/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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/Logging.h"
#include "nsTArray.h"
#include "nsITimer.h"
#include "nsCOMPtr.h"
#include "nsAutoPtr.h"
#include "nsComponentManagerUtils.h"
#include "nsIEventTarget.h"
#include "nsIObserver.h"
#include "nsIObserverService.h"
#include "nsThreadUtils.h"
#include "mozilla/Attributes.h"
#include "mozilla/Services.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;
};
/**
* ExpirationTracker classes:
* - ExpirationTrackerImpl (Thread-safe class)
* - nsExpirationTracker (Main-thread only class)
*
* These classes 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?
*/
/**
* Base class for ExiprationTracker implementations.
*
* nsExpirationTracker class below is a specialized class to be inherited by the
* instances to be accessed only on main-thread.
*
* For creating a thread-safe tracker, you can define a subclass inheriting this
* base class and specialize the Mutex and AutoLock to be used.
*
*/
template <typename T, uint32_t K, typename Mutex, typename AutoLock>
class ExpirationTrackerImpl {
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
* AgeOneGenerationLocked explicitly.
* @param aName the name of the subclass for telemetry.
* @param aEventTarget the optional event target on main thread to label the
* runnable of the asynchronous invocation to NotifyExpired().
*/
ExpirationTrackerImpl(uint32_t aTimerPeriod, const char* aName,
nsIEventTarget* aEventTarget = nullptr)
: mTimerPeriod(aTimerPeriod),
mNewestGeneration(0),
mInAgeOneGeneration(false),
mName(aName),
mEventTarget(aEventTarget) {
static_assert(K >= 2 && K <= nsExpirationState::NOT_TRACKED,
"Unsupported number of generations (must be 2 <= K <= 15)");
MOZ_ASSERT(NS_IsMainThread());
if (mEventTarget) {
bool current = false;
// NOTE: The following check+crash could be condensed into a
// MOZ_RELEASE_ASSERT, but that triggers a segfault during compilation in
// clang 3.8. Once we don't have to care about clang 3.8 anymore, though,
// we can convert to MOZ_RELEASE_ASSERT here.
if (MOZ_UNLIKELY(NS_FAILED(mEventTarget->IsOnCurrentThread(¤t)) ||
!current)) {
MOZ_CRASH("Provided event target must be on the main thread");
}
}
mObserver = new ExpirationTrackerObserver();
mObserver->Init(this);
}
virtual ~ExpirationTrackerImpl() {
MOZ_ASSERT(NS_IsMainThread());
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 AddObjectLocked(T* aObj, const AutoLock& aAutoLock) {
if (NS_WARN_IF(!aObj)) {
MOZ_DIAGNOSTIC_ASSERT(false, "Invalid object to add");
return NS_ERROR_UNEXPECTED;
}
nsExpirationState* state = aObj->GetExpirationState();
if (NS_WARN_IF(state->IsTracked())) {
MOZ_DIAGNOSTIC_ASSERT(false,
"Tried to add an object that's already tracked");
return NS_ERROR_UNEXPECTED;
}
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 = CheckStartTimerLocked(aAutoLock);
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 RemoveObjectLocked(T* aObj, const AutoLock& aAutoLock) {
if (NS_WARN_IF(!aObj)) {
MOZ_DIAGNOSTIC_ASSERT(false, "Invalid object to remove");
return;
}
nsExpirationState* state = aObj->GetExpirationState();
if (NS_WARN_IF(!state->IsTracked())) {
MOZ_DIAGNOSTIC_ASSERT(false,
"Tried to remove an object that's not tracked");
return;
}
nsTArray<T*>& generation = mGenerations[state->mGeneration];
uint32_t index = state->mIndexInGeneration;
MOZ_ASSERT(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);
MOZ_ASSERT(generation.Length() == 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 MarkUsedLocked(T* aObj, const AutoLock& aAutoLock) {
nsExpirationState* state = aObj->GetExpirationState();
if (mNewestGeneration == state->mGeneration) {
return NS_OK;
}
RemoveObjectLocked(aObj, aAutoLock);
return AddObjectLocked(aObj, aAutoLock);
}
/**
* The timer calls this, but it can also be manually called if you want
* to age objects "artifically". This can result in calls to
* NotifyExpiredLocked.
