How Much time recovery after Killing the Long Running Session will take

select state,undoblocksdone,undoblockstotal,undoblocksdone / undoblockstotal * 100
from v$fast_start_transactions;
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The FAST_START_PARALLEL_ROLLBACK parameter controls how much juice SMON will take. The three possible parameters are: FALSE, LOW, and HIGH. A setting of FALSE will tell SMON to do the recovery on its own without parallel assistance. This could work to allow your app to remain online while SMON does its job, but could still cause contention issues. In the end, it is better to let SMON finish the work while other processes try to lay off UNDO as much as possible. A setting of LOW will tell SMON to use 2 * CPU_COUNT parallel processes to do the work, and a setting of high will use 4 * CPU_COUNT. In order to allow SMON to finish its work as quickly as possible, we turned off their app and set the FAST_START_PARALLEL_ROLLBACK parameter to HIGH, allowing SMON all the resources it needed to finish. To determine how long it would take to finish, we consulted the V$FAST_START_TRANSACTIONS view