New Mandatory Background Processes In Oracle Database 12cR2
New Mandatory Background Processes In Oracle Database 12cR2-:
Mandatory background processes are present in all typical database configurations.
These processes run by default in a read/write database instance started with a minimally configured initialization parameter file.
BWnn (Database Writer Processes)
Cleanup Main Process (CLMN)
Cleanup Helper Processes (CLNN)
LGnn (Log Writer Worker)
Listener Registration Process (LREG)
Process Manager (PMAN)
BWnn(Database Writer Processes)-:
There can be 1 to 100 Database Writer Processes. The names of the first 36 Database Writer Processes are DBW0-DBW9 and DBWa-DBWz. The names of the 37th through 100th Database Writer Processes are BW36-BW99.
The DB_WRITER_PROCESSES initialization parameter specifies the number of Database Writer Processes.
Cleanup Main Process (CLMN)-:
Cleanup Main Process (CLMN), and Cleanup Helper Processes (CLnn). These processes are responsible for the monitoring and cleanup of other processes.
PMON delegates cleanup work to the cleanup main process (CLMN). The task of detecting abnormal termination remains with PMON.
CLMN periodically performs cleanup of terminated processes, terminated sessions, transactions, network connections, idle sessions, detached transactions, and detached network connections that have exceeded their idle timeout.
Cleanup Helper Processes (CLnn)-:
CLMN delegates cleanup work to the CLnn helper processes.
The CLnn processes assist in the cleanup of terminated processes and sessions. The number of helper processes is proportional to the amount of cleanup work to be done and the current efficiency of cleanup.
A cleanup process can become blocked, which prevents it from proceeding to clean up other processes. Also, if multiple processes require cleanup, then cleanup time can be significant. For these reasons, Oracle Database can use multiple helper processes in parallel to perform cleanup, thus alleviating slow performance.
The V$CLEANUP_PROCESS and V$DEAD_CLEANUP views contain metadata about CLMN cleanup. The V$CLEANUP_PROCESS view contains one row for every cleanup process. For example, if V$CLEANUP_PROCESS.STATE is BUSY, then the process is currently engaged in cleanup.
V$CLEANUP_PROCESS provides information on the PMON processes.
This view is available starting with Oracle Database 12c Release 2 (126.96.36.199).
V$DEAD_CLEANUP shows the dead processes and killed sessions present in the instance and their cleanup status.
LGnn (Log Writer Worker)-:
On multiprocessor systems, LGWR creates worker processes to improve the performance of writing to the redo log. LGWR workers are not used when there is a SYNC standby destination.
Possible processes include LG00-LG99.
Listener Registration Process (LREG)-:
The listener registration process (LREG) registers information about the database instance and dispatcher processes with the Oracle Net Listener.
When an instance starts, LREG polls the listener to determine whether it is running. If the listener is running, then LREG passes it relevant parameters. If it is not running, then LREG periodically attempts to contact it.
LREG notifies the listeners about instances,services, handlers, and endpoint.
In releases before Oracle Database 12c, PMON performed the listener registration.
Process Manager (PMAN)-:
Process Manager (PMAN) oversees several background processes including shared servers, pooled servers, and job queue processes.
PMAN monitors, spawns, and stops the following types of processes:
Dispatcher and shared server processes
Connection broker and pooled server processes for database resident connection pools
Job queue processes
Restartable background processes
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(OCE in 11gR2 RAC)
(OCP in 11gR2/10gR2/9iR2)