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Running yes > /dev/null takes my CPU usage to 99%.

Activity monitor

The Macbook Air 2018 have a 2 core CPU (1.6 GHz Core i5 [I5-8210Y]). Why is the "% CPU" taken by this process so high?

marked as duplicate by Allan, bmike macbook Mar 30 at 0:59

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  • What did you expect? It just dumps data into the void, so the process doesn’t have to wait for disk IO, display updates or similar. – nohillside Mar 29 at 11:01
  • This is expected behaviour from yes. – DisplayName Mar 29 at 11:47

It is because Activity Monitor displays CPU usage as a percentage of a single core. So 100% means that one core is fully occupied.

The percentage can thus be higher than 100% when a program utilizes multiple cores.

The “yes” program is designed to use only one core, and use it fully. Therefore the behavior you’re seeing is entirely normal.

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