The sudden spike in CPU and memory utilization alarms people, so I've
reduced how many cores native-comp will use. In non-interactive
sessions, it will use all of them, however (that is, when I later
introduce an AOT switch).
You can still override this by setting native-comp-async-jobs-number or
comp-num-cpus yourself.
I use advice instead of setting comp-num-cpus so that users to avoid
trampling on default behavior, or attempts by the user to change them.
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