usleep inside loop takes too long

Question

In the below C++ program, I am using the function usleep() to sleep for a 1.5 seconds. I implemented that in 2 equivalent methods as illustrated below:

#include <iostream>
#include <unistd.h>

using namespace std;

int main() {
    //METHOD #1
    cout<<"sleep"<<endl;
    usleep(1500000);
    cout<<"wake up"<<endl;

    //METHOD #2
    cout<<"sleep"<<endl;
    for(int i=0; i<1500000; i++)
        usleep(1);
    cout<<"wake up"<<endl;

    return 0;
}

however the results came as follows:

First method: takes exactly 1.5 seconds
Second method: takes around 1.5 minutes !

Actually, I will need the second method. According to this Answer, I think I need a more accurate function that usleep(). Could any one help ?

The link is right there at the top of your question. Maybe refresh the page using F5. — πάντα ῥεῖ, Dec 12 '18 at 13:41
OK, I edited the question, could you please check again to see whether it is still duplicate? Thanks for the notice :) — Ahmed Hussein, Dec 12 '18 at 13:56
I don't see why I should reopen the question. The behavior of `usleep()` is well documented, that there's no exact time measurement guaranteed. Also it's a well known fact that any _realtime_ computing depends on the operating system's time resolution. You might want to have a look at `std::crono` and the usagee of realtime clock devices. — πάντα ῥεῖ, Dec 12 '18 at 14:00
I found a way to solve my problem, and I would like to share it with people in an answer. I found a more accurate way. Could you please reopen it? — Ahmed Hussein, Dec 12 '18 at 14:02
_"Could you please reopen it?"_ I'll leave that for other's to do. ATM the duplicate and the existing answer, perfectly answer your question. — πάντα ῥεῖ, Dec 12 '18 at 14:04
The answer you provided explains the reason behind the inaccuracy, but doesn't say how to overcome it — Ahmed Hussein, Dec 12 '18 at 14:07

score 3 · Answer 1 · answered Dec 12 '18 at 13:36

3

From the documentation (emphasis mine)

The usleep() function suspends execution of the calling thread for (at least) usec microseconds. The sleep may be lengthened slightly by any system activity or by the time spent processing the call or by the granularity of system timers.

So in other words, the reason why it takes longer is because it's now "going to sleep" and "waking up" 1500000 times instead of just once, and with such a short sleep duration, that overhead may be much bigger than the actual microsecond sleep.

answered Dec 12 '18 at 13:36

Blaze

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So, how can I implement method #2 with a more accurate function? – Ahmed Hussein Dec 12 '18 at 13:38
2

The function is always inaccurate, so if you call it multiple times in a row, those inaccuracies add up. Why do you need the method #2 over the first one? It sounds like there might be a better solution to your problem. – Blaze Dec 12 '18 at 13:46

usleep inside loop takes too long

1 Answers1