Bug 1868530 - Lower parallel marking thresholds following recent improvements r=sfink

These thresholds were chosen based on local benchmarking results.

In general all macOS systems I've tested perform great and the 4MB threshold is
because none of the benchmarks I used have a heap smaller than that.

Recent Windows and Linux results are also better than previously although not
as good as macOS.

I haven't done any more Android benchmarking recently so I'm using the same
value as for Linux. We're not going to enable this on Android without specific
performance work there first.

For reference, according to telemetry about 10% of our GCs have < 1MB heap size
(the minimum size captured by telemetry). A 4MB threshold excludes 11.4% of
GCs, 8MB 14.3% and 16MB 24% of GCs.

Differential Revision: https://phabricator.services.mozilla.com/D195629

Author: jonco3

js/src/gc/Scheduling.h

@@ -519,7 +519,7 @@
    * JSGC_PARALLEL_MARKING_THRESHOLD_MB                                        \
    */                                                                          \
   _(JSGC_PARALLEL_MARKING_THRESHOLD_MB, size_t, parallelMarkingThresholdBytes, \
-    ConvertMB, NoCheck, 10 * 1024 * 1024)
+    ConvertMB, NoCheck, 4 * 1024 * 1024)
 
 namespace js {
 

modules/libpref/init/all.js

@@ -957,13 +957,13 @@ pref("javascript.options.mem.gc_parallel_marking", false);
 // JSGC_PARALLEL_MARKING_THRESHOLD_MB
 // Minimum heap size at which to use parallel marking, if enabled.
 #if defined(XP_WIN)
-pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 20);
+pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 8);
 #elif defined(XP_MACOSX)
 pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 4);
 #elif defined(ANDROID)
-pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 200);
+pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 16);
 #elif defined(XP_UNIX)
-pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 200);
+pref("javascript.options.mem.gc_parallel_marking_threshold_mb", 16);
 #endif
 
 // JSGC_HIGH_FREQUENCY_TIME_LIMIT