Path: utzoo!utgpu!watmath!att!cbnewsl!der
From: der@cbnewsl.ATT.COM (david.e.rorke)
Newsgroups: comp.unix.wizards
Subject: Re: Real Time UNIX (was: Re: How do you tell a wizard?)
Summary: RT Scheduling in SVR4.0
Message-ID: <2396@cbnewsl.ATT.COM>
Date: 20 Oct 89 22:05:55 GMT
References: <21153@adm.BRL.MIL> <17150@rpp386.cactus.org> <9509@june.cs.washington.edu>
Organization: AT&T Bell Laboratories
Lines: 45

In article <9509@june.cs.washington.edu>, ka@cs.washington.edu (Kenneth Almquist) writes:
> 
> jfh@rpp386.cactus.org (John F. Haugh II) writes:
> > I was under the impression there was a real-time UNIX from AT&T
> > as I saw references to UNIX/RT ages ago.
> >
> > Also, I saw references to MERT in the 1978 BSTJ UNIX edition.
> > Isn't this available for hosting real-time UNIX implementations?
> 
> MERT stands for "Multi-Environment Real Time".  UNIX/RT is just a
> later name for the same system.  MERT was a structured operating
> system, in contrast to UNIX, where the kernel was one big blob.
>	. 
> 	.
> 	.
> MERT support for the PDP-11 was dropped primarily because if you
> wanted to run UNIX processes, native UNIX was faster.  A variant of
> MERT called DMERT runs on AT&T's 3B20 Duplex machines.  MERT was never
> ported to any other machines.  System V picked up the process locking
> feature (see plock(2)) but not the fixed process priorities.
> 				Kenneth Almquist

Real time scheduling is available (or will be in a couple weeks)
in System V Release 4.0.  The SVR4.0 process scheduler supports
multiple concurrent scheduling policies through a switch mechanism
(somewhat like the file system switch).

Time sharing and real time policies are provided with SVR4.0 and
the interface between generic and policy specific scheduling code
is well defined to facilitate development of additional policies
(although AT&T is not currently promising to support this kernel
interface unchanged in future releases).

The real time policy provides priority scheduling where the priority
is completely controlled by the user level application.  Preemption
points have been added into long code paths in the kernel to reduce
the delay to a high priority real time process which becomes runnable
while a lower priority process is running in the kernel.


Dave Rorke
AT&T Bell Laboratories
Summit, NJ
att!attunix!der
201-522-6025