cap the number of GC threads to number of cpu cores (JuliaLang#52192)

src/threading.c

@@ -681,6 +681,7 @@ void jl_init_threading(void)
  }
  }
 
+ int cpu = jl_cpu_threads();
  jl_n_markthreads = jl_options.nmarkthreads - 1;
  jl_n_sweepthreads = jl_options.nsweepthreads;
  if (jl_n_markthreads == -1) { // --gcthreads not specified
@@ -709,8 +710,20 @@ void jl_init_threading(void)
  else {
  jl_n_markthreads = (nthreads / 2) - 1;
  }
+ // if `--gcthreads` or ENV[NUM_GCTHREADS_NAME] was not specified,
+ // cap the number of threads that may run the mark phase to
+ // the number of CPU cores
+ if (jl_n_markthreads + 1 >= cpu) {
+ jl_n_markthreads = cpu - 1;
+ }
  }
  }
+ // warn the user if they try to run with a number
+ // of GC threads which is larger than the number
+ // of physical cores
+ if (jl_n_markthreads + 1 > cpu) {
+ jl_safe_printf("WARNING: running Julia with %d GC threads on %d CPU cores\n", jl_n_markthreads + 1, cpu);
+ }
  int16_t ngcthreads = jl_n_markthreads + jl_n_sweepthreads;
 
  jl_all_tls_states_size = nthreads + nthreadsi + ngcthreads;