Why Does Sleep(1) Seem to Sleep for 15ms in Windows?

Sleep() Function Behavior in Windows

When calling the Sleep() function, a thread typically suspends execution for the specified duration. However, in some cases, it may seem that the thread sleeps for a longer period.

In Windows versions such as Windows 7, Sleep() operates on a time quantum. This means that any non-zero delay is rounded up to the nearest quantum, which may result in slightly longer sleep durations.

A common symptom of this behavior is that a call to Sleep(1) may actually cause a sleep of around 15ms. This is because the operating system scheduler typically uses a quantum of 15.6ms.

To illustrate this, a code snippet is provided:

<code class="cpp">#include <iostream>
#include <ctime>
#include <Windows.h>

void test(void)
{
    std::cout << "Testing 1ms sleep." << std::endl;

    for (unsigned int i = 0; i < 10; i++)
    {
        std::clock_t startClocks = std::clock();

        Sleep(1);

        std::clock_t clocksTaken = std::clock() - startClocks;
        std::cout << "Time: " << clocksTaken << "ms." << std::endl;
    }
}</code>

If executed on Windows 7, this code will likely report sleep durations around 15ms instead of 1ms.

Therefore, it's typical behavior for Sleep() to sleep for a slightly longer period than the specified delay due to the time quantum system employed by Windows.

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