Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!hplabs!hpda!hpdslab!hpiacla!scottg
From: scottg@hpiacla.HP.COM (Scott Gulland)
Newsgroups: comp.os.misc
Subject: Re: Re: timeliness of real-time [was: Re: Realtime OS's]
Message-ID: <6930001@hpiacla.HP.COM>
Date: 13 Jul 88 15:20:26 GMT
References: <4702@killer.UUCP>
Organization: Hewlett Packard
Lines: 27

> / hpiacla:comp.os.misc / stan@shell.com (Stan Hanks) / 10:18 am  Jul 11, 1988 /
> Just a quick question, while we're talking about realtime OS:
> 
> what is the nominal interrupt dispatch time for Unix (time from the 
> the hardware interrupt line being yanked until your interrupt handler
> is executing)? 

I can't remember that exact timing, but it is loooong.  Something on the order
of 2.0 milli-seconds on a totally quiescent system and 9.1 milli-seconds on a
reasonable busy system.  The averages are more like 0.8 and 1.3 milli-seconds
respectively.  Note that in real-time systems you should be asking for worst
case rather than nominal.  It does do much good if nine times out of ten that
critical interrupt is handled just fine but on the tenth try you melted down
your reactor because it took a hair too long to open a valve.

These figures are for an HP9000/S840 running under HP-UX 2.0, a real-time 
Unix operating system.  I am assumming that by interrupt handler you mean
a user process for handling the interrupt.  In this case the handler in these
measurements is a real-time process which has the highest priority on the
system.  These measurements were taken using logic analyzers.

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