Bug 862970 - Don't lock images on the active page on B2G. r=jlebar,joe

b2g/app/b2g.js

@@ -271,9 +271,10 @@ pref("media.video-queue.default-size", 3);
 
 // optimize images' memory usage
 pref("image.mem.decodeondraw", true);
-pref("content.image.allow_locking", true);
-pref("image.mem.min_discard_timeout_ms", 10000);
-pref("image.mem.max_decoded_image_kb", 5120); /* 5MB */
+pref("content.image.allow_locking", false); /* don't allow image locking */
+pref("image.mem.min_discard_timeout_ms", 86400000); /* 24h, we rely on the out of memory hook */
+pref("image.mem.max_decoded_image_kb", 30000); /* 30MB seems reasonable */
+pref("image.onload.decode.limit", 24); /* don't decode more than 24 images eagerly */
 
 // XXX this isn't a good check for "are touch events supported", but
 // we don't really have a better one at the moment.

content/base/src/nsDocument.cpp

@@ -1866,6 +1866,8 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsDocument)
   tmp->mInUnlinkOrDeletion = false;
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
+static bool sPrefsInitialized = false;
+static int32_t sOnloadDecodeLimit = 0;
 
 nsresult
 nsDocument::Init()
@@ -1874,6 +1876,11 @@ nsDocument::Init()
     return NS_ERROR_ALREADY_INITIALIZED;
   }
 
+  if (!sPrefsInitialized) {
+    sPrefsInitialized = true;
+    Preferences::AddIntVarCache(&sOnloadDecodeLimit, "image.onload.decode.limit", 0);
+  }
+
   mIdentifierMap.Init();
   mStyledLinks.Init();
   mRadioGroups.Init();
@@ -8919,9 +8926,11 @@ nsDocument::AddImage(imgIRequest* aImage)
 
   // If this is the first insertion and we're locking images, lock this image
   // too.
-  if (oldCount == 0 && mLockingImages) {
-    rv = aImage->LockImage();
-    if (NS_SUCCEEDED(rv))
+  if (oldCount == 0) {
+    if (mLockingImages)
+      rv = aImage->LockImage();
+    if (NS_SUCCEEDED(rv) && (!sOnloadDecodeLimit ||
+                             mImageTracker.Count() < sOnloadDecodeLimit))
       rv = aImage->StartDecoding();
   }
 

dom/base/ConsoleAPIStorage.jsm

@@ -52,10 +52,7 @@ this.ConsoleAPIStorage = {
       this.clearEvents(innerWindowID);
     }
     else if (aTopic == "memory-pressure") {
-      /* Handle both low-memory and low-memory-no-forward events */
-      if (aData.startsWith("low-memory")) {
-        this.clearEvents();
-      }
+      this.clearEvents();
     }
   },
 

dom/base/nsJSEnvironment.cpp

@@ -240,6 +240,12 @@ nsJSEnvironmentObserver::Observe(nsISupports* aSubject, const char* aTopic,
                                  const PRUnichar* aData)
 {
   if (sGCOnMemoryPressure && !nsCRT::strcmp(aTopic, "memory-pressure")) {
+    if(StringBeginsWith(nsDependentString(aData),
+                        NS_LITERAL_STRING("low-memory-ongoing"))) {
+      // Don't GC/CC if we are in an ongoing low-memory state since its very
+      // slow and it likely won't help us anyway.
+      return NS_OK;
+    }
     nsJSContext::GarbageCollectNow(JS::gcreason::MEM_PRESSURE,
                                    nsJSContext::NonIncrementalGC,
                                    nsJSContext::NonCompartmentGC,

dom/ipc/ContentParent.cpp

@@ -1443,7 +1443,8 @@ ContentParent::Observe(nsISupports* aSubject,
 
     // listening for memory pressure event
     if (!strcmp(aTopic, "memory-pressure") &&
-        !NS_LITERAL_STRING("low-memory-no-forward").Equals(aData)) {
+        !StringEndsWith(nsDependentString(aData),
+                        NS_LITERAL_STRING("-no-forward"))) {
         unused << SendFlushMemory(nsDependentString(aData));
     }
     // listening for remotePrefs...

