Activity Monitor confusing

The %CPU used by each process is shown in Activity Monitor's main window. The summary at the bottom describes the overall CPU load.

Read

Use Activity Monitor on your Mac - Apple Support

and

Activity Monitor for Mac: View the processes on your Mac in Activity Monitor

Cosman wrote:

I have a CAD program that is hogging CPU time and causing long waits with spinning wheel, “not responding” messages, etc. It is actually grinding its way through the task, but I wanted to monitor if it was actually working and not hung up.

So, I open Activity Monitor, set to CPU, and put it on a side screen to watch.

The CAD program registered

"% CPU" at 100-100.1%,

“System" about 5%

“Idle" about 85%. The

"CPU Load" ribbon about 5-10% of the height of the window.

None of this makes any sense. % CPU, System, and CPU Load should be about the same.

Is there some hidden definition not represented by the labels? These should make more sense.

Actually, it can make a lot of sense, but you need to be aware of a few things.

Your mac has several cores, physical or virtual. How a program uses those cores depends on how it is written. Many programs can use only one core, others can leverage several cores at once in parallel.

The 100% you see in %CPU means the program is fully utilizing ONE core.

The 85% refers to all the available cores. So for example if you have 8 logical cores (like most iMacs, or 15" MBP) one program utilizing one core at 100% plus other system processes and applications doing light work will translate to about 85% idle overall.

If you have 8 cores, 100% of one core equates to 12.5% overall.

A program that uses more than one core can show a cpu usage of more than 100%.

For example it is not uncommon to have FCP X or Motion when doing heavy stuff showing 200% or more in Activity monitor.

Just to add to what Luis said, your CAD program is not written to take advantage of multiple cores apparently, and since it's using 100% of one core it would tend to cause performance issues.

Assuming your MBP has an i7 with 4 cores and 8 threads, if the CPU was being used in its entirety your CPU Load would be 800%. The total of all cores and virtual cores (threads) being used.