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POC# Why are timer frequencies extremely off?
 

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  1. POC# Why are timer frequencies extremely off?
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    <p>Both <code>System.Timers.Timer</code> and <code>System.Threading.Timer</code> fire at intervals that are considerable different from the requested ones. For example:</p> <pre><code>new System.Timers.Timer(1000d / 20); </code></pre> <p>yields a timer that fires 16 times per second, not 20.</p> <p>To be sure that there are no side-effects from too long event handlers, I wrote this little test program:</p> <pre><code>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 &lt; 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); } </code></pre> <p>The output looks like this:</p> <p>Requested: 5 Hz; actual: 4,8 Hz<br> Requested: 10 Hz; actual: 9,1 Hz<br> Requested: 15 Hz; actual: 12,7 Hz<br> Requested: 20 Hz; actual: 16 Hz<br> Requested: 30 Hz; actual: 21,3 Hz<br> Requested: 50 Hz; actual: 31,8 Hz<br> Requested: 75 Hz; actual: 63,9 Hz<br> Requested: 100 Hz; actual: 63,8 Hz<br> Requested: 200 Hz; actual: 63,9 Hz<br> Requested: 500 Hz; actual: 63,9 Hz</p> <p>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 <code>System.Windows.Forms.Timer</code>, this puzzles me.</p> <p>Btw, these are not random deviations. They are the same values every time. And a similar test program for the other timer class, <code>System.Threading.Timer</code>, shows the exact same results.</p> <p>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.</p> <p>Any ideas?</p> <p>@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 <code>System.Threading.Timer</code> uses the thread pool, so each event is fired on a free thread.</p>
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    1. COI'm not sure that: "This should not yet require a real-time system." can be true, given : "I need to collect measurements at precisely 50 samples per second."
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    2. COI agree. Sounds like a hard real time requirement to me - exactly what a RTOS is meant to solve. Event every 20ms with a guaranteeed response of 1-2 ms.
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      1. POC# Why are timer frequencies extremely off?
      1. USTomTom
    3. COThe MultiMediaTimer (winmm.dll) usage presented by Justin Tanner works perfect. Thanks a lot man.
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