Why Does Sleep(1) in Windows Sometimes Sleep for Longer Than Expected?

Investigating Sleep() Sleeping More Than Expected

Windows API's Sleep() function is a versatile tool for suspending thread execution. However, some users encounter unexpected behavior where the function sleeps for longer than intended. This article will delve into the issue, examining its common occurrence and potential causes.

Behavior and Duration Disparity

When calling Sleep(1) in a loop 100 times, the total sleep time is observed to be 1500ms instead of 100ms. This disparity suggests that the actual sleep duration is approximately 15ms per iteration. Note that this behavior varies across systems, with some users reporting consistent 1ms sleep durations.

Possible Reasons

The excessive sleep duration may not necessarily indicate a hardware or software malfunction. Windows thread scheduling employs a "time quantum," a time interval to which thread execution is bound. For non-zero sleep durations, the actual delay is rounded up to the nearest complete quantum.

Default Time Quantum

By default, Windows 7 has a time quantum of 15.6ms. This means that Sleep(1) will actually suspend the thread for a minimum of 15.6ms, resulting in the observed behavior of 15ms sleep per iteration.

Alternative Explanation

In some cases, the observed 1ms sleep durations may be attributable to the presence of another program or process that has modified the system-wide timer resolution to 1ms. This override can lead to more precise sleep durations.

Conclusion

The Sleep() function's behavior in this scenario is typically expected. The time quantum mechanism of Windows' thread scheduler rounds up sleep durations to complete intervals, resulting in longer sleep times for small durations. However, external factors such as modified timer resolution settings can also influence the sleep behavior.

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