Huge CPU Wait spikes displayed in EM12c

In one 11.2.0.3 DB target's performance home page, the Average Active Sessions graph often shows huge temporary spikes in CPU Wait when on a Real Time display showing only Foreground sessions. On a 4-way P6 AIX box (8-cores total), this regularly spikes to 8-10, but as high as 30.

However, drilling into the Top Activity shows maybe a peak of 3, 4, or 5. This confuses me. I would expect the reverse to be more like likely, where the detail of the Top Activity would show occasional brief spikes that are smoothed over in the granularity of the Average Active Sessions.

I can find no explanation as to what the spike is or what -- if anything -- is causing it. This only started after upgrading the target DB from 10.1.0.5, and there is nothing else to indicate a true performance issue with either the DB or the AIX box it's on (that I can see anyway).

Thoughts?

Rich

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