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Enforcing process/thread priority in the face of resource starvation

My application must ping a (lan) server every 30 seconds in order to let it know I'm alive.
This works fine, except for when a separate process I do not control uses a very large amount of system resources and hangs the entire machine while it does its thing. As a result, my timer tick is delayed until the other process finishes its heavy lifting, and by this time the server already believes I have died.
The other process is set at low priority, and my process is set at normal priority.
My first attempt used a System.Timers.Timer to initiate the ping.
In a second attempt, I tried to roll my own PriorityTimer, to run on a dedicated high priority thread, but that also seems to have little effect.
public sealed class PriorityTimer : IDisposable
{
private int interval;
private bool active;
private Thread timerThread;
private AutoResetEvent resetEvent;
public event EventHandler Elapsed;
public PriorityTimer(int interval)
{
this.interval = interval;
resetEvent = new AutoResetEvent(false);
}
public bool Enabled
{
get { return active; }
set
{
if (active != value)
{
active = value;
if (value)
{
timerThread = new Thread(Wait);
timerThread.IsBackground = true;
timerThread.Priority = ThreadPriority.Highest;
timerThread.Start();
}
else
{
resetEvent.Set();
}
}
}
}
private void Wait()
{
resetEvent.Reset();
while (!resetEvent.WaitOne(interval))
{
if (Elapsed != null)
{
Elapsed(this, EventArgs.Empty);
}
}
}
public void Dispose()
{
Enabled = false;
}
}
Assuming I can not force the offending process to play nice, what can I do to assure my timer elapses at 30 seconds?

this does seem to happen when huge amounts of memory is paged to disk.
You can either try to keep your own working set in memory, or force the other (rogue) process(es) to consume less. The latter strategy is more reliable.
Create a Windows Job Object and configure its maximum memory usage. Assign the problematic processes into the Job. Jobs are capable of limiting CPU usage and memory. They are meant to "jail" processes.

Any operating system that doesn't enforce real-time constraints will generally miss timer requests by some normal delta under contention. Have you been keeping track of your deltas?
Also, I would ask if you're using a garbage collected language. If you are, pauses will definitely skew timer results.
Finally, what's the timer interval you're using to send the heartbeats? As a general rule of thumb, you should send heartbeats twice every interval to account for timing variances.

Monitor, lock or volatile?

I have a windows service (.NET 4) that periodically processes a queue, for example every 15 minutes. I use a System.Threading.Timer which is set when the service starts to fire a callback every X milliseconds. Typically each run takes seconds and never collides, but what if I could not assume that - then I want the next run to exit at once if processing is in progress.
This is easily solved with lock, volatile bool or a monitor, but what is actually the appropriate to use in this scenario, or simply the preferred option in general?
I've found other posts that answers almost this scenario (like Volatile vs. Interlocked vs. lock) but need some advice on extending this to a Timer example with immediate exit.

You don't need any locks for this, you should just reschedule next timer execution from within the timer delegate. That should ensure 100% no overlaps.
At the end of timer's event handler call timer.Change(nextRunInMilliseconds, Timeout.Infinite), that way the timer will fire only once, after nextRunInMilliseconds.
Example:
//Object that holds timer state, and possible additional data
private class TimerState
{
public Timer Timer { get; set; }
public bool Stop { get; set; }
}
public void Run()
{
var timerState = new TimerState();
//Create the timer but don't start it
timerState.Timer = new Timer(OnTimer, timerState, Timeout.Infinite, Timeout.Infinite);
//Start the timer
timerState.Timer.Change(1000, Timeout.Infinite);
}
public void OnTimer(object state)
{
var timerState = (TimerState) state;
try
{
//Do work
}
finally
{
//Reschedule timer
if (!timerState.Stop)
timerState.Timer.Change(1000, Timeout.Infinite);
}
}

Well, any of them will do the job. Monitor is usually pretty simple to use via lock, but you can't use lock in this case because you need to specify a zero timeout; as such, the simplest approach is probably a CompareExchange:
private int isRunning;
...
if(Interlocked.CompareExchange(ref isRunning, 1, 0) == 0) {
try {
// your work
} finally {
Interlocked.Exchange(ref isRunning, 0);
}
}
to do the same with Monitor is:
private readonly object syncLock = new object();
...
bool lockTaken = false;
try {
Monitor.TryEnter(syncLock, 0, ref lockTaken);
if (lockTaken) {
// your work
}
} finally {
if(lockTaken) Monitor.Exit(syncLock);
}

