Path: utzoo!utgpu!water!watmath!clyde!rutgers!sri-spam!ames!pasteur!ucbvax!hplabs!hpda!hpdslab!hpiacla!mlight
From: mlight@hpiacla.HP.COM (Mike Light )
Newsgroups: comp.unix.wizards
Subject: Re: UNIX for realtime
Message-ID: <4070009@hpiacla.HP.COM>
Date: 13 Jan 88 18:07:54 GMT
References: <449@aati.UUCP>
Organization: Hewlett Packard
Lines: 25

> Why is "plain vanilla" UNIX considered inferior for realtime applications
> and what can be done to improve its realtime capabilities?

The heart of the issue is quick servicing of interrupts and
transference of control to a process monitoring a device (let's
call it a "Control Rod" for example).

There are Windows of Time where an O/S's tables or state information
would appear "inconsistent" should a process be allowed to execute
right then.  Thus, to guarantee rapid notification and processing
of a Control Rod interrupt, any O/S Inconsistency Windows need to
be of very short duration.

In instrumenting a vanilla 4.2 BSD kernel, for example, there were
windows of up to 4 milliseconds (14,000 instructions on the particular
machine) where the O/S could not allow a process to execute.
Even one-half millisecond is too long for many device-control
processes.

Through some judicious reworking of algorithms in the 4.2 kernel,
the average Inconsistency Window was reduced to something like
300 instructions, which met the 100 microsecond goal.
We call it "HP-UX/RT", Playing at an HP Sales Office near You!

     -- Mike Light.