Session Management - Oracle 10g/11g Data and Database Management Utilities - page 185

Throughout these scenarios it was evident the advantage of controlling resource
allocation to the different database sessions through the service mapping. This is
particularly useful when there is no way to distinguish a session user in the database.
Once it was configured it becomes easy to access as the application only has to
change the connection descriptor to get into the database through an specific service.
When this concept is extended to the RAC environment the tuning possibilities get
multiplied as this allows a better workload balance, a dynamic node allocation, and
it allows the DBA to better comply with the service level agreements.
Active Session History (ASH)
Let's assume there was an outstanding user activity, the performance was reported
to be slow, but there is no more activity on the database, how would the DBA be able
to identify what the problem was ? The only possible way is by means of a tool that is
able to generate a report of the past user activity, the Active Session History Report,
known also as ASH.
You can view the Active Session History Report via the Performance tab. At the
Average Active Sessions section you should see a button named Run ASH Report,
clicking this will take you to the ASH report time frame specification, once the period
of time has been set, the report can be generated.