In this Document

Applies to:

Purpose

This document is intended to assist Database Administrators resolve issues
encountered involving Oracle Background Processes. It mainly applies to a single
instance environment and looks at a basic typical database configuration.

Troubleshooting Steps

Concepts

To maximize performance and accommodate many users, a multiprocess Oracle
Database system uses background processes. Background processes consolidate
functions that would otherwise be handled by multiple database programs running
for each user process. Background processes asynchronously perform I/O and
monitor other Oracle Database processes to provide increased parallelism for
better performance and reliability.

Issues Involving Oracle Background Processes

Oracle Database creates background processes automatically when a database
instance starts. An instance can have many background processes, not all of
which always exist in every database configuration.

The following information helps to outline some of the known issues involving
common Oracle background processes, and has been grouped by background process
where possible.

ORA- errors related to Background Processes

ORA-443 background process did not start

          See Note 18442.1 - OERR: ORA 443 background process
did not start

ORA-444 background process failed while starting

          See Note 18443.1 - OERR: ORA 444 background process
failed while starting

ORA-445 background process did not start after seconds

          See Note 18444.1 - OERR: ORA 445 background process
did not start after seconds

ORA-446 background process started when not expected

          See Note 18445.1 - OERR: ORA 446 background process started
when not expected

ORA-447 fatal error in background process

          See Note 18446.1 - OERR: ORA 447 fatal error in background
process

ORA-448 normal completion of background process

          See Note 18447.1 - OERR: ORA 448 normal completion of
background process

ORA-449 background process unexpectedly terminated with error

          See Note 18448.1 - OERR: ORA 449 background process
unexpectedly terminated with error

ORA-450 background process '%s' did not start

          See Note 287652.1 - OERR: ORA-450 background process '%s'
did not start

Generic issues not related to a specific background
process

CJQ0 / Jnnn ( Job Queue
Coordinator Process / Job Queue Slave Processes )

CJQ0 is automatically started and stopped as needed by Oracle Scheduler.

The JOB_QUEUE_PROCESSES initialization parameter specifies the
maximum number of processes that can be created for the execution of jobs. CJQ0
starts only as many job queue processes as required by the number of jobs to run
and available resources.

Job slave processes are created or awakened by the job coordinator when it is
time for a job to be executed.

Job slaves gather all the metadata required to run the job from the data
dictionary. The slave processes start a database session as the owner of the
job, execute triggers, and then execute the job. After the job is complete, the
slave processes commit and then execute appropriate triggers and close the
session. The slave can repeat this operation in case additional jobs need to be
run.

Issues Specific to CJQ0 / Jnnn

CKPT ( Checkpoint Process )

At specific times CKPT starts a checkpoint request by messaging DBWn to begin writing dirty buffers. On completion of
individual checkpoint requests, CKPT updates data file headers and control files
to record most recent checkpoint.

Issues Specific to CKPT

DBWn ( Database Writer Process )

The primary responsibility of DBWn is to
write data blocks to disk. DBWn also handles
checkpoints, file open synchronization, and logging of Block Written
records.

In many cases the blocks that DBWn writes
are scattered throughout the disk. Thus, the writes tend to be slower than the
sequential writes performed by LGWR. DBWn
performs multiblock writes when possible to improve efficiency. The number of
blocks written in a multiblock write varies by operating system.

The DB_WRITER_PROCESSES initialization parameter specifies the
number of DBWn processes (DBW0-DBW9 and
DBWa-DBWz). The database selects an appropriate default setting for this
parameter or adjusts a user-specified setting based on the number of CPUs and
processor groups.

Issues Specific to DBWn

EMNC / Ennn ( EMON
Coordinator Process / EMON Slave Process )

EMNC coordinates event management and notification activity in the database,
including Streams Event Notifications, Continuous Query Notifications, and Fast
Application Notifications.

The database event management and notification load is distributed among the
EMON slave processes. These processes work on the system notifications in
parallel, offering a capability to process a larger volume of notifications, a
faster response time, and a lower shared memory use for staging
notifications.

