By default, ASH is enabled on all Oracle Database 10g and above systems. With the instance parameter statistics_level=typical (default), ASH will be enabled. You can also directly turn ASH on or off by setting the instance parameter _ash_enable to either true or false. By default, when ASH buffers are written to the AWR tables, only one out of every ten samples is written. This is controlled by the _ash_disk_filter_ratio parameter, which has a default value of 10.

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.