C# Why are timer frequencies extremely off?

Question

Both System.Timers.Timer and System.Threading.Timer fire at intervals that are considerable different from the requested ones. For example:

new System.Timers.Timer(1000d / 20);

yields a timer that fires 16 times per second, not 20.

To be sure that there are no side-effects from too long event handlers, I wrote this little test program:

int[] frequencies = { 5, 10, 15, 20, 30, 50, 75, 100, 200, 500 };

// Test System.Timers.Timer
foreach (int frequency in frequencies)
{
    int count = 0;

    // Initialize timer
    System.Timers.Timer timer = new System.Timers.Timer(1000d / frequency);
    timer.Elapsed += delegate { Interlocked.Increment(ref count); };

    // Count for 10 seconds
    DateTime start = DateTime.Now;
    timer.Enabled = true;
    while (DateTime.Now < start + TimeSpan.FromSeconds(10))
        Thread.Sleep(10);
    timer.Enabled = false;

    // Calculate actual frequency
    Console.WriteLine(
        "Requested frequency: {0}\nActual frequency: {1}\n",
        frequency, count / 10d);
}

The output looks like this:

Requested: 5 Hz; actual: 4,8 Hz
Requested: 10 Hz; actual: 9,1 Hz
Requested: 15 Hz; actual: 12,7 Hz
Requested: 20 Hz; actual: 16 Hz
Requested: 30 Hz; actual: 21,3 Hz
Requested: 50 Hz; actual: 31,8 Hz
Requested: 75 Hz; actual: 63,9 Hz
Requested: 100 Hz; actual: 63,8 Hz
Requested: 200 Hz; actual: 63,9 Hz
Requested: 500 Hz; actual: 63,9 Hz

The actual frequency deviates by up to 36% from the requested one. (And evidently cannot exceed 64 Hz.) Given that Microsoft recommends this timer for its "greater accuracy" over System.Windows.Forms.Timer, this puzzles me.

Btw, these are not random deviations. They are the same values every time. And a similar test program for the other timer class, System.Threading.Timer, shows the exact same results.

In my actual program, I need to collect measurements at precisely 50 samples per second. This should not yet require a real-time system. And it is very frustrating to get 32 samples per second instead of 50.

Any ideas?

@Chris: You are right, the intervals all seem to be integer multiples of something around 1/64th second. Btw, adding a Thread.Sleep(...) in the event handler doesn't make any difference. This makes sense given that System.Threading.Timer uses the thread pool, so each event is fired on a free thread.

Answer 1

Well, I'm getting different number up to 100 Hz actually, with some big deviations, but in most cases closer to the requested number (running XP SP3 with most recent .NET SPs).

The System.Timer.Timer is implemented using System.Threading.Timer, so this explains why you see same results. I suppose that the timer is implemented using some kind of scheduling algorithm etc. (it's internal call, maybe looking at Rotor 2.0 might shed some light on it).

I would suggest to implement a kind of timer using another thread (or combination thereof) calling Sleep and a callback. Not sure about the outcome though.

Otherwise you might take a look at multimedia timers (PInvoke).