Bug 1821671 - Part 2: Add pref for setting the maximum number of marking threads to use r=sfink

Differential Revision: https://phabricator.services.mozilla.com/D207288

dom/base/nsJSEnvironment.cpp

@@ -2138,6 +2138,11 @@ void nsJSContext::EnsureStatics() {
       "javascript.options.mem.gc_parallel_marking_threshold_mb",
       (void*)JSGC_PARALLEL_MARKING_THRESHOLD_MB);
 
+  Preferences::RegisterCallbackAndCall(
+      SetMemoryPrefChangedCallbackInt,
+      "javascript.options.mem.gc_max_parallel_marking_threads",
+      (void*)JSGC_MAX_MARKING_THREADS);
+
   Preferences::RegisterCallbackAndCall(
       SetMemoryGCSliceTimePrefChangedCallback,
       "javascript.options.mem.gc_incremental_slice_ms");

dom/workers/RuntimeService.cpp

@@ -372,6 +372,7 @@ void LoadJSGCMemoryOptions(const char* aPrefName, void* /* aClosure */) {
       PREF("gc_parallel_marking", JSGC_PARALLEL_MARKING_ENABLED),
       PREF("gc_parallel_marking_threshold_mb",
            JSGC_PARALLEL_MARKING_THRESHOLD_MB),
+      PREF("gc_max_parallel_marking_threads", JSGC_MAX_MARKING_THREADS),
 #ifdef NIGHTLY_BUILD
       PREF("gc_experimental_semispace_nursery", JSGC_SEMISPACE_NURSERY_ENABLED),
 #endif
@@ -454,6 +455,7 @@ void LoadJSGCMemoryOptions(const char* aPrefName, void* /* aClosure */) {
       case JSGC_MAX_EMPTY_CHUNK_COUNT:
       case JSGC_HEAP_GROWTH_FACTOR:
       case JSGC_PARALLEL_MARKING_THRESHOLD_MB:
+      case JSGC_MAX_MARKING_THREADS:
         UpdateCommonJSGCMemoryOption(rts, pref->fullName, pref->key);
         break;
       default:

js/public/GCAPI.h

@@ -469,7 +469,7 @@ typedef enum JSGCParamKey {
    * helper threads and can be found by getting the JSGC_MARKING_THREAD_COUNT
    * parameter.
    *
-   * Pref: None.
+   * Pref: javascript.options.mem.gc_max_parallel_marking_threads
    * Default: 2.
    */
   JSGC_MAX_MARKING_THREADS = 52,

modules/libpref/init/all.js

@@ -983,6 +983,9 @@ pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 16);
 pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 16);
 #endif
 
+// JSGC_MAX_MARKING_THREADS
+pref("javascript.options.mem.gc_max_parallel_marking_threads", 2);
+
 // JSGC_HIGH_FREQUENCY_TIME_LIMIT
 pref("javascript.options.mem.gc_high_frequency_time_limit_ms", 1000);
 

toolkit/components/nimbus/FeatureManifest.yaml

@@ -2092,6 +2092,12 @@ gc:
       setPref:
         branch: user
         pref: "javascript.options.mem.gc_parallel_marking_threshold_mb"
+    gc_max_parallel_marking_threads:
+      description: The maximum number of threads to use for parallel marking, if enabled.
+      type: int
+      setPref:
+        branch: user
+        pref: "javascript.options.mem.gc_max_parallel_marking_threads"
     gc_per_zone:
       description: Whether per-zone GC is enabled. If not, all zones are collected every time.
       type: boolean