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Oracle Process Monitor (PMON) background process

The Process Monitor Process (PMON) periodically checks all processes to find any that have died abnormally. If it finds such a process, PMON initiates and coordinates the Cleanup Main Process (CLMN) and the Cleanup Slave Process slaves (CLnn).
PMON runs as an operating system process, and not as a thread. In addition to database instances, PMON also runs on ASM instances and Oracle ASM Proxy instances.
Every minute, PMON also terminates processes that have exceeded the resource manager's values for max_idle_time and max_idle_blocker_time.
PMON is a background process. 
select
   ses.event,                           -- Likely: pmon timer
   ses.p1text,                          -- Likely: duration,
   ses.p1,                              -- Likely: 300  (i. e. 3 seconds)
   ses.wait_time_micro / 1e6 in_wait_s, -- < 3
   ses.state,
   ses.status
from
   v$session ses
where
   ses.type  = 'BACKGROUND' and
   ses.program like '%(PMON)';

See also

SMON

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