Purge once random minutes(between 30min and 60min) after backgrounded.

chrome/browser/memory/tab_manager.h

Index: chrome/browser/memory/tab_manager.h
diff --git a/chrome/browser/memory/tab_manager.h b/chrome/browser/memory/tab_manager.h
index c384b7e054b07fa3eb20ad6b794bd748fcaf5665..ecc23053eba1797a40a22ada94b595b556cb5f47 100644
--- a/chrome/browser/memory/tab_manager.h
+++ b/chrome/browser/memory/tab_manager.h
@@ -147,7 +147,9 @@ class TabManager : public TabStripModelObserver {
   // Sets/clears the auto-discardable state of the tab.
   void SetTabAutoDiscardableState(content::WebContents* contents, bool state);
 
-  // Returns true when a given renderer can suspend when it is backgrounded.
+  // Returns true when a given renderer can be purged if the specified
+  // renderer is eligible for purging.
+  // TODO(tasak): rename this to CanPurgeBackgroundedRenderer.
   bool CanSuspendBackgroundedRenderer(int render_process_id) const;
 
   // Returns true if |first| is considered less desirable to be killed than
@@ -159,9 +161,10 @@ class TabManager : public TabStripModelObserver {
   static int64_t IdFromWebContents(content::WebContents* web_contents);
 
  private:
-  FRIEND_TEST_ALL_PREFIXES(TabManagerTest,
-                           ActivateTabResetPurgeAndSuspendState);
-  FRIEND_TEST_ALL_PREFIXES(TabManagerTest, NextPurgeAndSuspendState);
+  FRIEND_TEST_ALL_PREFIXES(TabManagerTest, PurgeBackgroundRenderer);
+  FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ActivateTabResetPurgeState);
+  FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ShouldPurgeAtDefaultTime);
+  FRIEND_TEST_ALL_PREFIXES(TabManagerTest, DefaultTimeToPurgeInCorrectRange);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, AutoDiscardable);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, CanOnlyDiscardOnce);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ChildProcessNotifications);
@@ -176,18 +179,26 @@ class TabManager : public TabStripModelObserver {
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ProtectVideoTabs);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, ReloadDiscardedTabContextMenu);
   FRIEND_TEST_ALL_PREFIXES(TabManagerTest, TabManagerBasics);
-  FRIEND_TEST_ALL_PREFIXES(TabManagerWebContentsDataTest, PurgeAndSuspendState);
 
   // The time of the first purging after a renderer is backgrounded.
   // The initial value was chosen because most of users activate backgrounded
   // tabs within 30 minutes. (c.f. Tabs.StateTransfer.Time_Inactive_Active)
-  static constexpr base::TimeDelta kDefaultTimeToFirstPurge =
+  static constexpr base::TimeDelta kDefaultMinTimeToPurge =
       base::TimeDelta::FromMinutes(30);
 
+  // The min/max time to purge ratio. The max time to purge is set to be
+  // min time to purge times this value.
+  const int kMinMaxTimeToPurgeRatio = 2;
+
   // This is needed so WebContentsData can call OnDiscardedStateChange, and
-  // can use PurgeAndSuspendState.
+  // can use PurgeState.
   friend class WebContentsData;
 
+  // Finds TabStripModel which has a WebContents whose id is the given
+  // web_contents_id, and returns the WebContents index and the TabStripModel.
+  int FindTabStripModelById(int64_t target_web_contents_id,
+                            TabStripModel** model) const;
+
   // Called by WebContentsData whenever the discard state of a WebContents
   // changes, so that observers can be informed.
   void OnDiscardedStateChange(content::WebContents* contents,
@@ -258,38 +269,27 @@ class TabManager : public TabStripModelObserver {
   // that need to be run periodically (see comment in implementation).
   void UpdateTimerCallback();
 
-  // Initially PurgeAndSuspendState is RUNNING.
-  // RUNNING => SUSPENDED
-  // - A tab has been backgrounded for more than purge-and-suspend-time
-  //   seconds.
-  // SUSPENDED => RESUMED
-  // - A suspended tab is still suspended (i.e. last active time < last
-  //   purge-and-suspend modified time), and
-  // - kMaxTimeRendererAllowedToBeSuspendedBeforeResume time passes since
-  //   since the tab was suspended.
-  // RESUMED => SUSPENDED
-  // - A resumed tab is still backgrounded (i.e. last active time < last
-  //   purge-and-suspend modified time), and
-  // - kSuspendedRendererLengthOfResumption time passes since the tab was
-  //   resumed.
-  // SUSPENDED, RESUMED, RUNNING => RUNNING
-  // - When ActiveTabChaged, the newly activated tab's state will be RUNNING.
-  enum PurgeAndSuspendState {
-    RUNNING,
-    RESUMED,
-    SUSPENDED,
-  };
   // Returns WebContents whose contents id matches the given tab_contents_id.
   content::WebContents* GetWebContentsById(int64_t tab_contents_id) const;
 
-  // Returns the next state of the purge and suspend.
-  PurgeAndSuspendState GetNextPurgeAndSuspendState(
-      content::WebContents* content,
-      base::TimeTicks current_time,
-      const base::TimeDelta& time_to_first_suspension) const;
-
-  // Purges and suspends renderers in backgrounded tabs.
-  void PurgeAndSuspendBackgroundedTabs();
+  // Returns a random time-to-purge whose min value is min_time_to_purge and max
+  // value is min_time_to_purge * kMinMaxTimeToPurgeRatio.
+  base::TimeDelta GetTimeToPurge(base::TimeDelta min_time_to_purge) const;
+
+  // Returns true if the tab specified by |content| is now eligible to have
+  // its memory purged.
+  bool ShouldPurgeNow(content::WebContents* content) const;
+
+  // Purges renderers in backgrounded tabs if the following conditions are
+  // satisfied:
+  // - the renderers are not purged yet,
+  // - the renderers are not playing media,
+  //   (CanPurgeBackgroundedRenderer returns true)
+  // - the renderers are left inactive and background for time-to-purge.
+  // If renderers are purged, their internal states become 'purged'.
+  // The state is reset to be 'not purged' only when they are activated
+  // (=ActiveTabChanged is invoked).
+  void PurgeBackgroundedTabsIfNeeded();
 
   // Does the actual discard by destroying the WebContents in |model| at |index|
   // and replacing it by an empty one. Returns the new WebContents or NULL if
@@ -372,8 +372,8 @@ class TabManager : public TabStripModelObserver {
   // backgrounded.
   base::TimeDelta minimum_protection_time_;
 
-  // A backgrounded renderer will be suspended when this time passes.
-  base::TimeDelta time_to_first_suspension_;
+  // A backgrounded renderer will be purged when this time passes.
+  base::TimeDelta min_time_to_purge_;
 
 #if defined(OS_CHROMEOS)
   std::unique_ptr<TabManagerDelegate> delegate_;