Use uv_thread_getaffinity when --threads=auto

NEWS.md

@@ -16,6 +16,8 @@ Compiler/Runtime improvements
 Command-line option changes
 ---------------------------
 
+* In Linux and Windows, `--threads=auto` now tries to infer usable number of CPUs from the
+  process affinity which is set typically in HPC and cloud environments ([#42340]).
 
 Multi-threading changes
 -----------------------

doc/man/julia.1

@@ -103,7 +103,13 @@ Load <file> immediately on all processors
 
 .TP
 -t, --threads <n>
-Enable n threads
+Enable n threads; "auto" tries to infer a useful default number
+of threads to use but the exact behavior might change in the future.
+Currently, "auto" uses the number of CPUs assigned to this julia
+process based on the OS-specific affinity assignment interface, if
+supported (Linux and Windows). If this is not supported (macOS) or
+process affinity is not configured, it uses the number of CPU
+threads.
 
 .TP
 -p, --procs <n>

doc/src/manual/command-line-options.md

@@ -89,7 +89,7 @@ The following is a complete list of command-line switches available when launchi
 |`-e`, `--eval <expr>`                  |Evaluate `<expr>`|
 |`-E`, `--print <expr>`                 |Evaluate `<expr>` and display the result|
 |`-L`, `--load <file>`                  |Load `<file>` immediately on all processors|
-|`-t`, `--threads {N\|auto`}            |Enable N threads; `auto` currently sets N to the number of local CPU threads but this might change in the future|
+|`-t`, `--threads {N\|auto`}            |Enable N threads; `auto` tries to infer a useful default number of threads to use but the exact behavior might change in the future.  Currently, `auto` uses the number of CPUs assigned to this julia process based on the OS-specific affinity assignment interface, if supported (Linux and Windows). If this is not supported (macOS) or process affinity is not configured, it uses the number of CPU threads.|
 |`-p`, `--procs {N\|auto`}              |Integer value N launches N additional local worker processes; `auto` launches as many workers as the number of local CPU threads (logical cores)|
 |`--machine-file <file>`                |Run processes on hosts listed in `<file>`|
 |`-i`                                   |Interactive mode; REPL runs and `isinteractive()` is true|
@@ -111,6 +111,8 @@ The following is a complete list of command-line switches available when launchi
 |`--track-allocation={none\|user\|all}` |Count bytes allocated by each source line|
 |`--track-allocation`                   |equivalent to `--track-allocation=user`|
 
+
+
 !!! compat "Julia 1.1"
     In Julia 1.0, the default `--project=@.` option did not search up from the root
     directory of a Git repository for the `Project.toml` file. From Julia 1.1 forward, it

doc/src/manual/multi-threading.md

@@ -19,8 +19,8 @@ The number of execution threads is controlled either by using the
 specified, then `-t`/`--threads` takes precedence.
 
 The number of threads can either be specified as an integer (`--threads=4`) or as `auto`
-(`--threads=auto`), where `auto` sets the number of threads to the number of local CPU
-threads.
+(`--threads=auto`), where `auto` tries to infer a useful default number of threads to use
+(see [Command-line Options](@id command-line-options) for more details).
 
 !!! compat "Julia 1.5"
     The `-t`/`--threads` command line argument requires at least Julia 1.5.

src/jloptions.c

@@ -108,8 +108,13 @@ static const char opts[]  =
     " -L, --load <file>         Load <file> immediately on all processors\n\n"
 
     // parallel options
-    " -t, --threads {N|auto}    Enable N threads; \"auto\" currently sets N to the number of local\n"
-    "                           CPU threads but this might change in the future\n"
+    " -t, --threads {N|auto}    Enable N threads; \"auto\" tries to infer a useful default number\n"
+    "                           of threads to use but the exact behavior might change in the future.\n"
+    "                           Currently, \"auto\" uses the number of CPUs assigned to this julia\n"
+    "                           process based on the OS-specific affinity assignment interface, if\n"
+    "                           supported (Linux and Windows). If this is not supported (macOS) or\n"
+    "                           process affinity is not configured, it uses the number of CPU\n"
+    "                           threads.\n"
     " -p, --procs {N|auto}      Integer value N launches N additional local worker processes\n"
     "                           \"auto\" launches as many workers as the number of local CPU threads (logical cores)\n"
     " --machine-file <file>     Run processes on hosts listed in <file>\n\n"
@@ -441,7 +446,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
         case 'p': // procs
             errno = 0;
             if (!strcmp(optarg,"auto")) {
-                jl_options.nprocs = jl_cpu_threads();
+                jl_options.nprocs = jl_effective_threads();
             }
             else {
                 long nprocs = strtol(optarg, &endptr, 10);

src/julia_internal.h

@@ -766,6 +766,7 @@ extern JL_DLLEXPORT ssize_t jl_tls_offset;
 extern JL_DLLEXPORT const int jl_tls_elf_support;
 void jl_init_threading(void);
 void jl_start_threads(void);
+int jl_effective_threads(void);
 
