Oracle instrumented its code starting with Oracle 7. The beauty of this instrumentation is that Oracle made the information available through the SQL interface. Oracle could have forced us to trace a specific process or processes, and perhaps use an additional command like tkprof to consolidate and format the information. By making the instrumentation details easily available through the performance views, Oracle has encouraged performance analysts and vendors to use the information.

I personally believe that Oracle instrumented its code for kernel optimization, not for DBAs or application developers. An obvious clue is the names given to describe the timing, such as db file scattered read to relate non-buffer cache multiblock reads and latch free to signify latch sleep time. No DBA would have come up with names like these!

Oracle has never guaranteed it has completely instrumented it code. This leaves open the possibility for unaccounted for time. If you are counting each millisecond and come up short, one of the possible reasons for this is that Oracle simply did not instrument a piece of code you ran. For this, and many other reasons, I don't expect to account for every little bit of time in my analyses. This may seem like a problem, but I don't believe it is.