Re: [2.4.17/18pre] VM and swap - it's really unusable

yodaiken
2002-01-14T22:50:04Z

On Tue, Jan 15, 2002 at 12:34:01AM +0200, Momchil Velikov wrote:
> One can consider a non-preemptible kernel as a special kind of
> priority inversion, preemptible kernel will eliminate _that_ case of
> priority inversion.

The problem here is that priority means something very different in
a time-shared system than in a hard real-time system. And even in real-time
systems, as Walpole and colleagues have pointed out, priority doesn't
really capture much of what is needed for good scheduling.

In a general purpose system, priorities are dynamic and "fair".
The priority of even the lowliest process increases while it waits
for time. In a raw real-time system, the low priority process can sit
forever and should wait until no higher priority thread needs the
processor. So it's absurd to talk of priority inversion in a non RT
system. When a low priority process is delaying a higher priority task
for reasons of fairness, increased throughput, or any other valid
objective, that is not a scheduling error.

>
> Regards,
> -velco

--
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Victor Yodaiken
Finite State Machine Labs: The RTLinux Company.
www.fsmlabs.com www.rtlinux.com