/* -*- 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 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& 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& 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& 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* mTracker; uint32_t mGeneration; uint32_t mIndex; public: Iterator(nsExpirationTracker* aTracker) : mTracker(aTracker), mGeneration(0), mIndex(0) {} T* Next() { while (mGeneration < K) { nsTArray* 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 mObserver; nsTArray mGenerations[K]; nsCOMPtr 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 *obj) { mOwner = obj; nsCOMPtr obs = mozilla::services::GetObserverService(); if (obs) { obs->AddObserver(this, "memory-pressure", false); } } void Destroy() { mOwner = nullptr; nsCOMPtr obs = mozilla::services::GetObserverService(); if (obs) obs->RemoveObserver(this, "memory-pressure"); } NS_DECL_ISUPPORTS NS_DECL_NSIOBSERVER private: nsExpirationTracker *mOwner; }; static void TimerCallback(nsITimer* aTimer, void* aThis) { nsExpirationTracker* tracker = static_cast(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 NS_IMETHODIMP nsExpirationTracker::ExpirationTrackerObserver::Observe(nsISupports *aSubject, const char *aTopic, const PRUnichar *aData) { if (!strcmp(aTopic, "memory-pressure") && mOwner) mOwner->AgeAllGenerations(); return NS_OK; } template NS_IMETHODIMP_(nsrefcnt) nsExpirationTracker::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 NS_IMETHODIMP_(nsrefcnt) nsExpirationTracker::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 NS_IMETHODIMP nsExpirationTracker::ExpirationTrackerObserver::QueryInterface(REFNSIID aIID, void** aInstancePtr) { NS_ASSERTION(aInstancePtr, "QueryInterface requires a non-NULL destination!"); nsresult rv = NS_ERROR_FAILURE; NS_INTERFACE_TABLE1(ExpirationTrackerObserver, nsIObserver) return rv; } #endif /*NSEXPIRATIONTRACKER_H_*/