Re: What to do with CPU seconds? -- MIDRANGE-L

Jim,

    I would have to disagree with you about what you said for the amount of CPU
seconds that a job takes up.  If any job on the system reports that it is using 
25%
of CPU, then its 25% of the Systems CPU time.  So, as an example, if two jobs 
over 10
seconds each use 4 seconds of CPU time, then the system during that 10 second
interval would be at 80% CPU used (assuming no other jobs were running during 
that
time).  Each job took 40%.  Now if you added a 3rd job, that used 4 seconds of 
CPU
time during that same interval, then each job would only be able to get 3.33 
seconds,
as you cant get more than 10 CPU seconds in 10 seconds (assuming we are on a 
single
processor).

    Thats how I have understood it to work all these years.


James W Kilgore wrote:

> 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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--
Pete Massiello
OS Solutions International
Phone: (203)-744-7854  Ext 11.
http://www.os-solutions.com
mailto:pmassiello@os-solutions.com



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