Bug 871005 - Add a pref for GC decommit threshold (r=gregor)

b2g/app/b2g.js

@@ -563,6 +563,7 @@ pref("javascript.options.mem.gc_high_frequency_low_limit_mb", 10);
 pref("javascript.options.mem.gc_low_frequency_heap_growth", 120);
 pref("javascript.options.mem.high_water_mark", 6);
 pref("javascript.options.mem.gc_allocation_threshold_mb", 1);
+pref("javascript.options.mem.gc_decommit_threshold_mb", 1);
 
 // Show/Hide scrollbars when active/inactive
 pref("ui.showHideScrollbars", 1);

dom/base/nsJSEnvironment.cpp

@@ -3430,6 +3430,11 @@ nsJSRuntime::Init()
   Preferences::RegisterCallbackAndCall(SetMemoryGCPrefChangedCallback,
                                        "javascript.options.mem.gc_allocation_threshold_mb",
                                        (void *)JSGC_ALLOCATION_THRESHOLD);
+
+  Preferences::RegisterCallbackAndCall(SetMemoryGCPrefChangedCallback,
+                                       "javascript.options.mem.gc_decommit_threshold_mb",
+                                       (void *)JSGC_DECOMMIT_THRESHOLD);
+
   nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
   if (!obs)
     return NS_ERROR_FAILURE;

js/src/gc/Heap.h

@@ -110,12 +110,6 @@ struct Cell
     inline Chunk *chunk() const;
 };
 
-/*
- * This is the maximum number of arenas we allow in the FreeCommitted state
- * before we trigger a GC_SHRINK to release free arenas to the OS.
- */
-const static uint32_t FreeCommittedArenasThreshold = (32 << 20) / ArenaSize;
-
 /*
  * The mark bitmap has one bit per each GC cell. For multi-cell GC things this
  * wastes space but allows to avoid expensive devisions by thing's size when

js/src/jsapi.cpp

@@ -781,6 +781,7 @@ JSRuntime::JSRuntime(JSUseHelperThreads useHelperThreads)
     gcLowFrequencyHeapGrowth(1.5),
     gcDynamicHeapGrowth(false),
     gcDynamicMarkSlice(false),
+    gcDecommitThreshold(32 * 1024 * 1024),
     gcShouldCleanUpEverything(false),
     gcGrayBitsValid(false),
     gcIsNeeded(0),
@@ -2891,6 +2892,9 @@ JS_SetGCParameter(JSRuntime *rt, JSGCParamKey key, uint32_t value)
       case JSGC_ALLOCATION_THRESHOLD:
         rt->gcAllocationThreshold = value * 1024 * 1024;
         break;
+      case JSGC_DECOMMIT_THRESHOLD:
+        rt->gcDecommitThreshold = value * 1024 * 1024;
+        break;
       default:
         JS_ASSERT(key == JSGC_MODE);
         rt->gcMode = JSGCMode(value);

js/src/jsapi.h

@@ -2699,7 +2699,14 @@ typedef enum JSGCParamKey {
     JSGC_ANALYSIS_PURGE_TRIGGER = 19,
 
     /* Lower limit after which we limit the heap growth. */
-    JSGC_ALLOCATION_THRESHOLD = 20
+    JSGC_ALLOCATION_THRESHOLD = 20,
+
+    /*
+     * We decommit memory lazily. If more than this number of megabytes is
+     * available to be decommitted, then JS_MaybeGC will trigger a shrinking GC
+     * to decommit it.
+     */
+    JSGC_DECOMMIT_THRESHOLD = 21
 } JSGCParamKey;
 
 typedef enum JSGCMode {

js/src/jscntxt.h

@@ -897,6 +897,7 @@ struct JSRuntime : public JS::shadow::Runtime,
     double              gcLowFrequencyHeapGrowth;
     bool                gcDynamicHeapGrowth;
     bool                gcDynamicMarkSlice;
+    uint64_t            gcDecommitThreshold;
 
     /* During shutdown, the GC needs to clean up every possible object. */
     bool                gcShouldCleanUpEverything;

js/src/jsgc.cpp

@@ -1982,7 +1982,7 @@ js::MaybeGC(JSContext *cx)
     int64_t now = PRMJ_Now();
     if (rt->gcNextFullGCTime && rt->gcNextFullGCTime <= now) {
         if (rt->gcChunkAllocationSinceLastGC ||
-            rt->gcNumArenasFreeCommitted > FreeCommittedArenasThreshold)
+            rt->gcNumArenasFreeCommitted > rt->gcDecommitThreshold)
         {
             JS::PrepareForFullGC(rt);
             GCSlice(rt, GC_SHRINK, JS::gcreason::MAYBEGC);

mobile/android/app/mobile.js

@@ -413,6 +413,7 @@ pref("javascript.options.mem.gc_high_frequency_low_limit_mb", 10);
 pref("javascript.options.mem.gc_low_frequency_heap_growth", 105);
 pref("javascript.options.mem.high_water_mark", 16);
 pref("javascript.options.mem.gc_allocation_threshold_mb", 3);
+pref("javascript.options.mem.gc_decommit_threshold_mb", 1);
 #else
 pref("javascript.options.mem.high_water_mark", 32);
 #endif

modules/libpref/src/init/all.js

@@ -827,6 +827,7 @@ pref("javascript.options.mem.gc_low_frequency_heap_growth", 150);
 pref("javascript.options.mem.gc_dynamic_heap_growth", true);
 pref("javascript.options.mem.gc_dynamic_mark_slice", true);
 pref("javascript.options.mem.gc_allocation_threshold_mb", 30);
+pref("javascript.options.mem.gc_decommit_threshold_mb", 32);
 
 pref("javascript.options.mem.analysis_purge_mb", 100);