/// <summary>
/// This method is executed on a dedicated a timer thread. Its purpose is
/// to handle timer requests and notify the TimerQueue when a timer expires.
/// </summary>
private static void TimerThread()
{
AutoResetEvent timerEvent = s_timerEvent;
List <TimerQueue> timersToFire = s_scheduledTimersToFire !;
List <TimerQueue> timers;
lock (timerEvent)
{
timers = s_scheduledTimers !;
}
int shortestWaitDurationMs = Timeout.Infinite;
while (true)
{
timerEvent.WaitOne(shortestWaitDurationMs);
int currentTimeMs = TickCount;
shortestWaitDurationMs = int.MaxValue;
lock (timerEvent)
{
for (int i = timers.Count - 1; i >= 0; --i)
{
TimerQueue timer = timers[i];
int waitDurationMs = timer._scheduledDueTimeMs - currentTimeMs;
if (waitDurationMs <= 0)
{
timer._isScheduled = false;
timersToFire.Add(timer);
int lastIndex = timers.Count - 1;
if (i != lastIndex)
{
timers[i] = timers[lastIndex];
}
timers.RemoveAt(lastIndex);
continue;
}
if (waitDurationMs < shortestWaitDurationMs)
{
shortestWaitDurationMs = waitDurationMs;
}
}
}
if (timersToFire.Count > 0)
{
foreach (TimerQueue timerToFire in timersToFire)
{
ThreadPool.UnsafeQueueUserWorkItemInternal(timerToFire, preferLocal: false);
}
timersToFire.Clear();
}
if (shortestWaitDurationMs == int.MaxValue)
{
shortestWaitDurationMs = Timeout.Infinite;
}
}
}
// Fire any timers that have expired, and update the native timer to schedule the rest of them.
// We're in a thread pool work item here, and if there are multiple timers to be fired, we want
// to queue all but the first one. The first may can then be invoked synchronously or queued,
// a task left up to our caller, which might be firing timers from multiple queues.
private void FireNextTimers()
{
// We fire the first timer on this thread; any other timers that need to be fired
// are queued to the ThreadPool.
TimerQueueTimer?timerToFireOnThisThread = null;
lock (this)
{
// Since we got here, that means our previous timer has fired.
_isTimerScheduled = false;
bool haveTimerToSchedule = false;
uint nextTimerDuration = uint.MaxValue;
long nowTicks = TickCount64;
// Sweep through the "short" timers. If the current tick count is greater than
// the current threshold, also sweep through the "long" timers. Finally, as part
// of sweeping the long timers, move anything that'll fire within the next threshold
// to the short list. It's functionally ok if more timers end up in the short list
// than is truly necessary (but not the opposite).
TimerQueueTimer?timer = _shortTimers;
for (int listNum = 0; listNum < 2; listNum++) // short == 0, long == 1
{
while (timer != null)
{
Debug.Assert(timer._dueTime != Timeout.UnsignedInfinite, "A timer in the list must have a valid due time.");
// Save off the next timer to examine, in case our examination of this timer results
// in our deleting or moving it; we'll continue after with this saved next timer.
TimerQueueTimer?next = timer._next;
long elapsed = nowTicks - timer._startTicks;
long remaining = timer._dueTime - elapsed;
if (remaining <= 0)
{
// Timer is ready to fire.
if (timer._period != Timeout.UnsignedInfinite)
{
// This is a repeating timer; schedule it to run again.
// Discount the extra amount of time that has elapsed since the previous firing time to
// prevent timer ticks from drifting. If enough time has already elapsed for the timer to fire
// again, meaning the timer can't keep up with the short period, have it fire 1 ms from now to
// avoid spinning without a delay.
timer._startTicks = nowTicks;
long elapsedForNextDueTime = elapsed - timer._dueTime;
timer._dueTime = (elapsedForNextDueTime < timer._period) ?
timer._period - (uint)elapsedForNextDueTime :
1;
// Update the timer if this becomes the next timer to fire.
if (timer._dueTime < nextTimerDuration)
{
haveTimerToSchedule = true;
nextTimerDuration = timer._dueTime;
}
// Validate that the repeating timer is still on the right list. It's likely that
// it started in the long list and was moved to the short list at some point, so
// we now want to move it back to the long list if that's where it belongs. Note that
// if we're currently processing the short list and move it to the long list, we may
// end up revisiting it again if we also enumerate the long list, but we will have already
// updated the due time appropriately so that we won't fire it again (it's also possible
// but rare that we could be moving a timer from the long list to the short list here,
// if the initial due time was set to be long but the timer then had a short period).
bool targetShortList = (nowTicks + timer._dueTime) - _currentAbsoluteThreshold <= 0;
if (timer._short != targetShortList)
{
MoveTimerToCorrectList(timer, targetShortList);
}
}
else
{
// Not repeating; remove it from the queue
DeleteTimer(timer);
}
// If this is the first timer, we'll fire it on this thread (after processing
// all others). Otherwise, queue it to the ThreadPool.
if (timerToFireOnThisThread == null)
{
timerToFireOnThisThread = timer;
}
else
{
ThreadPool.UnsafeQueueUserWorkItemInternal(timer, preferLocal: false);
}
}
else
{
// This timer isn't ready to fire. Update the next time the native timer fires if necessary,
// and move this timer to the short list if its remaining time is now at or under the threshold.
if (remaining < nextTimerDuration)
{
haveTimerToSchedule = true;
nextTimerDuration = (uint)remaining;
}
if (!timer._short && remaining <= ShortTimersThresholdMilliseconds)
{
MoveTimerToCorrectList(timer, shortList: true);
}
}
timer = next;
}
// Switch to process the long list if necessary.
if (listNum == 0)
{
// Determine how much time remains between now and the current threshold. If time remains,
// we can skip processing the long list. We use > rather than >= because, although we
// know that if remaining == 0 no timers in the long list will need to be fired, we
// don't know without looking at them when we'll need to call FireNextTimers again. We
// could in that case just set the next firing to 1, but we may as well just iterate the
// long list now; otherwise, most timers created in the interim would end up in the long
// list and we'd likely end up paying for another invocation of FireNextTimers that could
// have been delayed longer (to whatever is the current minimum in the long list).
long remaining = _currentAbsoluteThreshold - nowTicks;
if (remaining > 0)
{
if (_shortTimers == null && _longTimers != null)
{
// We don't have any short timers left and we haven't examined the long list,
// which means we likely don't have an accurate nextTimerDuration.
// But we do know that nothing in the long list will be firing before or at _currentAbsoluteThreshold,
// so we can just set nextTimerDuration to the difference between then and now.
nextTimerDuration = (uint)remaining + 1;
haveTimerToSchedule = true;
}
break;
}
// Switch to processing the long list.
timer = _longTimers;
// Now that we're going to process the long list, update the current threshold.
_currentAbsoluteThreshold = nowTicks + ShortTimersThresholdMilliseconds;
}
}
// If we still have scheduled timers, update the timer to ensure it fires
// in time for the next one in line.
if (haveTimerToSchedule)
{
EnsureTimerFiresBy(nextTimerDuration);
}
}
// Fire the user timer outside of the lock!
timerToFireOnThisThread?.Fire();
}