Re: Real Time Scheduling Methods

[cis.ohio-state.edu!zaphod.mps.ohio-state.edu!moe.ksu.ksu.edu!hobbes.physi cs.uiowa.edu!news.uiowa.edu!news@ucbvax.Berkeley.EDU (Douglas W. Jones,201H ML]

>From article <92302.34020.LJ10891@LMSC5.IS.LMSC.LOCKHEED.COM>,
by LJ10891@LMSC5.IS.LMSC.LOCKHEED.COM:
> 
>    Sorry about that guys, but it looks like Rate Monotonic is the only game
> in town.

No, there's deadline based scheduling.  Deadline based scheduling is
incompatable with Ada, because it requires that the priority of each
process be, at any instant, the deadline by which the process must
complete its next critical action.  Once the process completes the
action, it changes its deadline to the time of the next action (note
that in Ada, priorities are fixed).

Liu and Leyland, the people who proved the sufficiency of rate monotonic
scheduling as long as there is not too much over 60% cpu utilization,
also proved that deadline based real-time scheduling was sufficient if
there is up to 100% utilization.

Thus, deadline based scheduling can be superior to rate monotonic
scheduling, but it's illegal in Ada.

Deadline based scheduling may also be a bit more difficult than rate
monotonic scheduling because it requires that processes state their
deadlines explicitly instead of merely bumbling along, but it can also
be more flexible, because deadlines need not be strictly periodic, while
rate monotonic scheduling theory is only good for processes with strictly
periodic real-time deadlines.

					Doug Jones
					jones@cs.uiowa.edu