*/
void AgeOneGenerationLocked(const AutoLock& aAutoLock) {
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 MarkUsedLocked) inside NotifyExpiredLocked.
// 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.
size_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;
NotifyExpiredLocked(generation[index], aAutoLock);
}
// 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 AgeOneGenerationLocked K times. Under normal circumstances
* this will result in all objects getting NotifyExpiredLocked called on them,
* but if NotifyExpiredLocked 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 AgeAllGenerationsLocked(const AutoLock& aAutoLock) {
uint32_t i;
for (i = 0; i < K; ++i) {
AgeOneGenerationLocked(aAutoLock);
}
}
class Iterator {
private:
ExpirationTrackerImpl<T, K, Mutex, AutoLock>* mTracker;
uint32_t mGeneration;
uint32_t mIndex;
public:
Iterator(ExpirationTrackerImpl<T, K, Mutex, AutoLock>* aTracker,
AutoLock& aAutoLock)
: 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 IsEmptyLocked(const AutoLock& aAutoLock) {
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 AgeOneGenerationLocked or AgeAllGenerationsLocked 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 NotifyExpiredLocked is called is
* K*(mTimerPeriod + D).
*
* The NotifyExpiredLocked call is expected to remove the object from the
* tracker, but it need not. The object (or other objects) could be
* "resurrected" by calling MarkUsedLocked() 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 NotifyExpiredLocked(T*, const AutoLock&) = 0;
/**
* This may be overridden to perform any post-aging work that needs to be
* done while still holding the lock. It will be called once after each timer
* event, and each low memory event has been handled.
*/
virtual void NotifyHandlerEndLocked(const AutoLock&){};
/**
* This may be overridden to perform any post-aging work that needs to be
* done outside the lock. It will be called once after each
* NotifyEndTransactionLocked call.
*/
virtual void NotifyHandlerEnd(){};
virtual Mutex& GetMutex() = 0;
private:
class ExpirationTrackerObserver;
RefPtr<ExpirationTrackerObserver> mObserver;
nsTArray<T*> mGenerations[K];
nsCOMPtr<nsITimer> mTimer;
uint32_t mTimerPeriod;
uint32_t mNewestGeneration;
bool mInAgeOneGeneration;
const char* const mName; // Used for timer firing profiling.
const nsCOMPtr<nsIEventTarget> mEventTarget;
/**
* Whenever "memory-pressure" is observed, it calls AgeAllGenerationsLocked()
* to minimize memory usage.
*/
class ExpirationTrackerObserver final : public nsIObserver {
public:
void Init(ExpirationTrackerImpl<T, K, Mutex, AutoLock>* aObj) {
mOwner = aObj;
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:
ExpirationTrackerImpl<T, K, Mutex, AutoLock>* mOwner;
};
void HandleLowMemory() {
{
AutoLock lock(GetMutex());
AgeAllGenerationsLocked(lock);
NotifyHandlerEndLocked(lock);
}
NotifyHandlerEnd();
}
void HandleTimeout() {
{
AutoLock lock(GetMutex());
AgeOneGenerationLocked(lock);
// Cancel the timer if we have no objects to track
if (IsEmptyLocked(lock)) {
mTimer->Cancel();
mTimer = nullptr;
}
NotifyHandlerEndLocked(lock);
}
NotifyHandlerEnd();
}
static void TimerCallback(nsITimer* aTimer, void* aThis) {
ExpirationTrackerImpl* tracker = static_cast<ExpirationTrackerImpl*>(aThis);
tracker->HandleTimeout();
}
nsresult CheckStartTimerLocked(const AutoLock& aAutoLock) {
if (mTimer || !mTimerPeriod) {
return NS_OK;
}
nsCOMPtr<nsIEventTarget> target = mEventTarget;
if (!target && !NS_IsMainThread()) {
// TimerCallback should always be run on the main thread to prevent races
// to the destruction of the tracker.
