Application or Oracle tool: A database user runs a database application (such as a precompiler program) or an Oracle tool (such as SQL*Plus), which issues SQL statements to an Oracle database.
Oracle database server code: Each user has some Oracle database code executing on his or her behalf, which interprets and processes the application's SQL statements.
These code modules are run by processes. A process is a "thread of control" or a mechanism in an operating system that can run a series of steps. (Some operating systems use the terms job or task.) A process normally has its own private memory area in which it runs.
Multiple-Process Oracle Systems
Multiple-process Oracle (also called multiuser Oracle) uses several processes to run different parts of the Oracle code and additional processes for the users—either one process for each connected user or one or more processes shared by multiple users. Most database systems are multiuser, because one of the primary benefits of a database is managing data needed by multiple users at the same time.
Each process in an Oracle Database instance performs a specific job. By dividing the work of Oracle Database and database applications into several processes, multiple users and applications can connect to a single database instance simultaneously while the system maintains excellent performance.
Types of Processes
The processes in an Oracle Database system can be categorized into two major groups:
- User processes run the application or Oracle tool code.
- Oracle Database processes run the Oracle database server code. They include server processes and background processes.
The process structure varies for different Oracle Database configurations, depending on the operating system and the choice of Oracle Database options. The code for connected users can be configured as a dedicated server or a shared server.
With dedicated server, for each user, the database application is run by a different process (a user process) than the one that runs the Oracle database server code (a dedicated server process).
With shared server, the database application is run by a different process (a user process) than the one that runs the Oracle database server code. Each server process that runs Oracle database server code (a shared server process) can serve multiple user processes.
Following figure illustrate dedicated server. Each connected user has a separate user process, and several background processes run Oracle Database.
User Processes
When a user runs an application program (such as a Pro*C program) or an Oracle tool (such as Oracle Enterprise Manager or SQL*Plus), Oracle Database creates a user process to run the user's application.
Connections and Sessions
Connection and session are closely related to user process but are very different in meaning.
A connection is a communication pathway between a user process and an Oracle Database instance. A communication pathway is established using available interprocess communication mechanisms (on a computer that runs both the user process and Oracle Database) or network software (when different computers run the database application and Oracle Database, and communicate through a network).
A session is a specific connection of a user to an Oracle Database instance through a user process. For example, when a user starts SQL*Plus, the user must provide a valid user name and password, and then a session is established for that user. A session lasts from the time the user connects until the time the user disconnects or exits the database application.
Multiple sessions can be created and exist concurrently for a single Oracle Database user using the same user name. For example, a user with the user name/password of SCOTT/TIGER can connect to the same Oracle Database instance several times.
In configurations without the shared server, Oracle Database creates a server process on behalf of each user session. However, with the shared server, many user sessions can share a single server process.
This section describes the two types of processes that run the Oracle database server code (server processes and background processes). It also describes the trace files and alert logs, which record database events for the Oracle Database processes.
Oracle Database Server Processes
Oracle Database creates server processes to handle the requests of user processes connected to the instance. In some situations when the application and Oracle Database operate on the same computer, it is possible to combine the user process and corresponding server process into a single process to reduce system overhead. However, when the application and Oracle Database operate on different computers, a user process always communicates with Oracle Database through a separate server process.
Server processes (or the server portion of combined user/server processes) created on behalf of each user's application can perform one or more of the following:
- Parse and run SQL statements issued through the application
- Read necessary data blocks from datafiles on disk into the shared database buffers of the SGA, if the blocks are not already present in the SGA
- Return results in such a way that the application can process the information
Oracle Database Background Processes
To maximize performance and accommodate many users, a multiprocess Oracle Database system uses some additional Oracle Database processes called background processes.
An Oracle Database instance can have many background processes; not all are always present. There are numerous background processes. See the V$BGPROCESS view for more information on the background processes. The background processes in an Oracle Database instance can include the following:
1. Archiver Processes (ARCn)
The archiver processes (ARCn) copy redo log files to a designated storage device after a log switch has occurred. In addition, they can collect transaction redo data and transmit that data to standby destinations. ARCn processes are present only when the database is in ARCHIVELOG mode, and automatic archiving is enabled.
