Is 100% CPU Util a bad thing?

I was conducting a ITIL V2 Manager course earlier this month where there is a case study on capacity management. In this case study, the students were given a scenario where they were to analyse system performance data (CPU Util over a period of 1 week for a Unix server) and identify potential issues and propose some further actions.

All the students noted the CPU utilisation was above the recommended CPU threshold (which was given in case study) from 10 am to 12 noon where mostly online work is done and also 2 am to 4 am where nightly batch jobs were running.

Guess what? All students indicated that immediate solutions are needed for both situations. These include tuning, demand management, and finally CPU upgrade.

Actually, batch jobs using CPU at 100% may not be a bad thing. Batch jobs are mostly I/O bound. If they are not waiting for data to be fetch/posted to disk or tape, they should be using as much CPU time as possible to complete the processing. Hence using CPU at 100% is not a bad thing for a batch-oriented processing. It would be an issue if the jobs cannot be completed on time (e.g. by daybreak), despite having used close to 100% CPU resources.

As any performance consultant would say...it depends.