From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.5-pre1 (2020-06-20) on ip-172-31-74-118.ec2.internal X-Spam-Level: X-Spam-Status: No, score=-0.0 required=3.0 tests=BAYES_40 autolearn=ham autolearn_force=no version=3.4.5-pre1 Date: 3 Nov 92 14:25:19 GMT From: agate!doc.ic.ac.uk!uknet!yorkohm!minster!ken@ucbvax.Berkeley.EDU Subject: Re: Real Time Scheduling Methods Message-ID: <720800717.15514@minster.york.ac.uk> List-Id: Douglas W. Jones,201H MLH,3193350740,3193382879 (jones@pyrite.cs.uiowa.edu) wro te: : 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. : : 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. This 100% condition for schedulability only holds if the deadline of a task is equal to its period. : 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. Don't confuse the term "Rate monotonic" with "fixed priority scheduling". Rate monotonic is simply a priority assignment policy, guaranteed to be optimal for task sets where deadline=period. For arbitrary deadlines (either less than or greater than periods) this assignment policy isn't optimal. If you want other beahviours (e.g. deadline < period, or sporadic tasks with response deadline < inter-arrival time) then all you need to do is derive analysis for the behaviour of the fixed priority dispatcher and you get a new family of scheduling disciplines. Hence, "deadline monotonic", for periodic tasks with deadline