Default to 1 interactive thread

NEWS.md

@@ -29,6 +29,13 @@ New language features
 Language changes
 ----------------
 
+ - Julia now defaults to 1 "interactive" thread, in addition to the 1 "default" worker thread. i.e. `-t1,1`
+  This means in default configuration the main task and repl (when in interactive mode), which both run on
+  thread 1, now run within the `interactive` threadpool. Also the libuv IO loop runs on thread 1,
+  helping efficient utilization of the "default" worker threadpool, which is what `Threads.@threads` and a bare
+  `Threads.@spawn` uses. Use `0` to disable the interactive thread i.e. `-t1,0` or `JULIA_NUM_THREADS=1,0`, or
+  `-tauto,0` etc. The zero is explicitly required to disable it, `-t2` will set the equivalent of `-t2,1` ([#57087])
+
  - When methods are replaced with exactly equivalent ones, the old method is no
    longer deleted implicitly simultaneously, although the new method does take
    priority and become more specific than the old method. Thus if the new

base/channels.jl

@@ -130,8 +130,7 @@ julia> chnl = Channel{Char}(1, spawn=true) do ch
            for c in "hello world"
                put!(ch, c)
            end
-       end
-Channel{Char}(1) (2 items available)
+       end;
 
 julia> String(collect(chnl))
 "hello world"

doc/src/manual/faq.md

@@ -941,7 +941,7 @@ While the streaming I/O API is synchronous, the underlying implementation is ful
 
 Consider the printed output from the following:
 
-```jldoctest
+```
 julia> @sync for i in 1:3
            Threads.@spawn write(stdout, string(i), " Foo ", " Bar ")
        end
@@ -954,7 +954,7 @@ yields to other tasks while waiting for that part of the I/O to complete.
 `print` and `println` "lock" the stream during a call. Consequently changing `write` to `println`
 in the above example results in:
 
-```jldoctest
+```
 julia> @sync for i in 1:3
            Threads.@spawn println(stdout, string(i), " Foo ", " Bar ")
        end
@@ -965,7 +965,7 @@ julia> @sync for i in 1:3
 
 You can lock your writes with a `ReentrantLock` like this:
 
-```jldoctest
+```
 julia> l = ReentrantLock();
 
 julia> @sync for i in 1:3

doc/src/manual/methods.md

@@ -615,7 +615,7 @@ Start some other operations that use `f(x)`:
 julia> g(x) = f(x)
 g (generic function with 1 method)
 
-julia> t = Threads.@spawn f(wait()); yield();
+julia> t = @async f(wait()); yield();
 ```
 
 Now we add some new methods to `f(x)`:
@@ -640,7 +640,7 @@ julia> g(1)
 julia> fetch(schedule(t, 1))
 "original definition"
 
-julia> t = Threads.@spawn f(wait()); yield();
+julia> t = @async f(wait()); yield();
 
 julia> fetch(schedule(t, 1))
 "definition for Int"

doc/src/manual/multi-threading.md

@@ -5,11 +5,13 @@ of Julia multi-threading features.
 
 ## Starting Julia with multiple threads
 
-By default, Julia starts up with a single thread of execution. This can be verified by using the
-command [`Threads.nthreads()`](@ref):
+By default, Julia starts up with 2 threads of execution; 1 worker thread and 1 interactive thread.
+This can be verified by using the command [`Threads.nthreads()`](@ref):
 
 ```jldoctest
-julia> Threads.nthreads()
+julia> Threads.nthreads(:default)
+1
+julia> Threads.nthreads(:interactive)
 1
 ```
 
@@ -22,13 +24,20 @@ The number of threads can either be specified as an integer (`--threads=4`) or a
 (`--threads=auto`), where `auto` tries to infer a useful default number of threads to use
 (see [Command-line Options](@ref command-line-interface) for more details).
 
+See [threadpools](@ref man-threadpools) for how to control how many `:default` and `:interactive` threads are in
+each threadpool.
+
 !!! compat "Julia 1.5"
     The `-t`/`--threads` command line argument requires at least Julia 1.5.
     In older versions you must use the environment variable instead.
 
 !!! compat "Julia 1.7"
     Using `auto` as value of the environment variable [`JULIA_NUM_THREADS`](@ref JULIA_NUM_THREADS) requires at least Julia 1.7.
     In older versions, this value is ignored.
+
+!!! compat "Julia 1.12"
+    Starting by default with 1 interactive thread, as well as the 1 worker thread, was made as such in Julia 1.12
+
 Lets start Julia with 4 threads:
 
 ```bash
@@ -96,10 +105,17 @@ using Base.Threads
 Interactive tasks should avoid performing high latency operations, and if they
 are long duration tasks, should yield frequently.
 
