The Tor Browser: xpcom/ds/nsExpirationTracker.h@b8a032363ba2 (annotated)

xpcom/ds/nsExpirationTracker.h@b8a032363ba2 (annotated)

xpcom/ds/nsExpirationTracker.h

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #ifndef NSEXPIRATIONTRACKER_H_
 #define NSEXPIRATIONTRACKER_H_
 #include "mozilla/Attributes.h"
 #include "prlog.h"
 #include "nsTArray.h"
 #include "nsITimer.h"
 #include "nsCOMPtr.h"
 #include "nsAutoPtr.h"
 #include "nsComponentManagerUtils.h"
 #include "nsIObserver.h"
 #include "nsIObserverService.h"
 #include "mozilla/Services.h"
 #include "mozilla/Attributes.h"
 /**
  * Data used to track the expiration state of an object. We promise that this
  * is 32 bits so that objects that includes this as a field can pad and align
  * efficiently.
  */
 struct nsExpirationState {
   enum { NOT_TRACKED = (1U << 4) - 1,
          MAX_INDEX_IN_GENERATION = (1U << 28) - 1 };
   nsExpirationState() : mGeneration(NOT_TRACKED) {}
   bool IsTracked() { return mGeneration != NOT_TRACKED; }
   /**
    * The generation that this object belongs to, or NOT_TRACKED.
    */
   uint32_t mGeneration:4;
   uint32_t mIndexInGeneration:28;
 };
 /**
  * nsExpirationTracker can track the lifetimes and usage of a large number of
  * objects, and send a notification some window of time after a live object was
  * last used. This is very useful when you manage a large number of objects
  * and want to flush some after they haven't been used for a while.
  * nsExpirationTracker is designed to be very space and time efficient.
  *
  * The type parameter T is the object type that we will track pointers to. T
  * must include an accessible method GetExpirationState() that returns a
  * pointer to an nsExpirationState associated with the object (preferably,
  * stored in a field of the object).
  *
  * The parameter K is the number of generations that will be used. Increasing
  * the number of generations narrows the window within which we promise
  * to fire notifications, at a slight increase in space cost for the tracker.
  * We require 2 <= K <= nsExpirationState::NOT_TRACKED (currently 15).
  *
  * To use this class, you need to inherit from it and override the
  * NotifyExpired() method.
  *
  * The approach is to track objects in K generations. When an object is accessed
  * it moves from its current generation to the newest generation. Generations
  * are stored in a cyclic array; when a timer interrupt fires, we advance
  * the current generation pointer to effectively age all objects very efficiently.
  * By storing information in each object about its generation and index within its
  * generation array, we make removal of objects from a generation very cheap.
  *
  * Future work:
  * -- Add a method to change the timer period?
  */
 template <class T, uint32_t K> class nsExpirationTracker {
   public:
     /**
      * Initialize the tracker.
      * @param aTimerPeriod the timer period in milliseconds. The guarantees
      * provided by the tracker are defined in terms of this period. If the
      * period is zero, then we don't use a timer and rely on someone calling
      * AgeOneGeneration explicitly.
      */
     nsExpirationTracker(uint32_t aTimerPeriod)
       : mTimerPeriod(aTimerPeriod), mNewestGeneration(0),
         mInAgeOneGeneration(false) {
       static_assert(K >= 2 && K <= nsExpirationState::NOT_TRACKED,
                     "Unsupported number of generations (must be 2 <= K <= 15)");
       mObserver = new ExpirationTrackerObserver();
       mObserver->Init(this);
     }
     ~nsExpirationTracker() {
       if (mTimer) {
         mTimer->Cancel();
       }
       mObserver->Destroy();
     }
     /**
      * Add an object to be tracked. It must not already be tracked. It will
      * be added to the newest generation, i.e., as if it was just used.
      * @return an error on out-of-memory
      */
     nsresult AddObject(T* aObj) {
       nsExpirationState* state = aObj->GetExpirationState();
       NS_ASSERTION(!state->IsTracked(), "Tried to add an object that's already tracked");
       nsTArray<T*>& generation = mGenerations[mNewestGeneration];
       uint32_t index = generation.Length();
       if (index > nsExpirationState::MAX_INDEX_IN_GENERATION) {
         NS_WARNING("More than 256M elements tracked, this is probably a problem");
         return NS_ERROR_OUT_OF_MEMORY;
       }
       if (index == 0) {
         // We might need to start the timer
         nsresult rv = CheckStartTimer();
         if (NS_FAILED(rv))
           return rv;
       }
       if (!generation.AppendElement(aObj))
         return NS_ERROR_OUT_OF_MEMORY;
       state->mGeneration = mNewestGeneration;
       state->mIndexInGeneration = index;
       return NS_OK;
     }
     /**
      * Remove an object from the tracker. It must currently be tracked.
      */
     void RemoveObject(T* aObj) {
       nsExpirationState* state = aObj->GetExpirationState();
       NS_ASSERTION(state->IsTracked(), "Tried to remove an object that's not tracked");
