Why Does WinAPI Sleep() Function Call Sometimes Exceed the Expected Duration?

Unexpected Duration for WinAPI Sleep() Function Call

The WinAPI Sleep() function is utilized to suspend the execution of a thread for a specified duration. However, in certain cases, it has been observed that the sleep period exceeds the intended value.

In Windows 7, calling Sleep(1) may result in a delay of approximately 15ms instead of the expected 1ms. This has been verified by executing a loop of 100 Sleep(1) calls and observing a total sleep time of 1500ms.

Expected Behavior

It is important to note that this duration extension is typical behavior in the Windows operating system. The thread scheduler operates on time quanta, which are predefined intervals of time. Any non-zero delay request is adjusted to the nearest complete quantum.

Therefore, the sleep period is elevated to the nearest time quantum, which in this case is 15ms.

Dependency on External Factors

The duration of the sleep period can also be influenced by the system-wide timer resolution. By default, Windows 7 sets this resolution to 15.6ms. However, other programs or system configurations may modify this setting, resulting in lower timer resolutions (e.g., 1ms).

In such scenarios, Sleep(1) calls may experience shorter sleep durations.

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