Date: Sat, 6 Jun 1998 05:35:58 -0400 (EDT) From: Peter Dufault <dufault@hda.com> To: tlambert@primenet.com (Terry Lambert) Cc: mike@smith.net.au, calebm@uspaceware.com, hackers@FreeBSD.ORG Subject: Re: FreeBSD as a RTOS Message-ID: <199806060935.FAA09928@hda.hda.com> In-Reply-To: <199806060537.WAA29354@usr08.primenet.com> from Terry Lambert at "Jun 6, 98 05:37:23 am"
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> > > The scheduler would let give the highest rate > > > process time, then the next higher rate, and so on with the caveat that > > > a process gets no more cpu time than it's scheduled for, i.e. if process > > > A is scheduled for every 30 seconds and process B for every 15 seconds, > > > if A is not done by the next 15 second interval it is preempted and B is > > > allowed to run. > > This is called a "fixed scheduling class". This is what SVR4.2 implements > for X servers. The reality is that this is bogus, because what they are > doing is implementing a time division instead of a priority. The various > requisite pages are still swapped out (for example, with the SVR4 "ld" > program), and the time is spent swapping the pages back in. The problem > is *really* that the working set for a non-RT process does not have a quota > associated with it. This is useful for some people. I've seen this referred to as a frame scheduler and it is used in real time simulations - you have an update rate for the overall simulation, and then budget the time to the pieces of the simulation the way we used to assign people an 8K block of memory for a given task way back when. It is a simple reliable model that you can obviously achieve other ways. My ideas about real real time involve SMP or a degraded time multiplexed CPU, priority inheritence, and detection of non realtime safe conditions generating signals to the concerned processes so that you can detect and avoid unsafe situations with resource assignment. Obviously the last item is intended to happen only in a development stage. Peter -- Peter Dufault (dufault@hda.com) Realtime development, Machine control, HD Associates, Inc. Safety critical systems, Agency approval To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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