Re: What to do with CPU seconds?

[email@xxxxxxxxxxxxxxxxxxx (James W Kilgore)]

Bernd,

I don't have the book available, but to test my certification eligibility ;-)

The CPU utilization has to do with the % of usage with the measured time span.

Let's say you do a WRKACTJOB and at the top of the screen the system reports CPU
90% and elapsed time 00:00:15

That means that within the past 15 seconds, the CPU was active 90% of the time.
(or idle 10%)

At the job level, (where a job reports 90%) it does -not- mean that a job hogged
90% of the CPU cycles.  It means that within the measured time span (15 seconds)
the job was using CPU 90% of -its- time.

AFAIK, the measurements do not indicate the amount of CPU being used (that 
should
always be 100%) but the percentage of the jobs time that required CPU usage.  
The
remaining time is I/O waits (disk=short wait, human=long wait).

Interactive jobs may show a small CPU utilization as a result of a display panel
that takes 7 seconds for a user to fill in then there is a burst of CPU and disk
utilization when the ENTER key is pressed.  Now the CPU may only take 1 second
and add another 2 for disk I/O and you wind up with CPU used for 1 second out of
10 therefore a 10% reading and a 3 second response time.

Batch jobs work in a similar manner.  The job log will show xxx CPU seconds used
but that has nothing to do with human clock time.  If a job used 60 CPU seconds
it does not mean that it took 1 minute to complete.  It could very well have
taken 5 minutes clock to run.  It just required CPU 20% of the time.

Now before anyone gets picky, all numbers are for example only.

James-W-Kilgore
email@James-W-Kilgore.com


Bernd wrote:

> Hi, could someone explain what CPU usage expressed in 'CPU seconds'
> means? We've just started with job accounting... What does it mean in terms
> of %CPU used if we'd tell our users,  'you've been using xxx CPU seconds?'
> I can't find any documentation on this, any pointers appreciated.
>
> Best,
> --
> Bernd Schaefers
> besch@okay.net

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