If you anticipate a heavy workload for archiving, such as during bulk loading of data, you can increase the maximum number of archiver processes with the LOG_ARCHIVE_MAX_PROCESSES initialization parameter. The ALTER SYSTEM statement can change the value of this parameter dynamically to increase or decrease the number of ARCn processes.
2. Checkpoint Process (CKPT)
When a checkpoint occurs, Oracle Database must update the headers of all datafiles to record the details of the checkpoint. This is done by the CKPT process. The CKPT process does not write blocks to disk; DBWn always performs that work.
The statistic DBWR checkpoints displayed by the System_Statistics monitor in Oracle Enterprise Manager indicates the number of checkpoint requests completed.
3. Database Writer Process (DBWn)
The database writer process (DBWn) writes the contents of buffers to datafiles. The DBWn processes are responsible for writing modified (dirty) buffers in the database buffer cache to disk. Although one database writer process (DBW0) is adequate for most systems, you can configure additional processes (DBW1 through DBW9 and DBWa through DBWj) to improve write performance if your system modifies data heavily. These additional DBWn processes are not useful on uniprocessor systems.
When a buffer in the database buffer cache is modified, it is marked dirty. A cold buffer is a buffer that has not been recently used according to the least recently used (LRU) algorithm. The DBWn process writes cold, dirty buffers to disk so that user processes are able to find cold, clean buffers that can be used to read new blocks into the cache. As buffers are dirtied by user processes, the number of free buffers diminishes. If the number of free buffers drops too low, user processes that must read blocks from disk into the cache are not able to find free buffers. DBWn manages the buffer cache so that user processes can always find free buffers.
By writing cold, dirty buffers to disk, DBWn improves the performance of finding free buffers while keeping recently used buffers resident in memory. For example, blocks that are part of frequently accessed small tables or indexes are kept in the cache so that they do not need to be read in again from disk. The LRU algorithm keeps more frequently accessed blocks in the buffer cache so that when a buffer is written to disk, it is unlikely to contain data that will be useful soon.
The initialization parameter DB_WRITER_PROCESSES specifies the number of DBWn processes. The maximum number of DBWn processes is 20. If it is not specified by the user during startup, Oracle Database determines how to set DB_WRITER_PROCESSES based on the number of CPUs and processor groups.
The DBWn process writes dirty buffers to disk under the following conditions:
- When a server process cannot find a clean reusable buffer after scanning a threshold number of buffers, it signals DBWn to write. DBWn writes dirty buffers to disk asynchronously while performing other processing.
- DBWn periodically writes buffers to advance the checkpoint, which is the position in the redo thread (log) from which instance recovery begins. This log position is determined by the oldest dirty buffer in the buffer cache.
In all cases, DBWn performs batched (multiblock) writes to improve efficiency. The number of blocks written in a multiblock write varies by operating system.
4. Job Queue Processes
Job queue processes are used for batch processing. They run user jobs. They can be viewed as a scheduler service that can be used to schedule jobs as PL/SQL statements or procedures on an Oracle Database instance. Given a start date and an interval, the job queue processes try to run the job at the next occurrence of the interval.
Job queue processes are managed dynamically. This allows job queue clients to use more job queue processes when required. The resources used by the new processes are released when they are idle.
Dynamic job queue processes can run a large number of jobs concurrently at a given interval. The job queue processes run user jobs as they are assigned by the CJQ process. Here's what happens:
- The coordinator process, named CJQ0, periodically selects jobs that need to be run from the system JOB$ table. New jobs selected are ordered by time.
- The CJQ0 process dynamically spawns job queue slave processes (J000…J999) to run the jobs.
- The job queue process runs one of the jobs that was selected by the CJQ process for execution. The processes run one job at a time.
