tid, Int16, (Any,), t)+1)
 function Threads.threadpool(t::Task)
     tpid = ccall(:jl_get_task_threadpoolid, Int8, (Any,), t)
-    return tpid == 0 ? :default : :interactive
+    return Threads._tpid_to_sym(tpid)
 end
 
 task_result(t::Task) = t.result
@@ -786,7 +786,7 @@ function enq_work(t::Task)
         if Threads.threadpoolsize(tp) == 1
             # There's only one thread in the task's assigned thread pool;
             # use its work queue.
-            tid = (tp === :default) ? 1 : Threads.threadpoolsize(:default)+1
+            tid = (tp === :interactive) ? 1 : Threads.threadpoolsize(:interactive)+1
             ccall(:jl_set_task_tid, Cint, (Any, Cint), t, tid-1)
             push!(workqueue_for(tid), t)
         else

base/threadingconstructs.jl

@@ -39,14 +39,22 @@ function _nthreads_in_pool(tpid::Int8)
     return Int(unsafe_load(p, tpid + 1))
 end
 
+function _tpid_to_sym(tpid::Int8)
+    return tpid == 0 ? :interactive : :default
+end
+
+function _sym_to_tpid(tp::Symbol)
+    return tp === :interactive ? Int8(0) : Int8(1)
+end
+
 """
     Threads.threadpool(tid = threadid()) -> Symbol
 
 Returns the specified thread's threadpool; either `:default` or `:interactive`.
 """
 function threadpool(tid = threadid())
     tpid = ccall(:jl_threadpoolid, Int8, (Int16,), tid-1)
-    return tpid == 0 ? :default : :interactive
+    return _tpid_to_sym(tpid)
 end
 
 """
@@ -67,24 +75,39 @@ See also: `BLAS.get_num_threads` and `BLAS.set_num_threads` in the
 [`Distributed`](@ref man-distributed) standard library.
 """
 function threadpoolsize(pool::Symbol = :default)
-    if pool === :default
-        tpid = Int8(0)
-    elseif pool === :interactive
-        tpid = Int8(1)
+    if pool === :default || pool === :interactive
+        tpid = _sym_to_tpid(pool)
     else
         error("invalid threadpool specified")
     end
     return _nthreads_in_pool(tpid)
 end
 
+"""
+    threadpooltids(pool::Symbol)
+
+Returns a vector of IDs of threads in the given pool.
+"""
+function threadpooltids(pool::Symbol)
+    ni = _nthreads_in_pool(Int8(0))
+    if pool === :interactive
+        return collect(1:ni)
+    elseif pool === :default
+        return collect(ni+1:ni+_nthreads_in_pool(Int8(1)))
+    else
+        error("invalid threadpool specified")
+    end
+end
+
 function threading_run(fun, static)
     ccall(:jl_enter_threaded_region, Cvoid, ())
     n = threadpoolsize()
+    tid_offset = threadpoolsize(:interactive)
     tasks = Vector{Task}(undef, n)
     for i = 1:n
         t = Task(() -> fun(i)) # pass in tid
         t.sticky = static
-        static && ccall(:jl_set_task_tid, Cint, (Any, Cint), t, i-1)
+        static && ccall(:jl_set_task_tid, Cint, (Any, Cint), t, tid_offset + i-1)
         tasks[i] = t
         schedule(t)
     end
@@ -287,6 +310,15 @@ macro threads(args...)
     return _threadsfor(ex.args[1], ex.args[2], sched)
 end
 
+function _spawn_set_thrpool(t::Task, tp::Symbol)
+    tpid = _sym_to_tpid(tp)
+    if _nthreads_in_pool(tpid) == 0
+        tpid = _sym_to_tpid(:default)
+    end
+    ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), t, tpid)
+    nothing
+end
+
 """
     Threads.@spawn [:default|:interactive] expr
 
@@ -315,7 +347,7 @@ the variable's value in the current task.
     A threadpool may be specified as of Julia 1.9.
 """
 macro spawn(args...)
-    tpid = Int8(0)
+    tp = :default
     na = length(args)
     if na == 2
         ttype, ex = args
@@ -325,9 +357,9 @@ macro spawn(args...)
             # TODO: allow unquoted symbols
             ttype = nothing
         end
-        if ttype === :interactive
-            tpid = Int8(1)
-        elseif ttype !== :default
+        if ttype === :interactive || ttype === :default
+            tp = ttype
+        else
             throw(ArgumentError("unsupported threadpool in @spawn: $ttype"))
         end
     elseif na == 1
@@ -344,11 +376,7 @@ macro spawn(args...)
         let $(letargs...)
             local task = Task($thunk)
             task.sticky = false
-            local tpid_actual = $tpid
-            if _nthreads_in_pool(tpid_actual) == 0
-                tpid_actual = Int8(0)
-            end
-            ccall(:jl_set_task_threadpoolid, Cint, (Any, Int8), task, tpid_actual)
+            _spawn_set_thrpool(task, $(QuoteNode(tp)))
             if $(Expr(:islocal, var))
                 put!($var, task)
             end

src/threading.c

@@ -600,17 +600,16 @@ void jl_init_threading(void)
     // specified on the command line (and so are in `jl_options`) or by the
     // environment variable. Set the globals `jl_n_threadpools`, `jl_n_threads`
     // and `jl_n_threads_per_pool`.
-    jl_n_threadpools = 1;
+    jl_n_threadpools = 2;
     int16_t nthreads = JULIA_NUM_THREADS;
     int16_t nthreadsi = 0;
     char *endptr, *endptri;
 
     if (jl_options.nthreads != 0) { // --threads specified
-        jl_n_threadpools = jl_options.nthreadpools;
         nthreads = jl_options.nthreads_per_pool[0];
         if (nthreads < 0)
             nthreads = jl_effective_threads();
-        if (jl_n_threadpools == 2)
+        if (jl_options.nthreadpools == 2)
             nthreadsi = jl_options.nthreads_per_pool[1];
     }
     else if ((cp = getenv(NUM_THREADS_NAME))) { // ENV[NUM_THREADS_NAME] specified
@@ -635,15 +634,13 @@ void jl_init_threading(void)
                 if (errno != 0 || endptri == cp || nthreadsi < 0)
                     nthreadsi = 0;
             }
-            if (nthreadsi > 0)
-                jl_n_threadpools++;
         }
     }
 
     jl_all_tls_states_size = nthreads + nthreadsi;
     jl_n_threads_per_pool = (int*)malloc_s(2 * sizeof(int));
-    jl_n_threads_per_pool[0] = nthreads;
-    jl_n_threads_per_pool[1] = nthreadsi;
+    jl_n_threads_per_pool[0] = nthreadsi;
+    jl_n_threads_per_pool[1] = nthreads;
 
     jl_atomic_store_release(&jl_all_tls_states, (jl_ptls_t*)calloc(jl_all_tls_states_size, sizeof(jl_ptls_t)));
     jl_atomic_store_release(&jl_n_threads, jl_all_tls_states_size);

test/threadpool_use.jl

@@ -4,8 +4,10 @@ using Test
 using Base.Threads
 
 @test nthreadpools() == 2
-@test threadpool() === :default
-@test threadpool(2) === :interactive
+@test threadpool() === :interactive
+@test threadpool(2) === :default
 @test fetch(Threads.@spawn Threads.threadpool()) === :default
 @test fetch(Threads.@spawn :default Threads.threadpool()) === :default
 @test fetch(Threads.@spawn :interactive Threads.threadpool()) === :interactive
+@test Threads.threadpooltids(:interactive) == [1]
+@test Threads.threadpooltids(:default) == [2]