An RTOS defines a stricter latency envelope than a mainframe, almost into "deterministic latency" range.

For example, you can say "this operation will take at most 3ms" in an RTOS, and it'll never exceed that number. If you're running on a fixed-frequency system, you can even say that I expect an answer in 2.8ms all the time, every time.

In a mainframe this is a bit relaxed. You can say that latency is <3ms 99% of the time, <3.1ms in 99.999% of the time and <3.5ms in 100% of the time.

IOW, you never think/say "somebody is running a heavy load on the host, and I also slowed down because of them".

How is the mainframe's 100% number different from the upper boundary that RTOSes offer? It's not 100%, is it?

What I tried to say is, a mainframe might answer late, but rarely, and very slightly. a RTOS doesn't deviate from that number in either direction. It's neither early nor late.

Some RTOS even doesn't consider late answers as acceptable.

Yeah exactly. My arm64 Linux box also might answer late, but rarely.

That's true, but that jitter envelope gets bigger as the load increases. For a mainframe this jitter envelope is much narrower, and in a competent RTOS, that's basically 0.