After the process finishes execution of a single job, it polls for more jobs. If no jobs are scheduled for execution, then it enters a sleep state, from which it wakes up at periodic intervals and polls for more jobs. If the process does not find any new jobs, then it aborts after a preset interval.
The initialization parameter JOB_QUEUE_PROCESSES represents the maximum number of job queue processes that can concurrently run on an instance. However, clients should not assume that all job queue processes are available for job execution.
Note: The coordinator process is not started if the initialization parameter JOB_QUEUE_PROCESSES is set to 0.
5. Log Writer Process (LGWR)
The log writer process (LGWR) is responsible for redo log buffer management—writing the redo log buffer to a redo log file on disk. LGWR writes all redo entries that have been copied into the buffer since the last time it wrote.
The redo log buffer is a circular buffer. When LGWR writes redo entries from the redo log buffer to a redo log file, server processes can then copy new entries over the entries in the redo log buffer that have been written to disk. LGWR normally writes fast enough to ensure that space is always available in the buffer for new entries, even when access to the redo log is heavy.
LGWR writes one contiguous portion of the buffer to disk. LGWR writes:
- A commit record when a user process commits a transaction
- Redo log buffers
- Every three seconds
- When the redo log buffer is one-third full
- When a DBWn process writes modified buffers to disk, if necessary
Note: Before DBWn can write a modified buffer, all redo records associated with the changes to the buffer must be written to disk (the write-ahead protocol). If DBWn finds that some redo records have not been written, it signals LGWR to write the redo records to disk and waits for LGWR to complete writing the redo log buffer before it can write out the data buffers.
LGWR writes synchronously to the active mirrored group of redo log files. If one of the files in the group is damaged or unavailable, LGWR continues writing to other files in the group and logs an error in the LGWR trace file and in the system alert log. If all files in a group are damaged, or the group is unavailable because it has not been archived, LGWR cannot continue to function.
When a user issues a COMMIT statement, LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the transaction's redo entries. The corresponding changes to data blocks are deferred until it is more efficient to write them. This is called a fast commit mechanism. The atomic write of the redo entry containing the transaction's commit record is the single event that determines the transaction has committed. Oracle Database returns a success code to the committing transaction, although the data buffers have not yet been written to disk.
Note: Sometimes, if more buffer space is needed, LGWR writes redo log entries before a transaction is committed. These entries become permanent only if the transaction is later committed.
When a user commits a transaction, the transaction is assigned a system change number (SCN), which Oracle Database records along with the transaction's redo entries in the redo log. SCNs are recorded in the redo log so that recovery operations can be synchronized in Real Application Clusters and distributed databases.
In times of high activity, LGWR can write to the redo log file using group commits. For example, assume that a user commits a transaction. LGWR must write the transaction's redo entries to disk, and as this happens, other users issue COMMIT statements. However, LGWR cannot write to the redo log file to commit these transactions until it has completed its previous write operation. After the first transaction's entries are written to the redo log file, the entire list of redo entries of waiting transactions (not yet committed) can be written to disk in one operation, requiring less I/O than do transaction entries handled individually. Therefore, Oracle Database minimizes disk I/O and maximizes performance of LGWR. If requests to commit continue at a high rate, then every write (by LGWR) from the redo log buffer can contain multiple commit records.
6. Process Monitor Process (PMON)
The process monitor (PMON) performs process recovery when a user process fails. PMON is responsible for cleaning up the database buffer cache and freeing resources that the user process was using. For example, it resets the status of the active transaction table, releases locks, and removes the process ID from the list of active processes.
PMON periodically checks the status of dispatcher and server processes, and restarts any that have stopped running (but not any that Oracle Database has terminated intentionally). PMON also registers information about the instance and dispatcher processes with the network listener.
Like SMON, PMON checks regularly to see whether it is needed and can be called if another process detects the need for it.
7. Queue Monitor Processes (QMNn)
The queue monitor process is an optional background process for Oracle Streams Advanced Queuing, which monitors the message queues. You can configure up to 10 queue monitor processes. These processes, like the job queue processes, are different from other Oracle Database background processes in that process failure does not cause the instance to fail.
