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What do you do when waits don't seem to show anything?

Waits work for identifying 99% of the performance issues, but there are a few cases where waits don't work.

There are some areas which are missing wait instrumentation such as (from Richard Powell):
   establishing an outbound DB link connection, various cases
   involving talking to an ASM shadow (which can be confusing) etc...
   Some of these other cases we should, in my opinion, really have
   wait events for but writing to trace probably does not make sense
   for :)

Other examples
  reading from an external table.( identified by Tanel Poder. Graham Wood says this will have it's own wait event in ) 
Waits don't measure log on time so it's important to know the statistics logons per second.
When processing rollback, if it doesn't involve IO, it will simply show up as CPU time. Be sure and monitor the statistics "consistent changes".

Here is an article by James Morle on a case where waits didn't work

And the other side of the coin from Tanel