This is a summary / cheat-sheet for diagnosing performance problems with the "USE" method. It's explained in much more detail here: http://www.brendangregg.com/usemethod.html

Worth checking out the rest of the site too, really useful stuff for performance engineers.

For reference, the definitions are:

* utilization: the average time that the resource was busy servicing work

* saturation: the degree to which the resource has extra work which it can't service, often queued

* errors: the count of error events

He described saturation as how much the processes want to use the resource beyond the maximum 100% utilization. E.g. if you look at load average on Linux, that's the size of the run queue (processes that are ready to run + blocking on uninterruptable IO) versus just CPU usage which cannot go over the 100%. That requires there is a meaningful queue. There's lots more in his excellent "Systems Performance" book.