8. Recoverer Process (RECO)
The recoverer process (RECO) is a background process used with the distributed database configuration that automatically resolves failures involving distributed transactions. The RECO process of a node automatically connects to other databases involved in an in-doubt distributed transaction. When the RECO process reestablishes a connection between involved database servers, it automatically resolves all in-doubt transactions, removing from each database's pending transaction table any rows that correspond to the resolved in-doubt transactions.
If the RECO process fails to connect with a remote server, RECO automatically tries to connect again after a timed interval. However, RECO waits an increasing amount of time (growing exponentially) before it attempts another connection. The RECO process is present only if the instance permits distributed transactions. The number of concurrent distributed transactions is not limited.
9. System Monitor Process (SMON)
The system monitor process (SMON) performs recovery, if necessary, at instance startup. SMON is also responsible for cleaning up temporary segments that are no longer in use and for coalescing contiguous free extents within dictionary managed tablespaces. If any terminated transactions were skipped during instance recovery because of file-read or offline errors, SMON recovers them when the tablespace or file is brought back online. SMON checks regularly to see whether it is needed. Other processes can call SMON if they detect a need for it.
With Real Application Clusters, the SMON process of one instance can perform instance recovery for a failed CPU or instance.
10. Other Oracle Database Background Processes
There are several other background processes that might be running. You can view the background processes running on your system by issuing the following SQL query:
SELECT name, description
FROM v$bgprocess
WHERE paddr != '00'
ORDER BY name;
These background processes can include the following:
10.1 ACMS (atomic control file to memory service) per-instance process is an agent that contributes to ensuring a distributed SGA memory update is either globally committed on success or globally aborted in the event of a failure in an Oracle RAC environment.
10.2 DBRM (database resource manager) process is responsible for setting resource plans and other resource manager related tasks.
10.3 DIA0 (diagnosability process 0) (only 0 is currently being used) is responsible for hang detection and deadlock resolution.
10.4 DIAG (diagnosability) process performs diagnostic dumps and executes global oradebug commands.
10.5 EMNC (event monitor coordinator) is the background server process used for database event management and notifications.
10.6 FBDA (flashback data archiver process) archives the historical rows of tracked tables into flashback data archives. Tracked tables are tables which are enabled for flashback archive. When a transaction containing DML on a tracked table commits, this process stores the pre-image of the rows into the flashback archive. It also keeps metadata on the current rows.
10.7 FBDA is also responsible for automatically managing the flashback data archive for space, organization, and retention and keeps track of how far the archiving of tracked transactions has occurred.
10.8 GTX0-j (global transaction) processes provide transparent support for XA global transactions in an Oracle RAC environment. The database autotunes the number of these processes based on the workload of XA global transactions. Global transaction processes are only seen in an Oracle RAC environment.
10.9 MMAN is used for internal database tasks.
10.10 MMNL performs frequent and light-weight manageability-related tasks, such as session history capture and metrics computation.
10.11 MMON performs various manageability-related background tasks, for example:
Issuing alerts whenever a given metrics violates its threshold value
Taking snapshots by spawning additional process (MMON slaves)
Capturing statistics value for SQL objects which have been recently modified
10.12 ARBn performs the actual rebalance data extent movements in an Automatic Storage Management instance. There can be many of these at a time, called ARB0, ARB1, and so forth.
10.13 PSP0 (process spawner) spawns Oracle processes.
10.14 RBAL coordinates rebalance activity for disk groups in an Automatic Storage Management instance. It performs a global open on Automatic Storage Management disks.
10.15 SMCO (space management coordinator) process coordinates the execution of various space management related tasks, such as proactive space allocation and space reclamation. It dynamically spawns slave processes (Wnnn) to implement the task.
10.16 VKTM (virtual keeper of time) is responsible for providing a wall-clock time (updated every second) and reference-time counter (updated every 20 ms and available only when running at elevated priority).
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