I think, that if you find that you need to synchronize timer delegate - you are doing it wrong, and Timer is probably not the class you want to use. Imho its better to :
1) either keep the Timer, but increase the interval value to the point, where its safe to assume, that there will be no issues with threading,
2) or remove Timer and use simple Thread instead. You know, something like:
var t = new Thread();
t.Start(() =>
{
while (!_stopEvent.WaitOne(100))
{
..........
}
});

How to correctly implement a custom timer in .Net

I have a requirement for a timer that has the following behaviour:
Millisecond accuracy
I want the tick event handler to only be called once the current tick handler has completed (much like the winforms timer)
I want exceptions on the main UI thread not to be swallowed up by the thread timer so this requires Invoke/Send instead of BeginInvoke/Post
I've played around with CreateTimerQueueTimer and had some success but at the same time had problems with code reentrance and/or locks when deleting the timer.
I decided to create my own timer so that I could get a better idea of what is going on under the hood so that I can fix the locking and reentrance problems. My code seems to work fine leading me to believe that I may as well use it. Does it look sound?
I've put in a check if the timer is deleted to make sure that the deletion is complete before the timer can be created again. Does that look ok?
Note: I should say that I call timeBeginPeriod(1) and timeEndPeriod(1) inorder to achieve the millisecond accuracy.
(The following code is converted from vb.net to c#, so apologies for any missed mess-ups}
ETA: I've found a problem with it. If the timer is running at an interval of 1 millisecond, and I call, say, Change(300), it locks up # while (this.DeleteRequest). This
must be because the TimerLoop is in the this.CallbackDelegate.Invoke(null) call.
public class MyTimer : IDisposable
{
private System.Threading.TimerCallback CallbackDelegate;
private bool DeleteRequest;
private System.Threading.Thread MainThread;
public MyTimer(System.Threading.TimerCallback callBack)
{
this.CallbackDelegate = callBack;
}
public void Create(int interval)
{
while (this.DeleteRequest) {
System.Threading.Thread.Sleep(0);
}
if (this.MainThread != null) {
throw new Exception("");
}
this.MainThread = new System.Threading.Thread(TimerLoop);
// Make sure the thread is automatically killed when the app is closed.
this.MainThread.IsBackground = true;
this.MainThread.Start(interval);
}
public void Change(int interval)
{
// A lock required here?
if (!this.IsRunning()) {
throw new Exception("");
}
this.Delete();
this.Create(interval);
}
public void Delete()
{
this.DeleteRequest = true;
}
public bool IsRunning()
{
return (this.MainThread != null) && this.MainThread.IsAlive;
}
private void TimerLoop(object args)
{
int interval = (int)args;
Stopwatch sw = new Stopwatch();
sw.Start();
do {
if (this.DeleteRequest) {
this.MainThread = null;
this.DeleteRequest = false;
return;
}
long t1 = sw.ElapsedMilliseconds;
// I want to wait until the operation completes, so I use Invoke.
this.CallbackDelegate.Invoke(null);
if (this.DeleteRequest) {
this.MainThread = null;
this.DeleteRequest = false;
return;
}
long t2 = sw.ElapsedMilliseconds;
int temp = Convert.ToInt32(Math.Max(interval - (t2 - t1), 0));
sw.Reset();
if (temp > 0) {
System.Threading.Thread.Sleep(temp);
}
sw.Start();
} while (true);
}
// The dispose method calls this.Delete()
}

I would recommend using p/Invoke and using the timers from Win32's Timer Queues:
http://msdn.microsoft.com/en-us/library/ms686796(v=vs.85).aspx
One should be mindful that the managed CLR environment has a lot of non-determinism built into it, garbage collection, for instance. Just because your timer is has a period of 1 millisecond doesn't mean that that is necessarily what happens.
Also, the documentation doesn't mention it, but the callback invoked by the timer must be pinned in memory and not garbage collectable, via a GCHandle or other construct. When a timer (or timers, if you kill off a timer queue), the callback will be executed one last time. Not sure whether that happens by the internal wait expiring, or by signalling the internal event handle.
Execution of DeleteTimerQueueTimer() and DeleteTimerQueueEx() can be made synchronous, so they won't return until all timers have signalled and invoked their last callback, but doing that would be suboptimal.
If you don't pin the callbacks and prevent them from being garbage-collected, things will go swimmingly...most of the time. You'll encounter random exceptions.
Also, the callback should be smart enough to bail out if the timer is being deleted, lest it make reference to something that's already GC'd.

μTimer would be a better example!
You can find it here # https://stackoverflow.com/questions/15725711/obtaining-microsecond-precision-using-net-without-platform-invoke?noredirect=1#comment22341931_15725711
It provides accurate wait times down to 1µs and possibly lower depending on your NIC!
Let me know if you need anything else!

