The process monitor process (PMON) periodically scans all processes to find any that have terminated abnormally. PMON delegates cleanup work to the cleanup main process (CLMN), which periodically performs cleanup of terminated processes and sessions. CLMN delegates to cleanup worker processes (CLnn).
PMON runs as an operating system process and not as a thread. In addition to database instances, PMON also runs on Oracle Automatic Storage Management (Oracle ASM) instances and Oracle ASM Proxy instances.