-Julia may be started with one or more threads reserved to run interactive tasks:
+By default Julia starts with one interactive thread reserved to run interactive tasks, but that number can
+be controlled with:
 
 ```bash
 $ julia --threads 3,1
+julia> Threads.nthreads(:interactive)
+1
+
+$ julia --threads 3,0
+julia> Threads.nthreads(:interactive)
+0
 ```
 
 The environment variable [`JULIA_NUM_THREADS`](@ref JULIA_NUM_THREADS) can also be used similarly:

src/jloptions.c

@@ -623,8 +623,13 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
             break;
         case 't': // threads
             errno = 0;
-            jl_options.nthreadpools = 1;
-            long nthreads = -1, nthreadsi = 0;
+            jl_options.nthreadpools = 2;
+            // By default:
+            // default threads = -1 (== "auto")
+            long nthreads = -1;
+            // interactive threads = 1, or 0 if generating output
+            long nthreadsi = jl_generating_output() ? 0 : 1;
+
             if (!strncmp(optarg, "auto", 4)) {
                 jl_options.nthreads = -1;
                 if (optarg[4] == ',') {
@@ -633,10 +638,9 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
                     else {
                         errno = 0;
                         nthreadsi = strtol(&optarg[5], &endptr, 10);
-                        if (errno != 0 || endptr == &optarg[5] || *endptr != 0 || nthreadsi < 1 || nthreadsi >= INT16_MAX)
-                            jl_errorf("julia: -t,--threads=auto,<m>; m must be an integer >= 1");
+                        if (errno != 0 || endptr == &optarg[5] || *endptr != 0 || nthreadsi < 0 || nthreadsi >= INT16_MAX)
+                            jl_errorf("julia: -t,--threads=auto,<m>; m must be an integer >= 0");
                     }
-                    jl_options.nthreadpools++;
                 }
             }
             else {
@@ -650,17 +654,18 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
                         errno = 0;
                         char *endptri;
                         nthreadsi = strtol(&endptr[1], &endptri, 10);
-                        if (errno != 0 || endptri == &endptr[1] || *endptri != 0 || nthreadsi < 1 || nthreadsi >= INT16_MAX)
-                            jl_errorf("julia: -t,--threads=<n>,<m>; n and m must be integers >= 1");
+                        // Allow 0 for interactive
+                        if (errno != 0 || endptri == &endptr[1] || *endptri != 0 || nthreadsi < 0 || nthreadsi >= INT16_MAX)
+                            jl_errorf("julia: -t,--threads=<n>,<m>; m must be an integer ≥ 0");
+                        if (nthreadsi == 0)
+                            jl_options.nthreadpools = 1;
                     }
-                    jl_options.nthreadpools++;
                 }
                 jl_options.nthreads = nthreads + nthreadsi;
             }
             int16_t *ntpp = (int16_t *)malloc_s(jl_options.nthreadpools * sizeof(int16_t));
             ntpp[0] = (int16_t)nthreads;
-            if (jl_options.nthreadpools == 2)
-                ntpp[1] = (int16_t)nthreadsi;
+            ntpp[1] = (int16_t)nthreadsi;
             jl_options.nthreads_per_pool = ntpp;
             break;
         case 'p': // procs

src/threading.c

@@ -698,15 +698,15 @@ void jl_init_threading(void)
     // and `jl_n_threads_per_pool`.
     jl_n_threadpools = 2;
     int16_t nthreads = JULIA_NUM_THREADS;
-    int16_t nthreadsi = 0;
+    // if generating output default to 0 interactive threads, otherwise default to 1
+    int16_t nthreadsi = jl_generating_output() ? 0 : 1;
     char *endptr, *endptri;
 
