Why does the system record messages about “excessive wakeups” when System Monitor is running?

If you review the logs of the operating system, you might find notice messages containing the System Monitor label. The messages have the form

kernel[0]: process SystemMonitorMen[AAA] caught causing excessive wakeups.
Observed wakeups rate (per sec): BBB; Maximum permitted wakeups rate (per sec): 150;
Observation period: 300 seconds; Task lifetime number of wakeups: CCC

or similar, where AAA, BBB, and CCC are different numeric values. At the same time, macOS has created diagnostic reports of type wakeups_resource.spin for each of these messages.

These messages and reports are normal. They neither indicate a problem with the computer or the App. They only indicate that System Monitor is using services of the operating system core (kernel) more often as macOS is expecting it for average applications. Software developers can use such reports to reduce the energy consumption created by Apps.

You can avoid this effect as far as it is related to System Monitor (and at the same time slightly reduce the energy consumption of your computer), by switching off all features for sensor monitoring, unchecking the option Information Sources > Main Sensors > Monitor this item.

Although the reports have the System Monitor label, the direct cause of these messages cannot be found in the App itself. The aforementioned effect is unavoidable and affects all applications (including Apple’s own software) which acquire readings from sensors within a running operating system on Macs built after mid–2007. Only Apple could evade this by modifying the hardware or macOS.