Monday, March 28, 2011

Memory management in Oracle database

Memory management in Oracle database



Introduction to Database Memory Components


The basic memory structures associated with Oracle Database include:

■ System Global Area (SGA)

The SGA is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. The SGA is shared by all server and background processes.

■ Program Global Area (PGA)

A PGA is a memory region that contains data and control information for a server process. It is nonshared memory created by Oracle Database when a server process is started. Access to the PGA is exclusive to the server process. There is one PGA for each server process. Background processes also allocate their own PGAs. The total PGA memory allocated for all background and server processes attached to an Oracle Database instance is referred to as the total instance PGA memory, and the collection of all individual PGAs is referred to as the total instance PGA, or just instance PGA.

It contains global variables and data structures and control information for a server process. example of such information is the runtime area of a cursor. Each time a cursor is executed, a new runtime area is created for that cursor in the PGA memory region of the server process executing that cursor.

The performance of complex long running queries, typical in a DSS environment, depend to a large extent on the memory available in the Program Global Area (PGA) which is also called work area.


Evolution of Memory Management Features


Memory management has evolved with each database release

Oracle 9i


Beginning with Oracle9i, the dynamic SGA infrastructure allowed for the sizing of the Buffer Cache, Shared Pool and the Large Pool without having to shutdown the database. Key features being

* Dynamic Memory resizing


* DB_nK_CACHE_SIZE for multiple block sizes

* PGA_AGGREGATE_TARGET Introduction of Automatic PGA Memory management

Oracle Database 10g


Automatic Shared Memory Management (ASMM) was introduced in 10g. You enable the automatic shared memory management feature by setting the SGA_TARGET parameter to a non-zero value.

Oracle Database 11g


Automatic Memory Management is being introduced in 11g. This enables automatic tuning of PGA and SGA with use of two new parameters named MEMORY_MAX_TARGET and MEMORY_TARGET.

The most important SGA components are the following:




Database Buffer Cache

The database buffer cache is the portion of the SGA that holds copies of data blocks read from datafiles. All users concurrently connected to the instance share access to the database buffer cache.


Redo Log Buffer

The redo log buffer is a circular buffer in the SGA that holds information about changes made to the database. This information is stored in redo entries. Redo entries contain the information necessary to reconstruct, or redo, changes made to the database by INSERT, UPDATE, DELETE, CREATE, ALTER, or DROP operations. Redo entries are used for database recovery, if necessary.


Shared Pool

The shared pool portion of the SGA contains the library cache, the dictionary cache, the result cache, buffers for parallel execution messages, and control structures.


Large Pool

Used for allocating session memory for shared server, Oracle XA, or parallel query buffers or for RMAN.


Java Pool

Java pool memory is used in server memory for all session-specific Java code and data within the JVM.


Streams Pool

The streams pool is used exclusively by Oracle Streams. The Streams pool stores buffered queue messages, and it provides memory for Oracle Streams capture processes and apply processes.


* RESULT_CACHE_MAX_SIZE is new component which has been introduced as part of 11g Memory architecture. The result cache is composed of the SQL query result cache and PL/SQL function result cache, which share the same infrastructure.Results of queries and query fragments can be cached in memory in the SQL query result cache. The database can then use cached results to answer future executions of these queries and query fragments. Similarly PL/SQL Function Result can also be cached.

You have to use RESULT_CACHE_MODE initialization parameter which determines the SQL query result cache behavior. The possible initialization parameter values are MANUAL and FORCE.

Oracle Database 11g supports various memory management methods, which are chosen by initialization parameter settings. Oracle recommends that you enable the automatic memory management method.

1. Automatic Memory Management – For Both the SGA and Instance PGA

2. Automatic Shared Memory Management – For the SGA

3. Manual Shared Memory Management – For the SGA

4. Automatic PGA Memory Management – For the Instance PGA

5. Manual PGA Memory Management – For the Instance PGA

1.Automatic Memory Management – For Both the SGA and Instance PGA

Beginning with Oracle Database 11g, Oracle Database can manage the SGA memory and instance PGA memory completely automatically. You designate only the total memory size to be used by the instance, and Oracle Database dynamically exchanges memory between the SGA and the instance PGA as needed to meet processing demands. This capability is referred to as automatic memory management. With this memory management method, the database also dynamically tunes the sizes of the individual SGA components and the sizes of the individual PGAs.

