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From: oberman@LLL-ICDC.ARPA ("Oberman, Kevin")
Newsgroups: comp.os.vms
Subject: RE: Running images at priority 1
Message-ID: <8707110525.AA01423@ucbvax.Berkeley.EDU>
Date: Fri, 10-Jul-87 10:48:00 EDT
Article-I.D.: ucbvax.8707110525.AA01423
Posted: Fri Jul 10 10:48:00 1987
Date-Received: Sun, 12-Jul-87 14:23:07 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: "Oberman, Kevin" 
Distribution: world
Organization: The ARPA Internet
Lines: 33

>I've been submitting CPU intensive batch jobs on our VAX 8650 with the priority
>set at 1. It uses some pretty big arrays so it tends to page ALOT. When I run
>my job people say that they notice a degredation to the system. I say that my
>job shouldn't pose a problem because it's priority is lower than theirs.

>Am I wrong in assumming that since my job's priority is set at 1, it shouldn't
>interfere with anybody with a higher priority?

This seems to pop up fairly regularly. Yes, you are wrong. I once looked at all
of the gory details, but even a low priority process competes with higher
priority processes for many resources besides CPU cycles. The process priority
has no effect on these.

In your case I suspect that you are using a fair amount of virtual address
space which, in turn, requires lots of paging and, probably, a big chunk of
physical memory for the working set.

Because memory management is handled by the system kernal, outside of the
normal process context, the fact that the process is running at a low priority
makes no difference. A low priority job may not get many CPU strokes for
the user-mode code, but if it gets enough to cause paging, it can start
impacting the system in a very noticable way.

					R. Kevin Oberman
					Lawrence Livermore National Laboratory
					arpa: oberman@lll-icdc.arpa
   					(415) 422-6955

Disclaimer: Neither my employer nor myself can take resposibility for the
accuracy of this information. I believe it is correct, but if it's not I can
only say "Sorry". I'm a rotten typist and a worse speller, so forgive any silly
errors.
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