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I'm looking at the Top Activity view in OEM. The left axis is labeled "Active Sessions" (up to 24 in the graph I'm looking at) and there is a line labeled "Maximum CPU". The measured activities include: user i/o, concurrency, commit, etc...

My question: Is OEM simply reporting the number of sessions involved in the activity, as the axis indicates? Or is it measuring the total duration of the activity over all sessions?

To put a point to it, concurrency (we believe) is the activity of presenting a consistent block to each session that requires it. Session A may see one version block 33.444 where session B might see a different version of block 33.444. (Because session A cannot rows updated by B which are not yet committed, and vice versa.)

But presenting a consistent version of a given block might be a simple task, or it might be a complicate task depending on how many uncommitted changes have been made to a block, and how many sessions are interested in it.

So if "concurrency" simply counts the session involved (as the vertical axis indicates) then we get one interpretation of the graph. But if it counts the duration, we get a different view.

Any insights on what OEM specifically measures would be appreciated. Thanks in advance!

Chris Curzon

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  • "Concurrency" is the name of one of the wait classes. It's about locking in general (click on it to drill down). As for "two interpretations", not really, boils down to the same thing.
    – Mat
    Nov 4 '14 at 7:02
  • Y axis is 'Active Sessions', the value plotted is called 'Average active Sessions' (AAS). Say you have 24 CPUs, so your system can support 24 ACTIVE sessions at any given time. ashmasters.com/wp-content/uploads/2008/10/… would make a a very good reading.
    – Raj
    Nov 4 '14 at 16:25
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The graph is showing the number of sessions in wait. The colors represent a broad category of wait class, which assists in quickly identifying the primary type of wait faced by sessions in the database.

The maximum CPU line simply gives you a marker to determine if the amount of CPU wait is reaching extremely critical levels.

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