       nsTArray<T*>& generation = mGenerations[state->mGeneration];
       uint32_t index = state->mIndexInGeneration;
       NS_ASSERTION(generation.Length() > index &&
                    generation[index] == aObj, "Object is lying about its index");
       // Move the last object to fill the hole created by removing aObj
       uint32_t last = generation.Length() - 1;
       T* lastObj = generation[last];
       generation[index] = lastObj;
       lastObj->GetExpirationState()->mIndexInGeneration = index;
       generation.RemoveElementAt(last);
       state->mGeneration = nsExpirationState::NOT_TRACKED;
       // We do not check whether we need to stop the timer here. The timer
       // will check that itself next time it fires. Checking here would not
       // be efficient since we'd need to track all generations. Also we could
       // thrash by incessantly creating and destroying timers if someone
       // kept adding and removing an object from the tracker.
     }
     /**
      * Notify that an object has been used.
      * @return an error if we lost the object from the tracker...
      */
     nsresult MarkUsed(T* aObj) {
       nsExpirationState* state = aObj->GetExpirationState();
       if (mNewestGeneration == state->mGeneration)
         return NS_OK;
       RemoveObject(aObj);
       return AddObject(aObj);
     }
     /**
      * The timer calls this, but it can also be manually called if you want
      * to age objects "artifically". This can result in calls to NotifyExpired.
      */
     void AgeOneGeneration() {
       if (mInAgeOneGeneration) {
         NS_WARNING("Can't reenter AgeOneGeneration from NotifyExpired");
         return;
       }
       mInAgeOneGeneration = true;
       uint32_t reapGeneration =
         mNewestGeneration > 0 ? mNewestGeneration - 1 : K - 1;
       nsTArray<T*>& generation = mGenerations[reapGeneration];
       // The following is rather tricky. We have to cope with objects being
       // removed from this generation either because of a call to RemoveObject
       // (or indirectly via MarkUsed) inside NotifyExpired. Fortunately no
       // objects can be added to this generation because it's not the newest
       // generation. We depend on the fact that RemoveObject can only cause
       // the indexes of objects in this generation to *decrease*, not increase.
       // So if we start from the end and work our way backwards we are guaranteed
       // to see each object at least once.
       uint32_t index = generation.Length();
       for (;;) {
         // Objects could have been removed so index could be outside
         // the array
         index = XPCOM_MIN(index, generation.Length());
         if (index == 0)
           break;
         --index;
         NotifyExpired(generation[index]);
       }
       // Any leftover objects from reapGeneration just end up in the new
       // newest-generation. This is bad form, though, so warn if there are any.
       if (!generation.IsEmpty()) {
         NS_WARNING("Expired objects were not removed or marked used");
       }
       // Free excess memory used by the generation array, since we probably
       // just removed most or all of its elements.
       generation.Compact();
       mNewestGeneration = reapGeneration;
       mInAgeOneGeneration = false;
     }
     /**
      * This just calls AgeOneGeneration K times. Under normal circumstances this
      * will result in all objects getting NotifyExpired called on them, but
      * if NotifyExpired itself marks some objects as used, then those objects
      * might not expire. This would be a good thing to call if we get into
      * a critically-low memory situation.
      */
     void AgeAllGenerations() {
       uint32_t i;
       for (i = 0; i < K; ++i) {
         AgeOneGeneration();
       }
     }
     class Iterator {
     private:
       nsExpirationTracker<T,K>* mTracker;
       uint32_t                  mGeneration;
       uint32_t                  mIndex;
     public:
       Iterator(nsExpirationTracker<T,K>* aTracker)
         : mTracker(aTracker), mGeneration(0), mIndex(0) {}
       T* Next() {
         while (mGeneration < K) {
           nsTArray<T*>* generation = &mTracker->mGenerations[mGeneration];
           if (mIndex < generation->Length()) {
             ++mIndex;
             return (*generation)[mIndex - 1];
           }
           ++mGeneration;
           mIndex = 0;
         }
         return nullptr;
       }
     };
     friend class Iterator;
     bool IsEmpty() {
       for (uint32_t i = 0; i < K; ++i) {
         if (!mGenerations[i].IsEmpty())
           return false;
       }
       return true;
     }
   protected:
     /**
      * This must be overridden to catch notifications. It is called whenever
      * we detect that an object has not been used for at least (K-1)*mTimerPeriod
      * milliseconds. If timer events are not delayed, it will be called within
      * roughly K*mTimerPeriod milliseconds after the last use. (Unless AgeOneGeneration