Issues Specific to EMNC / Ennn

LGWR ( Log Writer Process )

Redo log entries are generated in the redo log buffer of the system global
area (SGA). LGWR writes the redo log entries sequentially into a redo log file.
If the database has a multiplexed redo log, then LGWR writes the redo log
entries to a group of redo log files.

Issues Specific to LGWR

MMON / Mnnn (
Manageability Monitor Process / MMON Slave Process )

MMON performs many tasks related to manageability, including taking Automatic
Workload Repository snapshots and performing Automatic Database Diagnostic
Monitor analysis.

Mnnn performs manageability tasks dispatched to them by MMON. Tasks
performed include taking Automatic Workload Repository snapshots and Automatic
Database Diagnostic Monitor analysis.

Issues Specific to MMON / Mnnn

PMON ( Process Monitor )

PMON periodically performs cleanup of all the following:

• 
  

  Processes that died abnormally



• 
  

  Sessions that were killed



• 
  

  Detached transactions that have exceeded their idle
  timeout



• 
  

  Detached network connections which have exceeded their
  idle timeout

In addition, PMON monitors, spawns, and stops the following as needed:

• 
  

  Dispatcher and shared server processes



• 
  

  Job queue processes



• 
  

  Pooled server processes for database resident connection
  pooling



• 
  

  Restartable background processes

PMON is also responsible for registering information about the instance and
dispatcher processes with the network listener.

Issues Specific to PMON

QMNC / QMNn ( AQ
Coordinator Process / AQ Server Class Process )

QMNC is responsible for facilitating various background activities required
by AQ and Oracle Streams: time management of messages, management of
nonpersistent queues, cleanup of resources, and so on. QMNC dynamically spawns
Qnnn processes as needed for performing these
tasks.

Note that if the AQ_TM_PROCESSES initialization parameter is set
to 0, this process will not start. The database writes the following message to
the alert log: WARNING: AQ_TM_PROCESSES is set to 0. System might be
adversely affected.

Qnnn acts as a slave process for QMNC and
carry out tasks assigned by QMNC. The number of these processes is dynamically
managed by QMNC based on load.

Issues Specific to QMNC / QMNn

RECO ( Recoverer Process )

RECO uses the information in the pending transaction table to finalize the
status of in-doubt transactions. At timed intervals, the local RECO attempts to
connect to remote databases and automatically complete the commit or rollback of
the local portion of any pending distributed transactions. All transactions
automatically resolved by RECO are removed from the pending transaction
table.

Issues Specific to RECO

SMON ( System Monitor Process )

SMON performs many database maintenance tasks, including the following:

• 
  

  Creates and manages the temporary tablespace metadata



• 
  

  Reclaims space used by orphaned temporary segments



• 
  

  Maintains the undo tablespace by onlining, offlining, and shrinking the undo
  segments based on undo space usage statistics



• 
  

  Cleans up the data dictionary when it is in a transient and inconsistent
  state



• 
  

  Maintains the SCN to time mapping table used to support Oracle Flashback
  features

Issues Specific to SMON

VKTM ( Virtual Keeper of Time Process )

VKTM acts as a time publisher for an Oracle instance. VKTM publishes two sets
of time: a wall clock time using a seconds interval and a higher resolution time
(which is not wall clock time) for interval measurements. The VKTM timer service
centralizes time tracking and offloads multiple timer calls from other
clients.

Issues Specific to VKTM

Wnnn ( Space Management Slave Process
)

Wnnn processes are slave processes dynamically spawned by SMCO to
perform space management tasks in the background. These tasks include
preallocating space into locally managed tablespace and SecureFiles segments
based on space usage growth analysis, and reclaiming space from dropped
segments. At most 10 Wnnn slaves can run on one database instance.
After being started, the slave acts as an autonomous agent. After it finishes
task execution, it automatically picks up another task from the queue. The
process terminates itself after being idle for a long time.

Issues Specific to Wnnn

Helpful Articles on Background Processes

Further Diagnostics

If you were not able to resolve the issue with the details provided in this
document, please raise a Service Request for further assistance from Oracle
Support.

References

Related …

Products …

• Oracle
  Database Products > Oracle
  Database Suite > Oracle
  Database > Oracle
  Database - Enterprise Edition > RDBMS
  > Background
  processes

Keywords …

BACKGROUND
PROCESSES