To achieve this, two new parameters have been introduced named MEMORY_MAX_TARGET and MEMORY_TARGET. To do so (on most platforms), you set only a target memory size initialization parameter (MEMORY_TARGET) and optionally a maximum memory size initialization parameter (MEMORY_MAX_TARGET).

If you create your database with Database Configuration Assistant (DBCA) and choose the basic installation option, automatic memory management is enabled. If you choose advanced installation, Database Configuration Assistant (DBCA) enables you to select automatic memory management.

Switching to Automatic Memory Management

1)Check the current values configured for SGA_TARGET and PGA_AGGREGATE_TARGET.

------------------------------ ----------- ----------------
archive_lag_target integer 0
db_flashback_retention_target integer 1440
fast_start_io_target integer 0
fast_start_mttr_target integer 0
memory_max_target big integer 0
memory_target big integer 0
pga_aggregate_target big integer 200M
sga_target big integer 500M

Add the values of pga_aggregate_target and sga_target. In our case it would sum to 700 M

2)Decide on a maximum amount of memory that you would want to allocate to the database which will determine the maximum value for the sum of the SGA and instance PGA sizes. In our case we decide to set to 808M

3)Change the parameter in initialization parameter file.

Using Spfile


Using Pfile
If you have started the instance with Pfile, then edit the pfile and set the parameters manually


In case you do not specify any value for MEMORY_MAX_TARGET and only use MEMORY_TARGET then database automatically sets MEMORY_MAX_TARGET to the value of MEMORY_TARGET.

If you omit the line for MEMORY_TARGET and include a value for MEMORY_MAX_TARGET, the MEMORY_TARGET parameter defaults to zero. After startup, you can then dynamically change MEMORY_TARGET to a non-zero value, provided that it does not exceed the value of MEMORY_MAX_TARGET.

MEMORY_MAX_TARGET is a static parameter i.e it cannot be changed Dynamically and Instance has to be bounced for modifying the value. So ensure that you have set it to appropriate value.

4)Shutdown and startup the database

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 845348864 bytes
Fixed Size 1303188 bytes
Variable Size 469765484 bytes
Database Buffers 369098752 bytes
Redo Buffers 5181440 bytes
SQL> show parameter target

------------------------------------ ----------- ------------------------------
archive_lag_target integer 0
db_flashback_retention_target integer 1440
fast_start_io_target integer 0
fast_start_mttr_target integer 0
memory_max_target big integer 808M
memory_target big integer 808M
pga_aggregate_target big integer 0
sga_target big integer 0

The preceding steps instruct you to set SGA_TARGET and PGA_AGGREGATE_TARGET to zero so that the sizes of the SGA and instance PGA are tuned up and down as required, without restrictions. You can omit the statements that set these parameter values to zero and leave either or both of the values as positive numbers. In this case, the values act as minimum values for the sizes of the SGA or instance PGA.

Note : - In case you set any parameter value to value which is higher then MEMORY_TARGET, then you get the following error. E.g Setting SGA_MAX_SIZE to value of 900 M results in following

SQL> startup
ORA-00844: Parameter not taking MEMORY_TARGET into account, see alert log for more information

00844, 00000, "Parameter not taking MEMORY_TARGET into account, see alert log for more information"
// *Cause: The parameter was larger than MEMORY_TARGET.
// *Action: Set the parameter to a lower value than MEMORY_TARGET.

Monitoring and Tuning Automatic Memory Management

The dynamic performance view V$MEMORY_DYNAMIC_COMPONENTS shows the current sizes of all dynamically tuned memory components, including the total sizes of the SGA and instance PGA.
The view V$MEMORY_TARGET_ADVICE provides tuning advice for the MEMORY_TARGET initialization parameter.