Waiting for a timer elapsed event to complete before application/service closes/stops

Summary:
Within a Windows service & Console Application I am calling a common library that contains a Timer that periodically triggers an action that takes around 30 seconds to complete. This works fine, however...
When a service stop or application exit is called and the timer is in the ElapsedEventHandler I need the service stop/application exit to wait until the event handler has completed.
I have implemented this functionality by having a Boolean InEvent property that is checked when the timer stop method is called.
While this is functional, the question is: Is this the best way to go about doing this? Is there an alternative approach that may serve this purpose better?
The other issue is that I need to avoid the service stop request failing with a "Service failed to respond to stop request"
This is my implementation
public sealed class TimedProcess : IDisposable
{
static TimedProcess singletonInstance;
bool InEvent;
Timer processTimer;
private TimedProcess()
{
}
public static TimedProcess Instance
{
get
{
if (singletonInstance == null)
{
singletonInstance = new TimedProcess();
}
return singletonInstance;
}
}
public void Start(double interval)
{
this.processTimer = new Timer();
this.processTimer.AutoReset = false;
this.processTimer.Interval = interval;
this.processTimer.Elapsed += new ElapsedEventHandler(this.processTimer_Elapsed);
this.processTimer.Enabled = true;
}
public void Stop()
{
if (processTimer != null)
{
while (InEvent)
{
}
processTimer.Stop();
}
}
void processTimer_Elapsed(object sender, ElapsedEventArgs e)
{
try
{
InEvent = true;
// Do something here that takes ~30 seconds
}
catch
{
}
finally
{
InEvent = false;
processTimer.Enabled = true;
}
}
public void Dispose()
{
if (processTimer != null)
{
Stop();
processTimer.Dispose();
}
}
}
And this is how it is called in the service OnStart / console application main:
TimedProcess.Instance.Start(1000);
This is how it is called in service OnStop and application main (pending keypress):
TimedProcess.Instance.Stop();

Probably the easiest and most reliable way is to use a Monitor. Create an object that the main program and the timer callback can access:
private object _timerLock = new object();
Your main program tries to lock that before shutting down:
// wait for timer process to stop
Monitor.Enter(_timerLock);
// do shutdown tasks here
And your timer callback locks it, too:
void processTimer_Elapsed(object sender, ElapsedEventArgs e)
{
if (!Monitor.TryEnter(_timerLock))
{
// something has the lock. Probably shutting down.
return;
}
try
{
// Do something here that takes ~30 seconds
}
finally
{
Monitor.Exit(_timerLock);
}
}
The main program should never release the lock once it's obtained it.
If you want the main program to go ahead and shut down after some period of time, regardless of whether it's obtained the lock, use Monitor.TryEnter. For example, this will wait 15 seconds.
bool gotLock = Monitor.TryEnter(_timerLock, TimeSpan.FromSeconds(15));
The return value is true if it was able to obtain the lock.
By the way, I strongly suggest that you use System.Threading.Timer rather than System.Timers.Timer. The latter squashes exceptions, which can end up hiding bugs. If an exception occurs in your Elapsed event, it will never escape, meaning that you never know about it. See my blog post for more information.

EDIT
Each callback to the System.Timers.Timer is queued on the ThreadPool. Be aware that the System.Timers.Timer can have a race condition (you can read more about it here.) System.Threading.Timer is a slightly nicer wrapper which I prefer to use due to it's simplicity.
You haven't described enough details to know if your particular application could handle that race condition, so it's hard to tell. But given your code, it is possible that there might be a callback queued up for processTimer_Elapsed after Stop() is called.
For the service timeout issue --
One way to do this is to make a call to the ServiceController method WaitForStatus with a timeout. I've done this in the past and it works reasonably well, although I recall there being some edge cases around waiting for a very long time.
See the MSDN reference. A sample use is described here.

One possible alternative seems to be to not do the actual work in the timer callback itself but to just queue a work item from there on the tread pool to do the work. Then you can go ahead and dispose of the timer - anything currently running on the thread pool will remain operational, and your service can respond to the stop request immediately but the thread pool item (if queued) will still get processed.

