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From: garry@TCGOULD.TN.CORNELL.EDU (Garry Wiegand)
Newsgroups: mod.computers.vax
Subject: Re: VAX CPU usage not reproducible
Message-ID: <8609221725.AA00488@tcgould.tn.cornell.edu>
Date: Mon, 22-Sep-86 13:25:25 EDT
Article-I.D.: tcgould.8609221725.AA00488
Posted: Mon Sep 22 13:25:25 1986
Date-Received: Tue, 23-Sep-86 18:49:45 EDT
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Reply-To: garry%cadif-oak@cu-arpa.cs.cornell.edu
Organization: Cornell Engineering && Flying Moose Graphics
Lines: 33
Approved: info-vax@sri-kl.arpa

In a recent article A105@UWOCC1.BITNET (Brent Sterner) wrote:
>
>   Help again.  It has been brought to my attention here that a user
>running a fixed job can be charged varying amounts...
>

Hear ye, hear ye, all those who want to charge for CPU time or make accurate
timing measurements:  the "CPU time" figure is *not accurate*. If there is
more than one user on the system, then it is also *not reproducible*.

My suspicion is that CPU only gets charged at "quantum end". If a task is
no longer running when that clock tick happens (for example, it runs for a
little while, then stops for a page fault or I/O completion just before the
tick), it doesn't get charged. Whichever task IS running at that instant
does get charged. So: VMS cpu timing is not measuring - it's sampling!

For charging, we just live with it.

For timing, what I do is:

    1) make sure everyone else is off the system
    2) log in on another terminal, set priority down, and start a little
       "pure-compute" program. Empty DO-loops are satisfactory.
    3) run the program I want to time, and
    4) ask the "pure-compute" program how many machine instructions it managed
       to get while 3) was in progress.

Context-switching between the foreground and the background may be soaking
up a little machine time, but this is the most accurate method I've found. Or
at least reproducible.

PS - don't trust low-priority batch jobs not to scramble your figures either -
     the page-faulter doesn't seem to care what the nominal job priority is!