modules/libpref/src/init/all.js

@@ -3936,6 +3936,9 @@ pref("image.multithreaded_decoding.limit", -1);
 // cache.
 pref("canvas.image.cache.limit", 0);
 
+// How many images to eagerly decode on a given page. 0 means "no limit".
+pref("image.onload.decode.limit", 0);
+
 // WebGL prefs
 pref("gl.msaa-level", 2);
 pref("webgl.force-enabled", false);

widget/gonk/GonkMemoryPressureMonitoring.cpp

@@ -26,7 +26,14 @@ namespace {
 
 class MemoryPressureRunnable : public nsRunnable
 {
+  const char *mTopic;
+  const PRUnichar *mData;
 public:
+  MemoryPressureRunnable(const char *aTopic, const PRUnichar *aData) :
+    mTopic(aTopic), mData(aData)
+  {
+  }
+
   NS_IMETHOD Run()
   {
     MOZ_ASSERT(NS_IsMainThread());
@@ -34,15 +41,20 @@ public:
 
     nsCOMPtr<nsIObserverService> os = services::GetObserverService();
     if (os) {
-      // We use low-memory-no-forward because each process has its own watcher
-      // and thus there is no need for the main process to forward this event.
-      os->NotifyObservers(nullptr, "memory-pressure",
-                          NS_LITERAL_STRING("low-memory-no-forward").get());
+      os->NotifyObservers(nullptr, mTopic, mData);
     }
     return NS_OK;
   }
 };
 
+static void
+Dispatch(const char *aTopic, const PRUnichar *aData)
+{
+  nsRefPtr<MemoryPressureRunnable> memoryPressureRunnable =
+    new MemoryPressureRunnable(aTopic, aData);
+  NS_DispatchToMainThread(memoryPressureRunnable);
+}
+
 /**
  * MemoryPressureWatcher watches sysfs from its own thread to notice when the
  * system is under memory pressure.  When we observe memory pressure, we use
@@ -172,13 +184,17 @@ public:
       // sometimes completes after the memory-pressure event is over, so let's
       // just believe the result of poll().
 
-      nsRefPtr<MemoryPressureRunnable> memoryPressureRunnable =
-        new MemoryPressureRunnable();
-      NS_DispatchToMainThread(memoryPressureRunnable);
+      // We use low-memory-no-forward because each process has its own watcher
+      // and thus there is no need for the main process to forward this event.
+      Dispatch("memory-pressure",
+               NS_LITERAL_STRING("low-memory-no-forward").get());
 
       // Manually check lowMemFd until we observe that memory pressure is over.
-      // We won't fire any more memory-pressure events until we observe that
-      // we're no longer under pressure.
+      // We won't fire any more low-memory events until we observe that
+      // we're no longer under pressure. Instead, we fire low-memory-ongoing
+      // events, which cause processes to keep flushing caches but will not
+      // trigger expensive GCs and other attempts to save memory that are
+      // likely futile at this point.
       bool memoryPressure;
       do {
         {
@@ -201,7 +217,13 @@ public:
         LOG("Checking to see if memory pressure is over.");
         rv = CheckForMemoryPressure(lowMemFd, &memoryPressure);
         NS_ENSURE_SUCCESS(rv, rv);
-      } while (memoryPressure);
+
+        if (memoryPressure) {
+          Dispatch("memory-pressure",
+                   NS_LITERAL_STRING("low-memory-ongoing-no-forward").get());
+          continue;
+        }
+      } while (false);
 
       LOG("Memory pressure is over.");
     }

xpcom/base/nsIMemory.idl

@@ -26,9 +26,14 @@
  * This will be passed as the extra data when the pressure 
  * observer is being asked to flush for low-memory conditions.
  *
- * "low-memory-no-forward"
- * This will be passed as the extra data when the pressure observer
- * is being asked to flush for low-memory conditions but the resulting
+ * "low-memory-ongoing"
+ * This will be passed when we continue to be in a low-memory
+ * condition and we want to flush caches and do other cheap
+ * forms of memory minimization, but heavy handed approaches like
+ * a GC are unlikely to succeed.
+ *
+ * "-no-forward"
+ * This is appended to the above two parameters when the resulting
  * notification should not be forwarded to the child processes.
  *
  * "heap-minimize"