diff --git a/js/src/gc/GCRuntime.h b/js/src/gc/GCRuntime.h
index 4bded5b5c09a..4a8dee8fc4ea 100644
--- a/js/src/gc/GCRuntime.h
+++ b/js/src/gc/GCRuntime.h
@@ -274,7 +274,8 @@ class GCRuntime
 
     bool triggerGC(JS::gcreason::Reason reason);
     bool triggerZoneGC(Zone *zone, JS::gcreason::Reason reason);
-    void maybeGC(Zone *zone);
+    bool maybeGC(Zone *zone);
+    void maybePeriodicFullGC();
     void minorGC(JS::gcreason::Reason reason);
     void minorGC(JSContext *cx, JS::gcreason::Reason reason);
     void gcIfNeeded(JSContext *cx);
@@ -487,7 +488,7 @@ class GCRuntime
     void getNextZoneGroup();
     void endMarkingZoneGroup();
     void beginSweepingZoneGroup();
-    bool releaseObservedTypes();
+    bool shouldReleaseObservedTypes();
     void endSweepingZoneGroup();
     bool sweepPhase(SliceBudget &sliceBudget);
     void endSweepPhase(JSGCInvocationKind gckind, bool lastGC);
@@ -567,7 +568,6 @@ class GCRuntime
     bool                  chunkAllocationSinceLastGC;
     int64_t               nextFullGCTime;
     int64_t               lastGCTime;
-    int64_t               jitReleaseTime;
 
     JSGCMode              mode;
 
@@ -589,6 +589,12 @@ class GCRuntime
      */
     volatile uintptr_t    isNeeded;
 
+    /* Incremented at the start of every major GC. */
+    uint64_t              majorGCNumber;
+
+    /* The major GC number at which to release observed type information. */
+    uint64_t              jitReleaseNumber;
+
     /* Incremented on every GC slice. */
     uint64_t              number;
 
@@ -624,6 +630,9 @@ class GCRuntime
     /* Whether any sweeping will take place in the separate GC helper thread. */
     bool                  sweepOnBackgroundThread;
 
+    /* Whether observed type information is being released in the current GC. */
+    bool                  releaseObservedTypes;
+
     /* Whether any black->gray edges were found during marking. */
     bool                  foundBlackGrayEdges;
 
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 4361ee3d86da..c2d15765045b 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -1879,7 +1879,9 @@ JS_GC(JSRuntime *rt)
 JS_PUBLIC_API(void)
 JS_MaybeGC(JSContext *cx)
 {
-    MaybeGC(cx);
+    GCRuntime &gc = cx->runtime()->gc;
+    if (!gc.maybeGC(cx->zone()))
+        gc.maybePeriodicFullGC();
 }
 
 JS_PUBLIC_API(void)
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index f85a1ee7d9c2..2d7f2dbdcba5 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -1112,12 +1112,13 @@ GCRuntime::GCRuntime(JSRuntime *rt) :
     chunkAllocationSinceLastGC(false),
     nextFullGCTime(0),
     lastGCTime(0),
-    jitReleaseTime(0),
     mode(JSGC_MODE_INCREMENTAL),
     decommitThreshold(32 * 1024 * 1024),
     cleanUpEverything(false),
     grayBitsValid(false),
     isNeeded(0),
+    majorGCNumber(0),
+    jitReleaseNumber(0),
     number(0),
     startNumber(0),
     isFull(false),
@@ -1248,8 +1249,11 @@ GCRuntime::initZeal()
 
 #endif
 
-/* Lifetime for type sets attached to scripts containing observed types. */
-static const int64_t JIT_SCRIPT_RELEASE_TYPES_INTERVAL = 60 * 1000 * 1000;
+/*
+ * Lifetime in number of major GCs for type sets attached to scripts containing
+ * observed types.
+ */
+static const uint64_t JIT_SCRIPT_RELEASE_TYPES_PERIOD = 20;
 
 bool
 GCRuntime::init(uint32_t maxbytes, uint32_t maxNurseryBytes)
@@ -1276,9 +1280,7 @@ GCRuntime::init(uint32_t maxbytes, uint32_t maxNurseryBytes)
     tunables.setParameter(JSGC_MAX_BYTES, maxbytes);
     setMaxMallocBytes(maxbytes);
 
-#ifndef JS_MORE_DETERMINISTIC
-    jitReleaseTime = PRMJ_Now() + JIT_SCRIPT_RELEASE_TYPES_INTERVAL;
-#endif
+    jitReleaseNumber = majorGCNumber + JIT_SCRIPT_RELEASE_TYPES_PERIOD;
 
 #ifdef JSGC_GENERATIONAL
     if (!nursery.init(maxNurseryBytes))
@@ -2415,13 +2417,7 @@ GCRuntime::triggerZoneGC(Zone *zone, JS::gcreason::Reason reason)
     return true;
 }
 
