bump number of GC threads to the number of compute threads (as opposed to half of it)

Author: d-netto

src/threading.c

@@ -708,14 +708,9 @@ void jl_init_threading(void)
         }
         else {
             // if `--gcthreads` or ENV[NUM_GCTHREADS_NAME] was not specified,
-            // set the number of mark threads to half of compute threads
+            // set the number of mark threads to the number of compute threads
             // and number of sweep threads to 0
-            if (nthreads <= 1) {
-                jl_n_markthreads = 0;
-            }
-            else {
-                jl_n_markthreads = (nthreads / 2) - 1;
-            }
+            jl_n_markthreads = nthreads - 1; // -1 for the master (mutator) thread which may also do marking
             // 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

test/cmdlineargs.jl

@@ -388,7 +388,7 @@ let exename = `$(Base.julia_cmd()) --startup-file=no --color=no`
     # --gcthreads
     code = "print(Threads.ngcthreads())"
     cpu_threads = ccall(:jl_effective_threads, Int32, ())
-    @test (cpu_threads == 1 ? "1" : string(div(cpu_threads, 2))) ==
+    @test string(cpu_threads) ==
           read(`$exename --threads auto -e $code`, String) ==
           read(`$exename --threads=auto -e $code`, String) ==
           read(`$exename -tauto -e $code`, String) ==