 // Whether the GC is running
 extern char *jl_safepoint_pages;

src/sys.c

@@ -661,6 +661,29 @@ JL_DLLEXPORT int jl_cpu_threads(void) JL_NOTSAFEPOINT
 #endif
 }
 
+int jl_effective_threads(void) JL_NOTSAFEPOINT
+{
+    int cpu = jl_cpu_threads();
+    int masksize = uv_cpumask_size();
+    if (masksize < 0 || jl_running_under_rr(0))
+        return cpu;
+    uv_thread_t tid = uv_thread_self();
+    char *cpumask = (char *)calloc(masksize, sizeof(char));
+    int err = uv_thread_getaffinity(&tid, cpumask, masksize);
+    if (err) {
+        free(cpumask);
+        jl_safe_printf("WARNING: failed to get thread affinity (%s %d)\n", uv_err_name(err),
+                       err);
+        return cpu;
+    }
+    int n = 0;
+    for (size_t i = 0; i < masksize; i++) {
+        n += cpumask[i];
+    }
+    free(cpumask);
+    return n < cpu ? n : cpu;
+}
+
 
 // -- high resolution timers --
 // Returns time in nanosec

src/threading.c

@@ -454,7 +454,7 @@ void jl_init_threading(void)
     // how many threads available, usable
     jl_n_threads = JULIA_NUM_THREADS;
     if (jl_options.nthreads < 0) { // --threads=auto
-        jl_n_threads = jl_cpu_threads();
+        jl_n_threads = jl_effective_threads();
     }
     else if (jl_options.nthreads > 0) { // --threads=N
         jl_n_threads = jl_options.nthreads;
@@ -463,7 +463,7 @@ void jl_init_threading(void)
         if (strcmp(cp, "auto"))
             jl_n_threads = (uint64_t)strtol(cp, NULL, 10); // ENV[NUM_THREADS_NAME] == "N"
         else
-            jl_n_threads = jl_cpu_threads(); // ENV[NUM_THREADS_NAME] == "auto"
+            jl_n_threads = jl_effective_threads(); // ENV[NUM_THREADS_NAME] == "auto"
     }
     if (jl_n_threads <= 0)
         jl_n_threads = 1;

test/threads.jl

@@ -93,13 +93,42 @@ else
 end
 # Note also that libuv does not support affinity in macOS and it is known to
 # hang in FreeBSD. So, it's tested only in Linux and Windows:
-if Sys.islinux() || Sys.iswindows()
-    if Sys.CPU_THREADS > 1 && !running_under_rr()
+const AFFINITY_SUPPORTED = (Sys.islinux() || Sys.iswindows()) && !running_under_rr()
+
+if AFFINITY_SUPPORTED
+    if Sys.CPU_THREADS > 1
         @test run_with_affinity([2]) == "2"
         @test run_with_affinity([1, 2]) == "1,2"
     end
 end
 
+function get_nthreads(options = ``; cpus = nothing)
+    cmd = `$(Base.julia_cmd()) --startup-file=no $(options)`
+    cmd = `$cmd -e "print(Threads.nthreads())"`
+    cmd = addenv(cmd, "JULIA_EXCLUSIVE" => "0", "JULIA_NUM_THREADS" => "auto")
+    if cpus !== nothing
+        cmd = setcpuaffinity(cmd, cpus)
+    end
+    return parse(Int, read(cmd, String))
+end
+
+@testset "nthreads determined based on CPU affinity" begin
+    if AFFINITY_SUPPORTED && Sys.CPU_THREADS ≥ 2
+        @test get_nthreads() ≥ 2
+        @test get_nthreads(cpus = [1]) == 1
+        @test get_nthreads(cpus = [2]) == 1
+        @test get_nthreads(cpus = [1, 2]) == 2
+        @test get_nthreads(`-t1`, cpus = [1]) == 1
+        @test get_nthreads(`-t1`, cpus = [2]) == 1
+        @test get_nthreads(`-t1`, cpus = [1, 2]) == 1
+
+        if Sys.CPU_THREADS ≥ 3
+            @test get_nthreads(cpus = [1, 3]) == 2
+            @test get_nthreads(cpus = [2, 3]) == 2
+        end
+    end
+end
+
 # issue #34769
 function idle_callback(handle)
     idle = @Base.handle_as handle UvTestIdle