Bug 797239 - Defer scheduling prelaunch app until child goes idle. r=cjones, a=blocking-basecamp

dom/ipc/ContentChild.cpp

@@ -470,10 +470,21 @@ ContentChild::AllocPImageBridge(mozilla::ipc::Transport* aTransport,
     return ImageBridgeChild::StartUpInChildProcess(aTransport, aOtherProcess);
 }
 
+static void FirstIdle(void)
+{
+    ContentChild::GetSingleton()->SendFirstIdle();
+}
+
 PBrowserChild*
 ContentChild::AllocPBrowser(const uint32_t& aChromeFlags,
                             const bool& aIsBrowserElement, const AppId& aApp)
 {
+    static bool firstIdleTaskPosted = false;
+    if (!firstIdleTaskPosted) {
+        MessageLoop::current()->PostIdleTask(FROM_HERE, NewRunnableFunction(FirstIdle));
+        firstIdleTaskPosted = true;
+    }
+
     nsRefPtr<TabChild> child =
         TabChild::Create(aChromeFlags, aIsBrowserElement, aApp.get_uint32_t());
     // The ref here is released below.

dom/ipc/ContentParent.cpp

@@ -227,10 +227,17 @@ ContentParent::MaybeTakePreallocatedAppProcess()
 {
     nsRefPtr<ContentParent> process = sPreallocatedAppProcess.get();
     sPreallocatedAppProcess = nullptr;
-    ScheduleDelayedPreallocateAppProcess();
     return process.forget();
 }
 
+/*static*/ void
+ContentParent::FirstIdle(void)
+{
+  // The parent has gone idle for the first time. This would be a good
+  // time to preallocate an app process.
+  ScheduleDelayedPreallocateAppProcess();
+}
+
 /*static*/ void
 ContentParent::StartUp()
 {
@@ -247,7 +254,10 @@ ContentParent::StartUp()
             "dom.ipc.processPrelaunch.delayMs", 1000);
 
         MOZ_ASSERT(!sPreallocateAppProcessTask);
-        ScheduleDelayedPreallocateAppProcess();
+
+        // Let's not slow down the main process initialization. Wait until
+        // the main process goes idle before we preallocate a process
+        MessageLoop::current()->PostIdleTask(FROM_HERE, NewRunnableFunction(FirstIdle));
     }
 }
 
@@ -1003,6 +1013,17 @@ ContentParent::RecvGetShowPasswordSetting(bool* showPassword)
     return true;
 }
 
+bool
+ContentParent::RecvFirstIdle()
+{
+    // When the ContentChild goes idle, it sends us a FirstIdle message
+    // which we use as a good time to prelaunch another process. If we
+    // prelaunch any sooner than this, then we'll be competing with the
+    // child process and slowing it down.
+    ScheduleDelayedPreallocateAppProcess();
+    return true;
+}
+
 NS_IMPL_THREADSAFE_ISUPPORTS3(ContentParent,
                               nsIObserver,
                               nsIThreadObserver,

dom/ipc/ContentParent.h

@@ -144,6 +144,7 @@ private:
     static void DelayedPreallocateAppProcess();
     static void ScheduleDelayedPreallocateAppProcess();
     static already_AddRefed<ContentParent> MaybeTakePreallocatedAppProcess();
+    static void FirstIdle();
 
     // Hide the raw constructor methods since we don't want client code
     // using them.
@@ -308,6 +309,8 @@ private:
 
     virtual bool RecvPrivateDocShellsExist(const bool& aExist);
 
+    virtual bool RecvFirstIdle();
+
     virtual void ProcessingError(Result what) MOZ_OVERRIDE;
 
     GeckoChildProcessHost* mSubprocess;

dom/ipc/PContent.ipdl

@@ -346,6 +346,9 @@ parent:
     // Notify the parent of the presence or absence of private docshells
     PrivateDocShellsExist(bool aExist);
 
+    // Tell the parent that the child has gone idle for the first time
+    async FirstIdle();
+
 both:
      AsyncMessage(nsString aMessage, ClonedMessageData aData);
 };