Wait time delay with condition_variable in C++

Mahesh C · 02-24-2021, 12:43 AM

Dear All,

In C++ project for linux, we have used the condition_variable to wait in a loop. Sometimes we are observing delay in wake up. e.g. we have set the interval of 10mins, getting the delay of ~2mins in between. here are the observations:
Wait until: [22-2-2021 19:0:58]
Wait until: [22-2-2021 19:10:58]
Wait until: [22-2-2021 19:20:58]
Wait until: [22-2-2021 19:30:58]
Wait until: [22-2-2021 19:42:20]
Wait until: [22-2-2021 19:52:20]

it is suppose to wake up 19:40:58 but woke up at 19:42:20. Could anybody help me to understand why this delay is occurring?

This is observed on only particular device variant, for rest of devices same code working as expected.

Please find the sample code:

Code:

auto nowp = std::chrono::system_clock::now();
    
    while(true)
    {
        std::unique_lock<std::mutex> lock(cv_mtx);    
        cv.wait_until(lock, nowp, [](){ return false;});
        nowp += std::chrono::minutes(delay);
        std::cout<<"Wait until: "<<get_formatted_time(nowp)<<std::endl;            
    }

Thanks
Mahesh

astrogeek · 02-24-2021, 01:06 AM

Welcome to LQ and the Programming forum!

Please wrap your code snippets in [CODE]...[/CODE] tags to preserve formatting and help to make it easier for others to read it. You may type those yourself or follow the Advanced edit option and click the # tool button at top of the edit box.

Without knowing anything about the device which is working differently it will be difficult for anyone to offer much help.

Is that device very busy? Is the RTC of the device stable? What type of device is this and what operating system is running on the device, what is it doing?

Please provide as much relevant information as you think would be helpful to others to understand your problem.

AnanthaP · 02-24-2021, 05:11 AM

Any other instance of the same programme running simultaneously?

pan64 · 02-24-2021, 05:25 AM

is this a VM ?

Mahesh C · 02-25-2021, 08:28 AM

Thanks Astrogeek,

Added the code under code blocks.
Mentioned sample code tried on various linux based embedded devices. On particular linux based variant only we are observing this delay.
Device is neither busy nor doing any time consuming work. but still delay is observed.
RTC is unstable means are you pointing to hardware issue ?

thanks
Mahesh