SQL> select * from v$memory_target_advice order by memory_size;

You can also use V$MEMORY_RESIZE_OPS which has a circular history buffer of the last 800 SGA resize requests.

2. Automatic Shared Memory Management – For the SGA

If you want to exercise more direct control over the size of the SGA, you can disable automatic memory management and enable automatic shared memory management.This feature was introduced in 10g with a parameter known as SGA_TARGET. When automatic SGA memory management is enabled, the sizes of the different SGA components are flexible and can adapt to the needs of current workload without requiring any additional configuration.

In case you have enabled Automatic Memory Management , then to switch to Automatic Shared Memory Management , please follow below procedure

SQL>Alter system set MEMORY_TARGET=0 scope=both;
SQL>Alter system set SGA_TARGET=500M scope=both;

3. Manual Shared Memory Management – For the SGA

If you want complete control of individual SGA component sizes, you can disable both automatic memory management and automatic shared memory management. In this mode, you need to set the sizes of several individual SGA components, thereby determining the overall SGA size. You then manually tune these individual SGA components on an ongoing basis.

In this case you set SGA_TARGET and MEMORY_TARGET to 0 and set value for other SGA components upto value of SGA_MAX_SIZE

4. Automatic PGA Memory Management – For the Instance PGA

While using Automatic memory management , PGA memory is allocated based upon value of MEMORY_TARGET. In case you enable automatic shared memory management or manual shared memory management, you also implicitly enable automatic PGA memory management.

Automatic/Manual PGA memory management is decided by initialization parameter WORKAREA_SIZE_POLICY which is a session- and system-level parameter that can take only two values: MANUAL or AUTO. The default is AUTO.

With automatic PGA memory management, you set a target size for the instance PGA by defining value for parameter named PGA_AGGREGATE_TARGET and sizing of SQL work areas is automatic and all *_AREA_SIZE initialization parameters are ignored for these sessions. This feature is available from 9i.

At any given time, the total amount of PGA memory available to active work areas on the instance is automatically derived from the parameter PGA_AGGREGATE_TARGET. This amount is set to the value of PGA_AGGREGATE_TARGET minus the PGA memory allocated for other purposes (for example, session memory). The resulting PGA memory is then allotted to individual active work areas based on their specific memory requirements.

5.Manual PGA Memory Management – For the Instance PGA

In case you wish to manually specify the maximum work area size for each type of SQL operator (such as sort or hash-join) then you can enable Manual PGA Memory management.

Set WORKAREA_SIZE_POLICY value to MANUAL and also specify values for *_area_size such as SORT_AREA_SIZE, HASH_AREA_SIZE, BITMAP_MERGE_AREA_SIZE, and CREATE_BITMAP_AREA_SIZE, etc.

Although the Oracle Database 11g supports this manual PGA memory management method, Oracle strongly recommends that you leave automatic PGA memory management enabled.

Table below summarizes the various memory management methods

Memory Management Mode


You Set

Oracle Database Automatically Tunes

Automatic memory management(AMM)


- Total memory target size for the Oracle instance (Memory_Target)

- (Optional) Maximum memory size for the Oracle instance(Memory_max_Target)

- Total SGA size
- SGA component sizes
- Instance PGA size
- Individual PGA sizes

Automatic shared memory management(ASMM)

(AMM disabled)


- SGA target size (SGA_TARGET)
- (Optional) SGA maximum size

SGA component sizes

Manual shared memory management

(AMM and ASMM disabled)


- Shared pool size (SHARED_POOL_SIZE)

-Buffer cache size

-Java pool size

-Large pool size

- (Optional) SGA maximum size


Automatic PGA memory management


Instance PGA target size (PGA_AGGREGATE_TARGET)

Individual PGA sizes

Manual PGA memory management

(not recommended)


Maximum work area size for each type of SQL operator


The Automatic Memory Management (AMM) feature uses background process named Memory Manager (MMAN). This process was introduced in 10g which assisted in Automatic Shared Memory Management (ASMM) using SGA_TARGET. MMAN serves as the SGA Memory Broker and coordinates the sizing of the memory components. The SGA Memory Broker keeps track of the sizes of the components and pending resize operations

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