Thread.Sleep(1) takes longer than 1ms [duplicate]

Answer 1

I expect that Thread.Sleep is nothing more than a managed wrapper around the Win32 Sleep API. It is well known that this API function has low resolution. The documentation says this:

This function causes a thread to relinquish the remainder of its time slice and become unrunnable for an interval based on the value of dwMilliseconds. The system clock "ticks" at a constant rate. If dwMilliseconds is less than the resolution of the system clock, the thread may sleep for less than the specified length of time. If dwMilliseconds is greater than one tick but less than two, the wait can be anywhere between one and two ticks, and so on. To increase the accuracy of the sleep interval, call the timeGetDevCaps function to determine the supported minimum timer resolution and the timeBeginPeriod function to set the timer resolution to its minimum. Use caution when calling timeBeginPeriod, as frequent calls can significantly affect the system clock, system power usage, and the scheduler. If you call timeBeginPeriod, call it one time early in the application and be sure to call the timeEndPeriod function at the very end of the application.

After the sleep interval has passed, the thread is ready to run. If you specify 0 milliseconds, the thread will relinquish the remainder of its time slice but remain ready. Note that a ready thread is not guaranteed to run immediately. Consequently, the thread may not run until some time after the sleep interval elapses. For more information, see Scheduling Priorities.

So, you could use timeBeginPeriod and timeEndPeriod as suggested to increase timer resolution, but that's really not to be recommended. If you really need to block for short periods of time then I suggest that you find a better solution. For instance a multimedia timer.

Answer 2

Searching for "thread sleep resolution" brings up these related SO questions:

• Can I improve the resolution of Thread.Sleep?
• What is the impact of Thread.Sleep(1) in C#?
• Problems with using Thread.Sleep for short times

Consensus is that by default, Sleep() takes at least 15ms, unless you set the resolution using timeBegin/EndPeriod() as in this answer.

Answer 3

Thread.Sleep(1) will put a thread to sleep for 1ms, but you usually can't get that precision.

However, once thread goes in the sleep state, it takes time for kernel to put it back into the ready queue, and then to put it into the running state when the thread will actually be run again.

All these factors depend on the platform, number of the CPUs, number of the other processes, etc. With non-RT operating system, you can't get any guarantee on this.

Answer 4

Timers other than Stopwatch are incremented by the clock interrupt. Which by default ticks 64 times per second on Windows. Or 15.625 milliseconds. So a Thread.Sleep() argument less than 16 doesn't give you the delay you are looking for, you'll always get at least that 15.625 interval. Similarly, if you read, say, Environment.TickCount or DateTime.Now and wait less than 16 millisecond then you'll read the same value back and think 0 msec has passed.

Always use Stopwatch for small increment measurements, it uses a different frequency source. Its resolution is variable, it depends on the chipset on the motherboard. But you can rely on it being better than a microsecond. Stopwatch.Frequency gives you the rate.

The clock interrupt rate can be changed, you have to pinvoke timeBeginPeriod(). That can get you down to a single millisecond and actually make Thread.Sleep(1) accurate. Best not to do this, it is very power unfriendly.

Answer 5

Thread.Sleep(n) means block the current thread for at least the number of timeslices (or thread quantums) that can occur within n milliseconds. The length of a timeslice is different on different versions/types of Windows and different processors and generally ranges from 15 to 30 milliseconds. This means the thread is almost guaranteed to block for more than n milliseconds. The likelihood that your thread will re-awaken exactly after n milliseconds is about as impossible as impossible can be. So, Thread.Sleep is pointless for timing.

Answer 6

Yes what you've figured out is absolutely correct.

Thread.Sleep doesn't guarantees to release at N milliseconds. It will make sure that Sleep will not release before N milliseconds.

In other words Thread.Sleep(1) means that Sleep for atleast 1 millisecond. Operating system will not schedule the time slice for specific thread for specific amount of time.

The thread will not be scheduled for execution by the operating system for the amount of time specified. This method changes the state of the thread to include WaitSleepJoin.

From Msdn