Automatic Memory Management (AMM) in 11G

posted Jul 26, 2011, 8:55 AM by Sachchida Ojha
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:

Component
Description
Parameter
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.
DB_CACHE_SIZE
DB_KEEP_CACHE_SIZE
DB_RECYCLE_CACHE_SIZE
DB_nK_CACHE_SIZE

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.
LOG_BUFFER
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.
SHARED_POOL_SIZE
SHARED_POOL_RESERVED_SIZE
RESULT_CACHE_SIZE *

Large Pool
Used for allocating session memory for shared server, Oracle XA, or parallel query buffers or for RMAN.
LARGE_POOL_SIZE
Java Pool
Java pool memory is used in server memory for all session-specific Java code and data within the JVM.
JAVA_POOL_SIZE
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.
STREAMS_POOL_SIZE

* 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.

SQL>SHOW PARAMETER TARGET
NAME TYPE VALUE
------------------------------ ----------- ----------------
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
========

SQL>ALTER SYSTEM SET MEMORY_MAX_TARGET = 808M SCOPE = SPFILE;
SQL>ALTER SYSTEM SET MEMORY_TARGET = 808M SCOPE = SPFILE;
SQL>ALTER SYSTEM SET SGA_TARGET =0 SCOPE = SPFILE;
SQL>ALTER SYSTEM SET PGA_AGGREGATE_TARGET = 0 SCOPE = SPFILE;

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


MEMORY_MAX_TARGET = 808M
MEMORY_TARGET = 808M
SGA_TARGET =0
PGA_AGGREGATE_TARGET = 0


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 mount
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

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
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.

Please refer to following document for setting SGA_TARGET

Note 295626.1 - How To Use Automatic Shared Memory Management (ASMM) In Oracle10g

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

Please refer to Note 148495.1 - Dynamic SGA

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.

Please refer to following document for more details on Automatic PGA Memory Management

Note 223730.1 - Automatic PGA Memory Managment in 9i and 10g

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
For
You Set
Oracle Database Automatically Tunes
Automatic memory management(AMM)


SGA and PGA
- 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
- SGA target size (SGA_TARGET)
- (Optional) SGA maximum size
(SGA_MAX_SIZE)
SGA component sizes
Manual shared memory management

(AMM and ASMM disabled)


SGA
- Shared pool size (SHARED_POOL_SIZE)

-Buffer cache size
(DB_CACHE_SIZE or DB_BLOCK_BUFFERS)

-Java pool size
(JAVA_POOL_SIZE)

-Large pool size
(LARGE_POOL_SIZE)

- (Optional) SGA maximum size
(SGA_MAX_SIZE)


None
Automatic PGA memory management

PGA
Instance PGA target size (PGA_AGGREGATE_TARGET)
Individual PGA sizes
Manual PGA memory management

(not recommended)


PGA
Maximum work area size for each type of SQL operator

None


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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