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!