target = do_GetMainThread();
NS_ENSURE_STATE(target);
}
return NS_NewTimerWithFuncCallback(
getter_AddRefs(mTimer), TimerCallback, this, mTimerPeriod,
nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY, mName, target);
}
};
namespace detail {
class PlaceholderLock {
public:
void Lock() {}
void Unlock() {}
};
class PlaceholderAutoLock {
public:
explicit PlaceholderAutoLock(PlaceholderLock&) {}
~PlaceholderAutoLock() = default;
};
template <typename T, uint32_t K>
using SingleThreadedExpirationTracker =
ExpirationTrackerImpl<T, K, PlaceholderLock, PlaceholderAutoLock>;
} // namespace detail
template <typename T, uint32_t K>
class nsExpirationTracker
: protected ::detail::SingleThreadedExpirationTracker<T, K> {
typedef ::detail::PlaceholderLock Lock;
typedef ::detail::PlaceholderAutoLock AutoLock;
Lock mLock;
AutoLock FakeLock() {
MOZ_DIAGNOSTIC_ASSERT(NS_IsMainThread());
return AutoLock(mLock);
}
Lock& GetMutex() override {
MOZ_DIAGNOSTIC_ASSERT(NS_IsMainThread());
return mLock;
}
void NotifyExpiredLocked(T* aObject, const AutoLock&) override {
NotifyExpired(aObject);
}
/**
* Since there are no users of these callbacks in the single threaded case,
* we mark them as final with the hope that the compiler can optimize the
* method calls out entirely.
*/
void NotifyHandlerEndLocked(const AutoLock&) final {}
void NotifyHandlerEnd() final {}
protected:
virtual void NotifyExpired(T* aObj) = 0;
public:
nsExpirationTracker(uint32_t aTimerPeriod, const char* aName,
nsIEventTarget* aEventTarget = nullptr)
: ::detail::SingleThreadedExpirationTracker<T, K>(aTimerPeriod, aName,
aEventTarget) {}
virtual ~nsExpirationTracker() {}
nsresult AddObject(T* aObj) {
return this->AddObjectLocked(aObj, FakeLock());
}
void RemoveObject(T* aObj) { this->RemoveObjectLocked(aObj, FakeLock()); }
nsresult MarkUsed(T* aObj) { return this->MarkUsedLocked(aObj, FakeLock()); }
void AgeOneGeneration() { this->AgeOneGenerationLocked(FakeLock()); }
void AgeAllGenerations() { this->AgeAllGenerationsLocked(FakeLock()); }
class Iterator {
private:
AutoLock mAutoLock;
typename ExpirationTrackerImpl<T, K, Lock, AutoLock>::Iterator mIterator;
public:
explicit Iterator(nsExpirationTracker<T, K>* aTracker)
: mAutoLock(aTracker->GetMutex()), mIterator(aTracker, mAutoLock) {}
T* Next() { return mIterator.Next(); }
};
friend class Iterator;
bool IsEmpty() { return this->IsEmptyLocked(FakeLock()); }
};
template <typename T, uint32_t K, typename Mutex, typename AutoLock>
NS_IMETHODIMP ExpirationTrackerImpl<T, K, Mutex, AutoLock>::
ExpirationTrackerObserver::Observe(nsISupports* aSubject,
const char* aTopic,
const char16_t* aData) {
if (!strcmp(aTopic, "memory-pressure") && mOwner) {
mOwner->HandleLowMemory();
}
return NS_OK;
}
template <class T, uint32_t K, typename Mutex, typename AutoLock>
NS_IMETHODIMP_(MozExternalRefCountType)
ExpirationTrackerImpl<T, K, Mutex, AutoLock>::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, typename Mutex, typename AutoLock>
NS_IMETHODIMP_(MozExternalRefCountType)
ExpirationTrackerImpl<T, K, Mutex,
AutoLock>::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, typename Mutex, typename AutoLock>
NS_IMETHODIMP ExpirationTrackerImpl<T, K, Mutex, AutoLock>::
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_*/