-void
-js::MaybeGC(JSContext *cx)
-{
-    cx->runtime()->gc.maybeGC(cx->zone());
-}
-
-void
+bool
 GCRuntime::maybeGC(Zone *zone)
 {
     JS_ASSERT(CurrentThreadCanAccessRuntime(rt));
@@ -2430,13 +2426,13 @@ GCRuntime::maybeGC(Zone *zone)
     if (zealMode == ZealAllocValue || zealMode == ZealPokeValue) {
         JS::PrepareForFullGC(rt);
         GC(rt, GC_NORMAL, JS::gcreason::MAYBEGC);
-        return;
+        return true;
     }
 #endif
 
     if (isNeeded) {
         GCSlice(rt, GC_NORMAL, JS::gcreason::MAYBEGC);
-        return;
+        return true;
     }
 
     double factor = schedulingState.inHighFrequencyGCMode() ? 0.85 : 0.9;
@@ -2447,15 +2443,25 @@ GCRuntime::maybeGC(Zone *zone)
     {
         PrepareZoneForGC(zone);
         GCSlice(rt, GC_NORMAL, JS::gcreason::MAYBEGC);
-        return;
+        return true;
     }
 
-#ifndef JS_MORE_DETERMINISTIC
+    return false;
+}
+
+void
+GCRuntime::maybePeriodicFullGC()
+{
     /*
+     * Trigger a periodic full GC.
+     *
+     * This is a source of non-determinism, but is not called from the shell.
+     *
      * Access to the counters and, on 32 bit, setting gcNextFullGCTime below
      * is not atomic and a race condition could trigger or suppress the GC. We
      * tolerate this.
      */
+#ifndef JS_MORE_DETERMINISTIC
     int64_t now = PRMJ_Now();
     if (nextFullGCTime && nextFullGCTime <= now) {
         if (chunkAllocationSinceLastGC ||
@@ -2881,7 +2887,7 @@ GCHelperState::onBackgroundThread()
 }
 
 bool
-GCRuntime::releaseObservedTypes()
+GCRuntime::shouldReleaseObservedTypes()
 {
     bool releaseTypes = false;
 
@@ -2890,13 +2896,12 @@ GCRuntime::releaseObservedTypes()
         releaseTypes = true;
 #endif
 
-#ifndef JS_MORE_DETERMINISTIC
-    int64_t now = PRMJ_Now();
-    if (now >= jitReleaseTime)
+    MOZ_ASSERT(majorGCNumber <= jitReleaseNumber);
+    if (majorGCNumber == jitReleaseNumber)
         releaseTypes = true;
+
     if (releaseTypes)
-        jitReleaseTime = now + JIT_SCRIPT_RELEASE_TYPES_INTERVAL;
-#endif
+        jitReleaseNumber = majorGCNumber + JIT_SCRIPT_RELEASE_TYPES_PERIOD;
 
     return releaseTypes;
 }
@@ -4157,10 +4162,9 @@ GCRuntime::beginSweepingZoneGroup()
             zone->discardJitCode(&fop);
         }
 
-        bool releaseTypes = releaseObservedTypes();
         for (GCCompartmentGroupIter c(rt); !c.done(); c.next()) {
             gcstats::AutoSCC scc(stats, zoneGroupIndex);
-            c->sweep(&fop, releaseTypes && !c->zone()->isPreservingCode());
+            c->sweep(&fop, releaseObservedTypes && !c->zone()->isPreservingCode());
         }
 
         for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
@@ -4174,7 +4178,7 @@ GCRuntime::beginSweepingZoneGroup()
             // code and new script information in the zone, the only things
             // whose correctness depends on the type constraints.
             bool oom = false;
-            zone->sweep(&fop, releaseTypes && !zone->isPreservingCode(), &oom);
+            zone->sweep(&fop, releaseObservedTypes && !zone->isPreservingCode(), &oom);
 
             if (oom) {
                 zone->setPreservingCode(false);
@@ -4264,6 +4268,8 @@ GCRuntime::beginSweepPhase(bool lastGC)
 
     sweepOnBackgroundThread = !lastGC && !TraceEnabled() && CanUseExtraThreads();
 
+    releaseObservedTypes = shouldReleaseObservedTypes();
+
 #ifdef DEBUG
     for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
         JS_ASSERT(!c->gcIncomingGrayPointers);
@@ -4986,6 +4992,8 @@ GCRuntime::gcCycle(bool incremental, int64_t budget, JSGCInvocationKind gckind,
     interFrameGC = true;
 
     number++;
+    if (incrementalState == NO_INCREMENTAL)
+        majorGCNumber++;
 
     // It's ok if threads other than the main thread have suppressGC set, as
     // they are operating on zones which will not be collected from here.
diff --git a/js/src/jsgc.h b/js/src/jsgc.h
index 3f3f4265b88b..71ae41121c7e 100644
--- a/js/src/jsgc.h
+++ b/js/src/jsgc.h
@@ -971,9 +971,6 @@ TriggerGC(JSRuntime *rt, JS::gcreason::Reason reason);
 extern bool
 TriggerZoneGC(Zone *zone, JS::gcreason::Reason reason);
 
-extern void
-MaybeGC(JSContext *cx);
-
 extern void
 ReleaseAllJITCode(FreeOp *op);
 
diff --git a/js/src/jsgcinlines.h b/js/src/jsgcinlines.h
index 2ae37ea6aba7..8e764d08d0d6 100644
--- a/js/src/jsgcinlines.h
+++ b/js/src/jsgcinlines.h
@@ -691,7 +691,7 @@ AllocateObjectForCacheHit(JSContext *cx, AllocKind kind, InitialHeap heap)
 
     JSObject *obj = AllocateObject<NoGC>(cx, kind, 0, heap);
     if (!obj && allowGC) {
-        MaybeGC(cx);
+        cx->runtime()->gc.maybeGC(cx->zone());
         return nullptr;
     }