Question

OS: Windows 7

When calling the WinAPI Sleep() function as Sleep(1) the thread actually sleeps for 15ms. I did it 100 times in a loop and the total sleep time was 1500ms instead of 100.

Is this common behavior or should I be concerced about something being wrong with my MOBO, CPU, Windows installion?

EDIT: If possible could you run this code and post how long the sleep time was. I let a friend of mine run this, and he actually had it all at 1ms.

#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;
    }
}

int main(void)
{
    test();

    std::cin.sync();
    std::cin.get();
    return 0;
}

EDIT2: It seems that the reason why some people are getting 1ms is that some other program is running that sets the system-wide timer resolution to 1ms. By default this should be 15.6ms on Windows 7.

Answer 1

Is this common behavior

It is.

Window’s thread scheduler works on a time quantum (exact length depends of various factors including Windows version and edition). Effectively any non-zero delay is rounded up to a complete quantum.