LREG (Listener Registration) is an Oracle background process created when you start an Oracle 12c or higher database instance. This function was handled by the PMON process in earlier releases.It registers the instance with the listeners.
The Listener Registration process attempts register the information about the database instance and services if the listener is running. If the listener is not running it will keep trying periodically connect to it. It may take some time till LREG makes all listener registration after you start it, if you want to make an immediate registration you must keep using “ALTER SYSTEM REGISTER;” exactly as in the older versions.
[oracle@PROD ]$ ps -ef | grep lreg oracle 18174 1 0 2018 ? 00:04:37 asm_lreg_+ASM1 oracle 1675 1 0 Jan27 ? 00:00:02 ora_lreg_PROD
To trace the service registration to Listener,
SQL> alter system set events='immediate trace name listener_registration level 3'; System altered. SQL> alter system register; System altered. SQL> SELECT VALUE FROM V$DIAG_INFO WHERE NAME = 'Diag Trace'; VALUE ---------------------------------------------- /oracle/app/oracle/diag/rdbms/PROD/PROD1/trace [oracle@PROD trace]$ cd /app/oracle/diag/rdbms/PROD/PROD1/trace [oracle@PROD trace]$ ls -lrt *lreg* -rw-r----- 1 oracle oinstall 175 Sep 19 23:44 PROD_lreg_9012.trm -rw-r----- 1 oracle oinstall 4972 Sep 19 23:44 PROD_lreg_9012.trc