     if (jl_options.nthreads != 0) { // --threads specified
         nthreads = jl_options.nthreads_per_pool[0];
         if (nthreads < 0)
             nthreads = jl_effective_threads();
-        if (jl_options.nthreadpools == 2)
-            nthreadsi = jl_options.nthreads_per_pool[1];
+        nthreadsi = (jl_options.nthreadpools == 1) ? 0 : jl_options.nthreads_per_pool[1];
     }
     else if ((cp = getenv(NUM_THREADS_NAME))) { // ENV[NUM_THREADS_NAME] specified
         if (!strncmp(cp, "auto", 4)) {
@@ -722,13 +722,16 @@ void jl_init_threading(void)
         }
         if (*cp == ',') {
             cp++;
-            if (!strncmp(cp, "auto", 4))
+            if (!strncmp(cp, "auto", 4)) {
                 nthreadsi = 1;
+                cp += 4;
+            }
             else {
                 errno = 0;
                 nthreadsi = strtol(cp, &endptri, 10);
                 if (errno != 0 || endptri == cp || nthreadsi < 0)
-                    nthreadsi = 0;
+                    nthreadsi = 1;
+                cp = endptri;
             }
         }
     }

stdlib/REPL/test/replcompletions.jl

@@ -1187,7 +1187,7 @@ let s, c, r
                     REPL.REPLCompletions.next_cache_update = 0
                 end
                 c,r = test_scomplete(s)
-                wait(REPL.REPLCompletions.PATH_cache_task::Task) # wait for caching to complete
+                timedwait(()->REPL.REPLCompletions.next_cache_update != 0, 5) # wait for caching to complete
                 c,r = test_scomplete(s)
                 @test "tmp-executable" in c
                 @test r == 1:9
@@ -1221,7 +1221,7 @@ let s, c, r
                     REPL.REPLCompletions.next_cache_update = 0
                 end
                 c,r = test_scomplete(s)
-                wait(REPL.REPLCompletions.PATH_cache_task::Task) # wait for caching to complete
+                timedwait(()->REPL.REPLCompletions.next_cache_update != 0, 5) # wait for caching to complete
                 c,r = test_scomplete(s)
                 @test ["repl-completion"] == c
                 @test s[r] == "repl-completio"

test/gcext/gcext-test.jl

@@ -31,7 +31,7 @@ end
     # @test success(p)
     errlines = fetch(err_task)
     lines = fetch(out_task)
-    @test length(errlines) == 0
+    @test isempty(errlines)
     # @test length(lines) == 6
     @test length(lines) == 5
     @test checknum(lines[2], r"([0-9]+) full collections", n -> n >= 10)

test/gcext/gcext.c

@@ -600,6 +600,13 @@ int main()
     jl_gc_set_cb_notify_external_alloc(alloc_bigval, 1);
     jl_gc_set_cb_notify_external_free(free_bigval, 1);
 
+    // single threaded mode
+    // Note: with -t1,1 a signal 10 occurs in task_scanner
+    jl_options.nthreadpools = 1;
+    jl_options.nthreads = 1;
+    int16_t ntpp[] = {jl_options.nthreads};
+    jl_options.nthreads_per_pool = ntpp;
+
     jl_init();
     if (jl_gc_enable_conservative_gc_support() < 0)
         abort();

test/threads.jl

@@ -72,11 +72,24 @@ let e = Event(true), started1 = Event(true), started2 = Event(true), done = Even
     end
 end
 
-let cmd = `$(Base.julia_cmd()) --depwarn=error --rr-detach --startup-file=no threads_exec.jl`
-    for test_nthreads in (1, 2, 4, 4) # run once to try single-threaded mode, then try a couple times to trigger bad races
+
+let cmd1 = `$(Base.julia_cmd()) --depwarn=error --rr-detach --startup-file=no threads_exec.jl`,
+    cmd2 = `$(Base.julia_cmd()) --depwarn=error --rr-detach --startup-file=no -e 'print(Threads.threadpoolsize(:default), ",", Threads.threadpoolsize(:interactive))'`
+    for (test_nthreads, test_nthreadsi) in (
+            (1, 0),
+            (1, 1),
+            (2, 0),
+            (2, 1),
+            (4, 0),
+            (4, 0)) # try a couple times to trigger bad races
         new_env = copy(ENV)
-        new_env["JULIA_NUM_THREADS"] = string(test_nthreads)
-        run(pipeline(setenv(cmd, new_env), stdout = stdout, stderr = stderr))
+        new_env["JULIA_NUM_THREADS"] = string(test_nthreads, ",", test_nthreadsi)
+        run(pipeline(setenv(cmd1, new_env), stdout = stdout, stderr = stderr))
+        threads_config = "$test_nthreads,$test_nthreadsi"
+        # threads set via env var
+        @test chomp(read(setenv(cmd2, new_env), String)) == threads_config
+        # threads set via -t
+        @test chomp(read(`$cmd2 -t$test_nthreads,$test_nthreadsi`, String)) == threads_config
     end
 end