Oracle Performance Tuning - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 360

Oracle Active Session History (ASH)
ASH is mechanisms by which we can get the real-time
information about the active sessions as the sessions get sampled
every second. We studied Automatic Database Diagnostics
Monitor earlier in this chapter; ADDM helps us identifying the
problems by analyzing the data captured in AWR. But the
problem in this scenario is that MMON process wakes up after
every hour to gather statistics and then saves in AWR. In case
there are some short-lived performance problems having a
lifespan of 10 minutes or in other words less than the duration in-
between the two taken snapshot then ADDM will not be able to
capture such performance issues. Such performance problems
are called as Transient Performance Problems and can be
addressed better using Oracle provided the ASH mechanism. Its
architecture is comprised of following components.
ASH Memory Buffer: Portion of RAM, gets utilized in a circular
fashion.
MMON Lite (MMNL) process: MMNL is a new Oracle
Instance's background process which gathers active sessions'
information and moves it over to ASH memory buffer and then
flushes the ASH memory buffer to AWR before it gets over
written (out of space).
You can view these ASH statistics using
V$ACTIVE_SESSION_HISTORY dynamic performance view.
In case the collected statistics are more than the size of ASH
Memory buffer then you can collect the information from
DBA_HIST_ACTIVE_SESS_HISTORY data dictionary view
(based on WRH$_ACTIVE_SESSION_HISTORY AWR table).
DBA_HIST_ACTIVE_SESS_HISTORY displays the history of
the contents of the in-memory ASH statistics or in other words
snapshots of V$ACTIVE_SESSION_HISTORY. Moreover,
using Oracle Enterprise Manager Database Control you can have
the ASH report generated at any time.