Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!samsung!munnari.oz.au!metro!cluster!necisa!boyd
From: boyd@necisa.ho.necisa.oz.au (Boyd Roberts)
Newsgroups: comp.unix.internals
Subject: Re: Nice
Message-ID: <2004@necisa.ho.necisa.oz.au>
Date: 31 Jan 91 03:33:33 GMT
References: <1991Jan22.184055.3641@demott.com> <rdavis.665087618@connie.convex.com> <richard.665126102@fafnir.la.locus.com>
Organization: NEC Information Systems Australia Pty. Ltd.
Lines: 19

In article <richard.665126102@fafnir.la.locus.com> richard@locus.com (Richard M. Mathews) writes:
>...  In many (most?) versions of Unix if an interrupt
>makes a process runnable at a higher priority than the running process,
>a context switch will be forced immediately.

No, UNIX does not have pre-emptive scheduling.  When the above event occurs
the kernel context switches at the next user-mode trap, system call, sleep()
or the once-a-second context switch (time quantum expiry).

The low level scheduling is done by code executed by the running process
which checks whether a context switch is required.  If it is, the process
gives up the CPU.  A higher priority process being made runnable doesn't
_force_ the current process to give up the CPU, it gives it up when it
notices that it has to.


Boyd Roberts			boyd@necisa.ho.necisa.oz.au

``When the going gets wierd, the weird turn pro...''