System.Threading.Timer keep reference to it

According to [http://msdn.microsoft.com/en-us/library/system.threading.timer.aspx][1] you need to keep a reference to a System.Threading.Timer to prevent it from being disposed.
I've got a method like this:
private void Delay(Action action, Int32 ms)
{
if (ms <= 0)
{
action();
}
System.Threading.Timer timer = new System.Threading.Timer(
(o) => action(),
null,
ms,
System.Threading.Timeout.Infinite);
}
Which I don't think keeps a reference to the timer, I've not seen any problems so far, but that's probably because the delay periods used have been pretty small.
Is the code above wrong? And if it is, how to I keep a reference to the Timer? I'm thinking something like this might work:
class timerstate
{
internal volatile System.Threading.Timer Timer;
};
private void Delay2(Action action, Int32 ms)
{
if (ms <= 0)
{
action();
}
timerstate state = new timerstate();
lock (state)
{
state.Timer = new System.Threading.Timer(
(o) =>
{
lock (o)
{
action();
((timerstate)o).Timer.Dispose();
}
},
state,
ms,
System.Threading.Timeout.Infinite);
}
The locking business is so I can get the timer into the timerstate class before the delegate gets invoked. It all looks a little clunky to me. Perhaps I should regard the chance of the timer firing before it's finished constructing and assigned to the property in the timerstace instance as negligible and leave the locking out.

Your second approach wouldn't keep the reference either. After the end of the Delay2-block, the reference to state is gone so the Garbage Collector will collect it ... then your reference to Timer is gone, too and it will be collected and disposed.
class MyClass
{
private System.Threading.Timer timer;
private void Delay(Action action, Int32 ms)
{
if (ms <= 0)
{
action();
}
timer = new System.Threading.Timer(
(o) => action(),
null,
ms,
System.Threading.Timeout.Infinite);
}
}

Update
Thinking about your problem a bit more generally, I think what you're actually trying to accomplish here is achievable in a much simpler way, without using a System.Threading.Timer at all.
Is this basically what you want your method to do? Perform action after a specified number of milliseconds? If so, I would suggest something like the following alternative implementation instead:
private void Delay(Action action, int ms)
{
if (ms <= 0)
{
action();
return;
}
System.Threading.WaitCallback delayed = state =>
{
System.Threading.Thread.Sleep(ms);
action();
};
System.Threading.ThreadPool.QueueUserWorkItem(delayed);
}
...by the way, are you aware that in the code you posted, specifying a non-zero value for ms will cause action to be executed twice?
Original Answer
The timerstate class really isn't necessary. Just add a System.Threading.Timer member to whatever class contains your Delay method; then your code should look like this:
public class Delayer
{
private System.Threading.Timer _timer;
private void Delay(Action action, Int32 ms)
{
if (ms <= 0)
{
action();
}
_timer = new System.Threading.Timer(
(o) => action(),
null,
ms,
System.Threading.Timeout.Infinite);
}
}
Now, I see that you are specifying the period argument of the timer's constructor as System.Threading.Timeout.Infinite (-1). What this means is that you intend for your timer to call action once, after ms has elapsed; am I right? If this is the case, then there's actually not much need to worry about the timer being disposed anyway (i.e., it will be, and that's fine), assuming a relatively low value for ms.
Anyway, if you're going to hold onto an instance of an IDisposable object (like System.Threading.Timer), you should generally dispose of that member when your object (i.e., this instance) is disposed of. I believe System.Threading.Timer has a finalizer that will cause it to be disposed of eventually anyway, but it's best to dispose of things as soon as you don't need them anymore. So:
public class Delayer : IDisposable
{
// same code as above, plus...
public void Dispose()
{
_timer.Dispose();
}
}

I read from your comments to the existing answers that you can have 0..n Actions and so you would have 0..n Timers, too. Is that right? In this case you should do one of the following:
Keep a List/Dictionary of timers, but in this case you have to remove the timer after firing.
Build a scheduler: have 1 Timer, that fired regulary, for each Delay-call add the action an the calculated time when it should run into a List/Dictionary, each time the timer fired, check you list and run&remove the Action. You can even build this scheduler that it sorts the Actions by execution time an sets the Timer to an adequate interval.

The code "working" is indeed a side-effect of a non-deterministic Garbage Collection / finalizers.
This code, running in LINQ Pad as C# Statements, shows the issue - no messages will be logged because the Timer is GC'ed (and the finalizer is called and it cleans up the internal timer resources..)
new System.Threading.Timer((o) => { "Hi".Dump(); }, this, 100, 100);
GC.Collect();
Thread.Sleep(2000);
However, comment out the "GC.Collect" statement and messages will be logged for 2 seconds as Garbage Collection is not [immediately] performed the Timer's finalizer is not called prior to the program ending.
Since the behavior is non-deterministic, it should also be considered a bug to rely on :}
The same issue exists in the follow on code because a strong reference is required to ensure an object is not GC'ed - in that example, there is still no reference kept to the timer wrapper object so the same issue exists, albeit with one more level of indirection..