Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!ucbvax!decwrl!hplabs!hpl-opus!hpccc!hp-sde!hpcea!hpcilzb!hpcuhb!hpda!hpcupt1!kluft From: kluft@hpcupt1.HP.COM (Ian Kluft) Newsgroups: comp.unix.questions Subject: Re: Real-time UNIX - what is it & who has it? Message-ID: <6060013@hpcupt1.HP.COM> Date: 24 Jun 88 20:34:30 GMT References: <649@necis.UUCP> Organization: HP Network Systems Group, Cupertino CA Lines: 38 / adamm@necis.UUCP (Adam Moskowitz) / writes: > A friend of mine has asked to help him locate a "real-time UN*X or UN*X-like > operating system". [ ... ] > Assuming that you are doing something like data acquisition or > process control, what is required to make an O/S "real-time"? > > The answer he and I came up with was this: the ability to have absolute > control over the scheduling of processes. Two issues to note when looking for a real-time OS are kernel pre-emption context switching time Kernel pre-emption is an important one to consider on Unix because most Unix systems do not allow system calls to be interrupted. Of course, context-switching time is somewhat obvious because it affects the time between arrival of an interrupt and (re)starting a user process to handle the event. That's really what it comes down to. If a guarantee can be made of the amount of time it takes between an event and entering the code to handle it, an OS is considered real-time. Of course, if that time is too long, it isn't worth mentioning. [End of unbiased material] While everyone was mentioning the real-time Unix's they knew of, I thought I'd mention the one I work with. Hewlett-Packard's HP-UX is a full System V with extentions for BSD 4.2 and real-time. While I don't have the exact numbers for measured real-time response, I remember that they are measured in milliseconds, as would be expected. ------------------------------------------------------------ Ian Kluft RAS Lab hplabs!hprasor!kluft HP Network Systems Group kluft@hpda.hp.com Cupertino, CA ------------------------------------------------------------