DB Time much higher compared to (Elapsed time * CPU)

Question

11.2.0.2 on Linux x86 64-bit, Single Instance
AWR shows
CPUs : 16
Cores : 8
Sockets : 2
Begin Snap: 10-Jan-12 01:00:17
End Snap: 10-Jan-12 02:00:40
Elapsed: 60.38 (mins)
DB Time: 1,627.47 (mins)
My understanding is that (Elapsed * CPUs) is an upper bound for DB Time. DB Time is much higher though (60.38 * 16 = 966.08).

Answer 1

Databases are rarely single user systems.
My understanding is that (Elapsed * CPUs) is an upper bound for DB TimeNo, this isn't correct. That's the maximum CPU time you have.
DB_TIME gives you the amount of time spent by active sessions actively working or actively waiting.
DB_TIME/ELAPSED_TIME gives you a figure of Average Active Sessions - i.e. on average you have 26 active sessions over this period.
If you think about it, if you have a 60 minute report and you have 1 user actively working or actively waiting, your elapsed time would be 60 minutes, maximum cpu time on 16 cpus is 960 minutes your db time would be 60 minutes.
Double the number of active users to 2, elapsed time still 60 minutes, max cpu time still 960 minutes, db time 120.
Double the number of active users to 4, elapsed time still 60 minutes, max cpu time still 960 minutes, db time 240.
Mulitply the number of active by 10 to 40, elapsed time still 60 minutes, max cpu time still 960 minutes, db time 2400.
Of course, you're unlikely to have 40 users who are constantly active for 60 minutes, but you get the picture.
Edited by: Dom Brooks on Jan 10, 2012 10:54 AM
Reworded