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The number given as % CPU in Activity Monitor

One of the most common reasons for opening Activity Monitor is to check the % CPU of processes that might be running out of control. How accurate are those figures, though, and should you believe t…

Tests To examine this more clearly, I have an app that runs tight loops of assembly code to load CPU cores on Apple silicon chips and measure their performance. If full load at maximum frequency is 100% of their capability, then at low QoS they were really running at 27%, not the 100% given by Activity Monitor, and that largely accounts for the difference in time that those threads took to complete their floating point calculations. The next time that you open Activity Monitor on an Apple silicon Mac, to look at % CPU figures, bear in mind that while those numbers aren’t completely meaningless, they don’t mean what you might think, and what’s shown as 100% could be anything between 27-100%.

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