Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!agate!ziploc!eps
From: eps@toaster.SFSU.EDU (Eric P. Scott)
Newsgroups: comp.sys.next
Subject: Re: Realtime Mach
Message-ID: <1254@toaster.SFSU.EDU>
Date: 30 Jan 91 03:14:41 GMT
References: <1991Jan29.102439.6966@canon.co.uk>
Reply-To: eps@cs.SFSU.EDU (Eric P. Scott)
Organization: San Francisco State University
Lines: 47

In article <1991Jan29.102439.6966@canon.co.uk> wu@canon.co.uk
	(Siu-Fan WU) writes:
>Is realtime computing possible on the NeXT? I couldn't find anything
>in SysRef Chapter 16 (Mach Operating System).

This is addressed in NextAnswers os.623, attached.

					-=EPS=-
-------
real time programming

Q:  What support does the NeXT environment supply for real-time
programs?

A:  NeXT's support for real-time programming is enhanced in 2.0.
This capability is also extended by using the DSP as a dedicated,
very programmable serial port.  Actual real-time performance for
code running on the 68030 or 68040 is not guaranteed, because the
Mach kernel itself is not preemptable at this point.  This topic
is a source of continuing work at NeXT, so that we may achieve
some guaranteed real-time responsiveness in the future.

The primary support in 2.0 for real-time programming is the
addition of scheduling policies that can be set on a per-thread
basis.  We currently have three scheduling policies:
POLICY_TIMESHARE for traditional UNIX scheduling,
POLICY_INTERACTIVE designed for interactive behavior for
applications, and POLICY_FIXEDPRI for non-degrading realtime
priorities.  Additionally, thread_switch() can be used to do
user-level scheduling among a set of threads.

The DSP is inherently a real-time programming environment.  It is
best used for low-level interrupt processing and data reduction/
expansion.  It's currently used this way for real-time expansion
of sound before it's played, as well as data reduction of input
from the DSP serial port.  The DSP can then communicate with a
Mach task using the Sound/DSP driver interface.  The combination
of these features makes the NeXT a suitable platform for a wide
range of real-time programming tasks.  The key is to require no
more than 100ms responsiveness from Mach-level programs with a
light virtual memory load.

In the future we hope to remove the remaining barriers to Mach-
level real-time programming by enhancing the scheduler behavior
and supporting user-level wiring of virtual memory resources.

QA623