Re: [uCsimm] Hard Real time Linux

From: Larry Doolittle (
Date: Thu Feb 24 2000 - 01:17:52 EST

Guys -

> > The problem is context switch time. With Hard real time systems
> > it is <10 microseconds. I am trying to find out context switch times in
> > Linux also. There will be several processors and DSP's
> > many pegs many holes
> First of all , in my guess, 10 microseconds is really hard!.
> One of the solution for this HARD realtime application could be a
> bufferring scheme. This idea is simple, add another single chip
> (like scenex? 100 mips PIC) doing simple processing
> and do main processing at 1 millsecond interval. But this is
> only useful for periodic request.

On a normal Pentium-class computer, RTLinux folks measure latencies
of about 2 microseconds typical. There is a tail, though. Look
at the Linux Journal article about flying a radar set into a hurricane.

The final extreme is to throw a gate array at the problem. That
can be very inexpensive and general, just imagine Linux running
on the same gate array, see OR1K project.
That would give you the flexibility to put the "right" amount of
reflexes in hardware (often that means 10 +/- 0.1 ns delay), and
the remainder in c code, with arbitrary coupling between the two.

    - Larry Doolittle <>
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