When idle the GC holds on to unused chunks indefinitely (bug 631733, r=brendan, a=blocker). (relanding in a CLOSED TREE)

dom/base/nsJSEnvironment.cpp

@@ -3480,6 +3480,12 @@ DOMGCCallback(JSContext *cx, JSGCStatus status)
         nsJSContext::PokeCC();
       }
     }
+
+    // If we didn't end up scheduling a GC, and there are unused
+    // chunks waiting to expire, make sure we will GC again soon.
+    if (!sGCTimer && JS_GetGCParameter(cx->runtime, JSGC_UNUSED_CHUNKS) > 0) {
+      nsJSContext::PokeGC();
+    }
   }
 
   JSBool result = gOldJSGCCallback ? gOldJSGCCallback(cx, status) : JS_TRUE;

js/src/jsapi.cpp

@@ -2644,6 +2644,8 @@ JS_GetGCParameter(JSRuntime *rt, JSGCParamKey key)
         return rt->gcBytes;
       case JSGC_MODE:
         return uint32(rt->gcMode);
+      case JSGC_UNUSED_CHUNKS:
+        return uint32(rt->gcChunksWaitingToExpire);
       default:
         JS_ASSERT(key == JSGC_NUMBER);
         return rt->gcNumber;

js/src/jsapi.h

@@ -1785,7 +1785,10 @@ typedef enum JSGCParamKey {
     JSGC_MAX_CODE_CACHE_BYTES = 6,
 
     /* Select GC mode. */
-    JSGC_MODE = 7
+    JSGC_MODE = 7,
+
+    /* Number of GC chunks waiting to expire. */
+    JSGC_UNUSED_CHUNKS = 8
 } JSGCParamKey;
 
 typedef enum JSGCMode {

js/src/jscntxt.h

@@ -1047,6 +1047,7 @@ struct JSRuntime {
     size_t              gcLastBytes;
     size_t              gcMaxBytes;
     size_t              gcMaxMallocBytes;
+    size_t              gcChunksWaitingToExpire;
     uint32              gcEmptyArenaPoolLifespan;
     uint32              gcNumber;
     js::GCMarker        *gcMarkingTracer;

js/src/jsgc.cpp

@@ -360,14 +360,6 @@ Chunk::releaseArena(Arena<T> *arena)
         info.age = 0;
 }
 
-bool
-Chunk::expire()
-{
-    if (!unused())
-        return false;
-    return info.age++ > MaxAge;
-}
-
 JSRuntime *
 Chunk::getRuntime()
 {
@@ -456,16 +448,22 @@ PickChunk(JSRuntime *rt)
 static void
 ExpireGCChunks(JSRuntime *rt)
 {
+    static const size_t MaxAge = 3;
+
     /* Remove unused chunks. */
     AutoLockGC lock(rt);
 
+    rt->gcChunksWaitingToExpire = 0;
     for (GCChunkSet::Enum e(rt->gcChunkSet); !e.empty(); e.popFront()) {
         Chunk *chunk = e.front();
         JS_ASSERT(chunk->info.runtime == rt);
-        if (chunk->expire()) {
-            e.removeFront();
-            ReleaseGCChunk(rt, chunk);
-            continue;
+        if (chunk->unused()) {
+            if (chunk->info.age++ > MaxAge) {
+                e.removeFront();
+                ReleaseGCChunk(rt, chunk);
+                continue;
+            }
+            rt->gcChunksWaitingToExpire++;
         }
     }
 }

js/src/jsgc.h

@@ -340,7 +340,6 @@ struct Chunk {
                                         sizeof(MarkingDelay);
 
     static const size_t ArenasPerChunk = (GC_CHUNK_SIZE - sizeof(ChunkInfo)) / BytesPerArena;
-    static const size_t MaxAge = 3;
 
     Arena<FreeCell> arenas[ArenasPerChunk];
     ArenaBitmap     bitmaps[ArenasPerChunk];
@@ -362,7 +361,6 @@ struct Chunk {
     void releaseArena(Arena<T> *a);
 
     JSRuntime *getRuntime();
-    bool expire();
 };
 JS_STATIC_ASSERT(sizeof(Chunk) <= GC_CHUNK_SIZE);
 JS_STATIC_ASSERT(sizeof(Chunk) + Chunk::BytesPerArena > GC_CHUNK_SIZE);