Personally, I would never change ASH parameters unless I had a very good reason for doing so. Even more risky would be to disable ASH. As Oracle turns increasingly to automated performance management, it needs the best performance data available, and it gets much of this data from ASH.

The two background processes MMON and MMNL are deeply involved with ASH activity. MMON, short for Manageability Monitor, wakes up every few seconds (but like DBWR and LGWR, it can be woken up by another process) and is involved in writing ASH buffers to the AWR tables. MMNL, short for Management Monitor Light, is responsible for gathering active session details once every second (default). It gathers operating system information (for example, from /proc/stat) and is also involved with writing ASH buffers to the AWR tables.

One way of visualizing the ASH buffers is to think of a ring structure. This model helps you understand how new ASH records are written into memory, how old ASH records are overwritten, how ASH buffers are referenced via the v$active_session_history view, and how ASH records are written to the AWR tables.