New API (GC_set_markers_count) to control number of parallel markers

Issue #320 (bdwgc).

Note that GC_set_markers_count() has effect only if called before GC
initialization. For convenience, alternatively the client could define
GC_MARKERS macro with the appropriate value before include of gc.h.

* doc/README.macros (GC_MARKERS): Document.
* include/gc.h [GC_THREADS] (GC_parallel): Update comment.
* include/gc.h [GC_THREADS] (GC_set_markers_count): New API function
declaration.
* include/gc.h (GC_INIT_CONF_MARKERS): New internal macro (defined to
GC_set_markers_count(GC_MARKERS) if GC_THREADS and GC_MARKERS are
defined).
* include/gc.h (GC_INIT): Call GC_INIT_CONF_MARKERS before GC_init.
* pthread_support.c [PARALLEL_MARK] (required_markers_cnt): New static
variable (initialized to 0).
* win32_threads.c [PARALLEL_MARK] (required_markers_cnt): Likewise.
* pthread_support.c (GC_set_markers_count): New API function definition
(empty unless PARALLEL_MARK).
* win32_threads.c [PARALLEL_MARK] (GC_set_markers_count): Likewise.
* pthread_support.c [PARALLEL_MARK] (GC_thr_init): Initialize markers
local variable to required_markers_cnt; set markers to GC_nprocs only
if markers_string is null and required_markers_cnt (markers) value is
zero.
* tests/test.c [GC_PTHREADS] (main): Call GC_set_markers_count(0)
before GC_COND_INIT().
* win32_threads.c [PARALLEL_MARK] (GC_thr_init): Initialize markers
local variable to required_markers_cnt; set markers to ncpu only if
markers_string is null and required_markers_cnt (markers) value is 0.

1 files changed, 16 insertions, 2 deletions

diff --git a/win32_threads.c b/win32_threads.c
index 72406d92..ee2222b0 100644
--- a/win32_threads.c
+++ b/win32_threads.c
@@ -2500,6 +2500,9 @@ GC_INNER void GC_get_next_stack(char *start, char *limit,
 
 # endif /* ! GC_PTHREADS_PARAMARK */
 
+  static unsigned required_markers_cnt = 0;
+                        /* The default value (0) means the number of    */
+                        /* markers should be selected automatically.    */
 #endif /* PARALLEL_MARK */
 
   /* We have no DllMain to take care of new threads.  Thus we   */
@@ -2737,6 +2740,14 @@ GC_INNER void GC_get_next_stack(char *start, char *limit,
 
 #endif /* GC_WINMAIN_REDIRECT */
 
+GC_API void GC_CALL GC_set_markers_count(unsigned markers GC_ATTR_UNUSED)
+{
+  /* The same implementation as in pthread_support.c.   */
+# ifdef PARALLEL_MARK
+    required_markers_cnt = markers < MAX_MARKERS ? markers : MAX_MARKERS;
+# endif
+}
+
 GC_INNER void GC_thr_init(void)
 {
   struct GC_stack_base sb;
@@ -2792,7 +2803,7 @@ GC_INNER void GC_thr_init(void)
 # if defined(PARALLEL_MARK)
     {
       char * markers_string = GETENV("GC_MARKERS");
-      int markers;
+      int markers = required_markers_cnt;
 
       if (markers_string != NULL) {
         markers = atoi(markers_string);
@@ -2801,7 +2812,10 @@ GC_INNER void GC_thr_init(void)
                "; using maximum threads\n", (signed_word)markers);
           markers = MAX_MARKERS;
         }
-      } else {
+      } else if (0 == markers) {
+        /* Unless the client sets the desired number of         */
+        /* parallel markers, it is determined based on the      */
+        /* number of CPU cores.                                 */
 #       ifdef MSWINCE
           /* There is no GetProcessAffinityMask() in WinCE.     */
           /* GC_sysinfo is already initialized.                 */