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Observing Activity Monitor, the per-process CPU is often very different to the total CPU.

Example

Here the sum of individual processes is > 50%, yet the total said to be (100 - 91.43) 8.57%.

enter image description here

Why is this? And which is correct?

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    Even if the measurements are exact (they are not - they are samples), the rounding error of 50 measurements can add up to dozens of percent. I saw one case where 100% CPU error - like a whole core was mis counted - due to just rounding on a mac with thousands of processes and doing the math on each and every one. The overall System / User / Idle numbers are the only ones that are mathematically significant on any one snap, here. Usually rounding is fine, but your adding up the errors ensures some data input won’t make sense at first glance. – bmike Jul 18 '20 at 14:15
  • have you tried organizing the processes "Hierarchically"? its on option under view.. this maybe is "threading related" when you dont list them "hierarchically"? – Asher Jul 18 '20 at 16:55
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The %CPU column is percentage of one [virtual] core. The total summary is percentage of entire CPU. So if, for instance you have 4 real cores, 8 virtual, the maximum possible total figure in the %CPU column would be 800%

if you want to check how many cores you have, Use either Window menu > CPU usage or CPU history. The number of stripes is the number of virtual [hyper-threaded] cores. Halve that to get your total physical core count.

This shows 12 cores, 24 HT cores…

enter image description here

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  • Thanks! Where is the Window menu? – stevec Jul 18 '20 at 11:30
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    Same place as in every Mac app, menu bar, left of the Help menu – Tetsujin Jul 18 '20 at 11:32
  • Ah see it now. Thanks again! – stevec Jul 18 '20 at 11:34

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