Oracle 10g Processes

Background Processes Revisited

An Oracle database instance can have many background processes, but all of them are not always needed. When the database instance is started, these background processes are automatically created. The important background processes are given next, with brief explanations of each:
Database Writer process (DBWn). The database writer process (DBWn) is responsible for writing modified (dirty) buffers in the database buffer cache to disk. Although one process (DBW0) is sufficient for most systems, you can have additional processes up to a maximum of 20 processes (DBW1 through DBW9 and DBWa through DBWj) to improve the write performance on heavy online transaction processing (OLTP) systems. By moving the data in dirty buffers to disk, DBWn improves the performance of finding free buffers for new transactions, while retaining the recently used buffers in the memory.
Log Writer process (LGWR). The log writer process (LGWR) manages the redo log buffers by writing the redo log buffer to a redo log file on disk in a circular fashion. After the LGWR moves the redo entries from the redo log buffer to a redo log file, server processes can overwrite new entries in to the redo log buffer. It writes fast enough to disk to have space available in the buffer for new entries.
Checkpoint process (CKPT). Checkpoint is the database event to synchronize the modified data blocks in memory with the data files on disk. It helps to establish data consistency and allows faster database recovery. When a checkpoint occurs, Oracle updates the headers of all data files using the CKPT process and records the details of the checkpoint. The dirtied blocks are written to disk by the DBWn process.
System Monitor process (SMON). SMON coalesces the contiguous free extents within dictionary managed tablespaces, cleans up the unused temporary segments, and does the database recovery at instance startup (as needed). During the instance recovery, SMON also recovers any skipped transactions. SMON checks periodically to see if the instance or other processes need its service.
Process Monitor process (PMON). When a user process fails, the process monitor (PMON) does process recovery by cleaning up the database buffer cache and releasing the resources held by that user process. It also periodically checks the status of dispatcher and server processes, and restarts the stopped process. PMON conveys the status of instance and dispatcher processes to the network listener and is activated (like SMON) whenever its service is needed.
Job Queue processes (Jnnn). Job queue processes run user jobs for batch processing like a scheduler service. When a start date and a time interval are assigned, the job queue processes run the job at the next occurrence of the interval. These processes are dynamically managed, allowing the job queue clients to demand more job queue processes (J000J999) when needed. The job queue processes are spawned as required by the coordinator process (CJQ0 or CJQnn) for completion of scheduled jobs.
Archiver processes (ARCn). The archiver processes (ARCn) copy the redo log files to an assigned destination after the log switch. These processes are present only in databases in ARCHIVELOG mode. An Oracle instance can have up to 10 ARCn processes (ARC0ARC9). Whenever the current number of ARCn processes becomes insufficient for the workload, the LGWR process invokes additional ARCn processes, which are recorded in the alert file. The number of archiver processes is set with the initialization parameter LOG_ARCHIVE_MAX_PROCESSES and changed with the ALTER SYSTEM command.
Queue Monitor processes (QMNn). The queue monitor process is an optional background process that monitors the message queues for Oracle Advanced Queuing in Streams.
Memory Monitor process (MMON). MMON is the acronym for Memory Monitor, a new process introduced in Oracle Database 10g associated with the Automatic Workload Repository (AWR). AWR gets the necessary statistics for automatic problem detection and tuning with the help of MMON. MMON writes out the required statistics for AWR to disk on a regularly scheduled basis.
Memory Monitor Light process (MMNL). Memory Monitor Light (MMNL) is another new process in Oracle Database 10g; it assists the AWR with writing the full statistics buffers to disk on an as-needed basis.