What does Usage mean?

What does Usage mean?

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My objective today is to clarify the meaning of one of the most used metric in sizing and in performance analysis: the usage.

First things first

The Usage exposes the state of the service center. It is a binary quantity:

• usage=0 when idle (not working)
• usage=1 when busy (working)

Any service center at any point in time is idle (usage=0) or busy (usage=1).

Usage Percentage (Usage%)

This is the "usage" metric we are familiarized with. It is the average of the usage over a certain time interval, called the averaging time. Typically is expressed as percentage.

In example usage%=20% (for a certain time interval) means that for that time interval the service center has been:

• 20% of the time busy
• 80% of the time idle

Averaging Time

The averaging time for calculating the usage% is of capital importance. Saying 20% is not enough, saying 20% over an hour is right.

Theory is very simple, but in practice the averaging time is frequently dropped. Always ask for it (but don't expect to have a crystal clear response).

To stress its importance look at these graphs representing the time evolution of the usage% metric for the same workload and different averaging time.

Averaging Time = 10 s

Averaging Time = 10 min

Averaging Time = 500 min

I insist: in the three graphs the workload is the same (and the total amount of work done -the area under the usage% curve- remains the same). The short averaging time is best suited for performance analysis, the medium is for sizing, and the long is for trend analysis.