c# - Timer takes 10 ms more than interval

Question

I am using a timer with interval 1 second. But in the timer's tick event when I print the time it's always 62 or 65 ms. I don't understand why it's taking 10 ms more.

Please can some one have look into this.

Here is the code I am using:

static int _counter;
var _timer = new System.Timers.Timer(1000);
public Form1()
{
    InitializeComponent();           
    _timer.Elapsed += new ElapsedEventHandler(_timer_Elapsed);
    _timer.Start();            
}

private void _timer_Elapsed(object sender, ElapsedEventArgs e)
{
    Console.WriteLine(DateTime.Now.ToString("{hh:mm:ss.fff}"));          
    _counter++;
    if (_counter == 20)
        _timer.Stop();
}

And this the output:

{01:59:08.381}
{01:59:09.393}
{01:59:10.407}
{01:59:11.421}
{01:59:12.435}
{01:59:13.449}
{01:59:14.463}
{01:59:15.477}
{01:59:16.491}
{01:59:17.505}
{01:59:18.519}
{01:59:19.533}
{01:59:20.547}
{01:59:21.561}
{01:59:22.575}
{01:59:23.589}
{01:59:24.603}
{01:59:25.615}
{01:59:26.629}
{01:59:27.643}

Answer 1

You need to understand that Windows is not a real-time operating system. Real-time operating systems have timer mechanisms that allow the system to make hard guarantees about when timer-initiated events occur and the overhead associated with them, and allow you to specify what behavior should occur when the deadline is missed -- for example if the previous execution took longer than the interval.

I would characterize the Windows timers as "best effort" when it comes to smaller intervals. When the interval is sufficiently long you don't notice that you aren't getting the exact interval that you requested. As you get closer and closer to the resolution of the timer (the frequency at which the timer runs), you start seeing the overhead as a percentage of the interval increase. Real-time systems take special care to minimize the software overhead, relying on more sophisticated and faster hardware solutions. The exact frequency of the Windows timer depends on the timing services that the underlying hardware provides and so may differ from system to system.

If you have real-time needs -- and doing something every 50ms may fall into that category -- then you may need to look at specialized hardware and/or a real-time OS.