From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=0.2 required=5.0 tests=BAYES_00,INVALID_MSGID, REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,a3ca574fc2007430 X-Google-Attributes: gid103376,public X-Google-Thread: 115aec,f41f1f25333fa601 X-Google-Attributes: gid115aec,public From: ken@nrtt.demon.co.uk (Ken Tindell) Subject: Re: Ada and Automotive Industry Date: 1996/11/11 Message-ID: #1/1 X-Deja-AN: 195759113 distribution: world x-nntp-posting-host: nrtt.demon.co.uk references: <55ea3g$m1j@newsbf02.news.aol.com> <3280DA96.15FB@hso.link.com> organization: Northern Real-Time Technologies Ltd. reply-to: ken@nrtt.demon.co.uk newsgroups: comp.lang.ada,comp.realtime Date: 1996-11-11T00:00:00+00:00 List-Id: In article eachus@spectre.mitre.org (Robert I. Eachus) wrote: > > In Ada 83 you can't schedule a periodic event reliably -- no Ada > > task was guaranteed to run at the time requested. Everyone I knew > > who used Ada for avionics in the 80s wrote their own scheduler. > > A HUGE amount of wasted effort because people couldn't be bothered > to read the reference manual. What the Ada 83 RM said, and pretty > plainly at that, was that a critical task would run exactly (within > the limits of accuracy of the physical clock, etc.) when scheduled, > unless there was an equal or higher priority task using every > available processor. There was even an AI, published as a > ramification, titled "Preemptive scheduling is required" (AI-32). > Can it get any clearer? Well, yes, it can. It's got little to do with pre-emption (in fact, preemption is often a bad thing in real-time systems). It's got to do with "delay" vs "delay until". Delay creeps the time forwards so that time events get jittered and periodic events drift. I think we should have a little respect for the Boeing point-of-view. It doesn't help them to be continuously pushed by academics to adopting unsound techniques. They have to live with the consequences of screwing up big time.