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I'm writing a simple application in C to handle switching relays on an old Pi 1B. When the Pi is supposed to wait for the next action to handle, I use the sleep() system call. I checked with top, only to find out that my application still uses 10% CPU! Why?? So I tried to rewrite the code using alarm() and pause(). Of course the same: 10% CPU. Again: why??

Is there a way to put the process really to sleep, 0% CPU? Tips maybe?

Proof of a simple application using 10%: enter image description here

#include <json-c/json.h>
#include <json-c/json_util.h>

main() {
    gpioInitialise();
    sleep(10000);
}
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  • We need the source code of a simple example of your application which uses 10%.
    – joan
    Feb 20 at 16:11
  • sleep pauses execution of a process, but the original process may have spawned other processes, and these child processes won't necessarily pause. You may have to pause at the "kernel level" to reach your objective; here's a paper discussing some ways to do that
    – Seamus
    Feb 21 at 1:52
  • In the sample C-program above nothing is forked (by me), only a library is initialised and that seems to be enough to cause a constant 10% CPU load on a Raspberry Pi 1B. My objective is not to put the whole system in sleep-mode, just the process will do. I bet that if I remove the gpioInitialise() call the load will drop to <1%...
    – D.Bugger
    Feb 21 at 9:18

2 Answers 2

Reset to default
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If you are using pigpio this is normal behaviour.

pigpio uses about 10% of 1 CPU to do the DMA sampling of GPIO and the generation of PWM. This is a constant overhead when using pigpio.

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  • Thanks, it's what crossed my mind. Obvious questions: are there other libraries that do not use 10% CPU? Or can I tell pigpio somehow not to sample when I don't need it? The thing is supposed to do nothing...
    – D.Bugger
    Feb 20 at 21:51
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I adopted a different library, and CPU utilisation during the sleep() call went down to 0: libgpiod to the rescue! It might not be the best gpio library around but at least there's no overhead in my case. Sample: https://www.ics.com/blog/gpio-programming-exploring-libgpiod-library

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