More detail on the "it depends". The guarantees are of the form "if the utilization is less than X%, then an arbitrary collection of periodic tasks can be sche

More detail on the "it depends". The guarantees are of the form "if the utilization is less than X%, then an arbitrary collection of periodic tasks can be scheduled without missing deadlines". The key term there is "arbitrary". For example, rate monotonic scheduling provides this guarantee up to ~70% utilization. However, manually scheduled tasks can walk up into the 90's. A UPS application I worked on scheduled all of its periodic tasks manually by ensuring that each slower rate task had a rate that was always an integer divisor of (and lower preemptable priority than) the next higher rate task. This proceeded all the way until you get down to much slower rate periodic "housekeeping" tasks that weren't even realtime.