Exact Sleep time in C++

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I try to measure the 'exact' sleep time when I use Sleep() in C++.

So, I used this code

C++

#include <stdio.h>
#include <intrin.h>
#include <windows.h>
#include <iostream>
using namespace std;

unsigned __int64 testSleepExactTime(int relax);
unsigned __int64 getFreqFrom__rdtsc();

int main()
{
	unsigned int relax = 0;
	unsigned __int64 freq = 0;
	freq = getFreqFrom__rdtsc();
	cout <<"Input Relax time (millisec): ";
	cin >> relax;
	cout <<"relax Time : " << relax * 1000<<" Micro Sec."<<endl;
	cout <<"System frequency : "<<freq<<" Ticks per sec."<<endl;
	for(int i = 0;i<10;i++)
		cout <<"Exact relax time : " << testSleepExactTime(relax)*1000000/freq <<" Micro Sec."<< endl;
	for(int i = 0;i<10;i++)
		cout <<"Exact relax time(Double) : " << ((double)testSleepExactTime(relax))*1000000/freq <<" Micro Sec."<< endl;

	system("pause");
	return 0;
}
unsigned __int64 testSleepExactTime(int relax)
{
	unsigned __int64 start = 0;
	unsigned __int64 stop = 0;
	start = __rdtsc();
	Sleep(relax);
	stop = __rdtsc();
	return stop - start;
}
unsigned __int64 getFreqFrom__rdtsc()
{
	 unsigned __int64 start = 0;
	 unsigned __int64 stop = 0;
	 start = __rdtsc();
     Sleep(1000);
	 stop = __rdtsc();
	return stop-start;
}

when I input 1 millisecond, it resuted 10000 Microsecond
when I input 1000 millisecond, it resuted 995000 Microsecond

why when I use 1 millisecond that result seem incorrect?

note:
Using MSVC++2010 as tool
Base on Windows7
CPU i3 3.07GHz
PS. I'm not using 'QueryPerformanceFrequency()' because it returns ~2.9m ticks per sec.

Posted 20-Dec-11 2:50am

windyl3ig

Updated 20-Dec-11 2:52am

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Albert Holguin 20-Dec-11 10:18am

By the way, in Windows, there's no accurate reading to the microsecond level, so your results are not really valid anyway.

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#realJSOP · Accepted Answer · 2011-12-20T03:01:00

Solution 1

Generally, when you try to sleep for 1 millisecond, the cpu sleeps for about 20-25 milliseconds due to the resolution of the clock on the CPU. If you google "cpu clock resolution" you'll probably find more in-depth descriptions of what's going on.

Posted 20-Dec-11 3:01am

#realJSOP

Comments

windyl3ig 20-Dec-11 9:58am

ok, That's time resolution problem. thanks you very much >w<

Albert Holguin 20-Dec-11 11:25am

Yep, typically, when there's an operating system involved, you're not going to be able to have a great amount of resolution due to the amount of overhead going on and the time-division multiplexing of instructions used by the OS to divide processor time amongst all processes.

Chuck O'Toole · Accepted Answer · 2011-12-20T05:15:00

Sleep() takes an argument specifying the number of milliseconds before returning. Let's call that argument "x"

Sleep() (or any other form of timeout) in Windows does not guarantee that you will resume in exactly 'x' milliseconds. It does not even guarantee that you will resume in anywhere near 'x' milliseconds. Here's why.

Your process (thread) is just one of a number of processes / threads running in your PC. Open Task Manager and look at the processes tab. Click the View menu and add the "Threads" and "Base Priority" items. Now look at the list.

Your process is competing for compute resources with every thread inside every process running in your PC. Look at the Base Priority column and you'll see that there are many processes listed as having a higher priority than yours. These will all prempt you if they need the cpu. There are a number of processes listed as having the same priority as yours. You will take turns with these processes.

So, when your Sleep() is finished, your process (thread) is once again allowed to compete with these other processes for the cpu's attention. You are not guaranteed to get it first.

To summarize:

1) as others have pointed out to you, the clock resolution is not exactly at 1 millisecond granularity so your Sleep() will not complete in exactly 'x' milliseconds, but rather will complete sometime after 'x' milliseconds have elapsed.

2) once the time has elapsed and your process (thread) is eligible to run again, it has to compete with everything else that is happening inside your PC at that moment and may have to wait its turn to resume execution.

And to your other issue in your question:

Your algorithm for figuring out freq is flawed because it relies on Sleep(1000) to calibrate the value and, as I've demonstrated, Sleep() cannot be used for this purpose and is inherently imprecise.

Chuck O'Toole · Accepted Answer · 2011-12-20T06:55:00

Re: QueryPeroformanceFrequency():

Read Microsoft's description and the "community comments" on it.

QueryPerformanceFrequency()[^]

As one person noted, you are querying the frequency of the timer, not the frequency of the cpu processor. So regardless of what your CPU Chip speed is, this is a different "frequency".

Consider if the performance counter was, in fact, the cpu cycle speed. Then would the number change when you switched to battery power? Certainly your processor chip speed does. How about "turbo boost" mode? If the cpu cycle speed were variable as it is in modern chips and under certain circumstances, the high percision timer would be useless.

Microsoft's documentation clearly states :

Quote:
The frequency cannot change while the system is running.

which implies that it does not follow the ups and downs of the cpu chip clock speed.

Chuck O'Toole · Accepted Answer · 2011-12-20T07:11:00

If your question is "How do I find out the CPU Chip Speed", which seems to be what you are asking in one of your replies, Microsoft has an article which describes the various ways to find it in the registry or other places.

http://support.microsoft.com/kb/888282[^]

Again with the proviso that the cpu chip cycle speed can vary during the course of the day. Given that, the information is "interesting" but hardly "useful".

Exact Sleep time in C++

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