Active Session History (ASH) performed an emergency flush. This may mean that ASH is undersized. If emergency flushes are a recurring issue, you may consider increasing ASH size by setting the value of _ASH_SIZE to a sufficiently large value. Currently, ASH size is 123...7 bytes.
This alert indicate sudden increase of database active sessions, that fill-up ASH buffer faster then usual and therefore raise this alert. Actually this is not a problem but an indication of more ASH buffer is needed to support peak activity on DB.
_ASH_SIZE is a hidden parameter and oracle not suggested to set such parameter without consulting their support. There is a meta link doc ( id 1385872.1) suggest we can increase 50% of current ASH buffer, if we repeatedly receive this alert. To monitor use this
emergency_count FROM v$ash_info;