gecko-dev/xpcom/ds/nsExpirationTracker.h

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/* -*- 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.
*/
explicit 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.
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;
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:
explicit 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>* 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:
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_*/