How Busy Is Busy
Last timeThings Arriving, and Things Being Served
Utilisation is the share of time the servers are occupied. It is the number everybody reports and, taken on its own, it says almost nothing about how long anything waits.
Computing utilisation
Utilisation is the share of the time the servers are occupied. If ten workers
are busy on average seven and a half of the time, the utilisation is nought
point seven five, usually written as seventy five per cent.
It is computed from the two rates of the previous lesson. The work arriving each
second occupies the servers for the arrival rate multiplied by the service time,
and dividing that by the number of servers gives the share of each one.
- the utilisation, a share between nought and one
- the number of servers
- the service time in seconds
- the arrival rate in units of work per second
Two properties make it the number everybody reports. It has no units, having
cancelled them in the division, so a web service, a disk and a support desk can
be compared on one scale. And it is bounded between nought and one, so it fits a
dial and a dashboard without anybody choosing a scale.
Rearranging it for capacity
Read in the planning direction, the relation answers the question capacity
planning actually asks, which is not how busy the system is but how much more it
can take.
The lesson stops here
4 more paragraphs to go
You have read the opening. The rest of the argument, the problems that check whether it landed, and the lines worth keeping at the end all come with a plan.
The first lesson of every course in the library reads the whole way through, free, so you can see exactly what the rest of them are.
See the planThe contentsThis is the reading half
Starting the course gives you your own copy of it. Every idea on every page has problems standing under it, marked with a reason rather than a tick, and any sentence you do not believe can be opened and argued with. None of that can happen on a page nobody owns.
The contents