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Comment by deepsquirrelnet | Hacker News Reader

What a blast from the past. I compiled a kernel for Debian with RT_PREEMPT about 17-18 years ago to use with scientific equipment that needed tighter timings. I was very impressed at the latencies and jitter.

I haven’t really thought about it since then, but I can imagine lots of used cases for something like an embedded application with raspberry pi where you don’t quite want to make the leap into a microcontroller running an RTOS.

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Interesting to mention the Raspberry Pi. I saw an article just a day or two ago that claimed that the RpiOS was stated by and ran on top of RTOS. That's particularly interesting because at one time years ago, I saw suggestions that Linux could run as a task on an RTOS. Things that required hard real time deadlines could run on the RTOS and not be subject to the delays that a virtual memory system could entail.

I don't recall if this was just an idea or was actually implemented. I also have seen only the one mention of RpiOS on an RTOS so I'm curious about that.

>That's particularly interesting because at one time years ago, I saw suggestions that Linux could run as a task on an RTOS.

I've worked with systems that ran Linux as a task of uITRON as well as threadX, both on somewhat obscure ARM hardware. Linux managed the MMU but had a large carveout for the RTOS code. They had some strange interrupt management so that Linux could 'disable interrupts' but while Linux IRQs were disabled, an RTOS IRQ could still fire and context switch back to an RTOS task. I haven't seen anything like this on RPi though, but it's totally doable.

Interesting to know that it was more than just an idea - thanks!

You could probably do something simpler now since the new ARMs have good hardware virtualization support.

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