Bug 957346: Extend grace period for perceivable background processes. r=fabrice

b2g/app/b2g.js

@@ -619,9 +619,11 @@ pref("ui.useOverlayScrollbars", 1);
 
 // Enable the ProcessPriorityManager, and give processes with no visible
 // documents a 1s grace period before they're eligible to be marked as
-// background.
+// background. Background processes that are perceivable due to playing
+// media are given a longer grace period to accomodate changing tracks, etc.
 pref("dom.ipc.processPriorityManager.enabled", true);
 pref("dom.ipc.processPriorityManager.backgroundGracePeriodMS", 1000);
+pref("dom.ipc.processPriorityManager.backgroundPerceivableGracePeriodMS", 5000);
 pref("dom.ipc.processPriorityManager.temporaryPriorityLockMS", 5000);
 
 // Number of different background levels for background processes.  We use

dom/ipc/ProcessPriorityManager.cpp

@@ -807,7 +807,16 @@ ParticularProcessPriorityManager::ResetPriority()
   ProcessPriority processPriority = ComputePriority();
   if (mPriority == PROCESS_PRIORITY_UNKNOWN ||
       mPriority > processPriority) {
-    ScheduleResetPriority("backgroundGracePeriodMS");
+    // Apps set at a perceivable background priority are often playing media.
+    // Most media will have short gaps while changing tracks between songs,
+    // switching videos, etc.  Give these apps a longer grace period so they
+    // can get their next track started, if there is one, before getting
+    // downgraded.
+    if (mPriority == PROCESS_PRIORITY_BACKGROUND_PERCEIVABLE) {
+      ScheduleResetPriority("backgroundPerceivableGracePeriodMS");
+    } else {
+      ScheduleResetPriority("backgroundGracePeriodMS");
+    }
     return;
   }