      * or AgeAllGenerations have been called to accelerate the aging process.)
      *
      * NOTE: These bounds ignore delays in timer firings due to actual work being
      * performed by the browser. We use a slack timer so there is always at least
      * mTimerPeriod milliseconds between firings, which gives us (K-1)*mTimerPeriod
      * as a pretty solid lower bound. The upper bound is rather loose, however.
      * If the maximum amount by which any given timer firing is delayed is D, then
      * the upper bound before NotifyExpired is called is K*(mTimerPeriod + D).
      *
      * The NotifyExpired call is expected to remove the object from the tracker,
      * but it need not. The object (or other objects) could be "resurrected"
      * by calling MarkUsed() on them, or they might just not be removed.
      * Any objects left over that have not been resurrected or removed
      * are placed in the new newest-generation, but this is considered "bad form"
      * and should be avoided (we'll issue a warning). (This recycling counts
      * as "a use" for the purposes of the expiry guarantee above...)
      *
      * For robustness and simplicity, we allow objects to be notified more than
      * once here in the same timer tick.
      */
     virtual void NotifyExpired(T* aObj) = 0;
   private:
     class ExpirationTrackerObserver;
     nsRefPtr<ExpirationTrackerObserver> mObserver;
     nsTArray<T*>       mGenerations[K];
     nsCOMPtr<nsITimer> mTimer;
     uint32_t           mTimerPeriod;
     uint32_t           mNewestGeneration;
     bool               mInAgeOneGeneration;
     /**
      * Whenever "memory-pressure" is observed, it calls AgeAllGenerations()
      * to minimize memory usage.
      */
     class ExpirationTrackerObserver MOZ_FINAL : public nsIObserver {
     public:
       void Init(nsExpirationTracker<T,K> *obj) {
         mOwner = obj;
         nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
         if (obs) {
           obs->AddObserver(this, "memory-pressure", false);
         }
       }
       void Destroy() {
         mOwner = nullptr;
         nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
         if (obs)
           obs->RemoveObserver(this, "memory-pressure");
       }
       NS_DECL_ISUPPORTS
       NS_DECL_NSIOBSERVER
     private:
       nsExpirationTracker<T,K> *mOwner;
     };
     static void TimerCallback(nsITimer* aTimer, void* aThis) {
       nsExpirationTracker* tracker = static_cast<nsExpirationTracker*>(aThis);
       tracker->AgeOneGeneration();
       // Cancel the timer if we have no objects to track
       if (tracker->IsEmpty()) {
         tracker->mTimer->Cancel();
         tracker->mTimer = nullptr;
       }
     }
     nsresult CheckStartTimer() {
       if (mTimer || !mTimerPeriod)
         return NS_OK;
       mTimer = do_CreateInstance("@mozilla.org/timer;1");
       if (!mTimer)
         return NS_ERROR_OUT_OF_MEMORY;
       mTimer->InitWithFuncCallback(TimerCallback, this, mTimerPeriod,
                                    nsITimer::TYPE_REPEATING_SLACK);
       return NS_OK;
     }
 };
 template<class T, uint32_t K>
 NS_IMETHODIMP
 nsExpirationTracker<T, K>::ExpirationTrackerObserver::Observe(nsISupports     *aSubject,
                                                               const char      *aTopic,
                                                               const char16_t *aData)
 {
   if (!strcmp(aTopic, "memory-pressure") && mOwner)
     mOwner->AgeAllGenerations();
   return NS_OK;
 }
 template <class T, uint32_t K>
 NS_IMETHODIMP_(MozExternalRefCountType)
 nsExpirationTracker<T,K>::ExpirationTrackerObserver::AddRef(void)
 {
   MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
   NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
   ++mRefCnt;
   NS_LOG_ADDREF(this, mRefCnt, "ExpirationTrackerObserver", sizeof(*this));
   return mRefCnt;
 }
 template <class T, uint32_t K>
 NS_IMETHODIMP_(MozExternalRefCountType)
 nsExpirationTracker<T,K>::ExpirationTrackerObserver::Release(void)
 {
   MOZ_ASSERT(int32_t(mRefCnt) > 0, "dup release");
   NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
   --mRefCnt;
   NS_LOG_RELEASE(this, mRefCnt, "ExpirationTrackerObserver");
   if (mRefCnt == 0) {
     NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
     mRefCnt = 1; /* stabilize */
     delete (this);
     return 0;
   }
   return mRefCnt;
 }
 template <class T, uint32_t K>
 NS_IMETHODIMP
 nsExpirationTracker<T,K>::ExpirationTrackerObserver::QueryInterface(REFNSIID aIID,
                                                                     void** aInstancePtr)
 {
   NS_ASSERTION(aInstancePtr,
                "QueryInterface requires a non-NULL destination!");
   nsresult rv = NS_ERROR_FAILURE;
   NS_INTERFACE_TABLE(ExpirationTrackerObserver, nsIObserver)
   return rv;
 }
 #endif /*NSEXPIRATIONTRACKER_H_*/

The Tor Browser / annotate

xpcom/ds/nsExpirationTracker.h@b8a032363ba2 (annotated)

xpcom/ds/nsExpirationTracker.h