Is CPU usage 100% really okay?

Even the great ones do misspeak from time to time. And when they do, this can lead to a terrible mess in people’s heads.

Here is one example:

Are you at 100% utilization? If not, you haven’t accomplished your job yet. You cannot
put CPU in the bank and save it for later. So, if you are running with idle cycles you
should be looking for more ways to use it.

This statement is, at best, very confusing (and you can see some examples of this confusion in this recent OTN thread).

Let’s apply it to a specific example:

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ASH basics


Active session history (ASH), available since 10g, is one of the greatest performance troubleshooting tools available for Oracle databases. And one of most underutilized, too. In this article I will give some basic guidelines regarding ASH usage, and will list some of my favorite ASH queries.

ASH is a view (V$ACTIVE_SESSION_HISTORY). It can give most of the information found in trace files, but unlike tracing, it’s always on!

Using ASH is extremely simple, because you don’t have enable any events or go to database server filesystem to retrieve the results: you just query the view. However, there are a few basic rules that allow to use ASH with maximum efficiency, and minize the risk of misinterpreting its output.

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