The thing is, long term sheduling isn't simpler. It works fine when you have to deal with one kind of workload on one type of system, but the variety of hardware and workloads that Linux has to deal with is way bigger than the variety any of the commercial Unixen has had to deal with.
I've tried implementing good long term scheduling (aka load control) strategies for Linux in the past, but those efforts have always ran into trouble when it comes to the magic knobs needed for such a system. The 64MB RAM system with a few apps behaves very differently from the 1GB RAM system with many apps, or the 1GB RAM system with a few scientific calculations, or ...
The token based thrashing prevention, OTOH, seems to just work. No magic involved. It's turned out to be a lot simpler than any long term scheduling algorithm I've ever seen.
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