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 Oracle Automatic Shared Memory Management
Oracle Tips by Burleson Consulting

Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Automatic Shared Memory Management

Automatic Shared Memory Management (ASMM) adjusts the sizes of the SGA components on the fly as the workload changes.  Automatic Shared Memory Management (ASMM), a 10g and 11g feature, is meant to simplify SGA management.  Do not confuse ASMM with AMM (Automatic Memory Management).  ASMM was introduced in Oracle 10g.  AMM is new in 11g and will be addressed in this chapter.

 

The DBA specifies the amount of memory available to an instance via the sga_target parameter.  The Oracle database periodically redistributes memory between the components according to workload requirements.  This solves the allocation issues that are normally faced when using the manual method.  Under-sizing memory settings can lead to poor performance and out-of memory errors (ORA-4031).

 

If a non-zero value is specified for sga_target, the following six memory pools are automatically sized by Oracle: 

  • Database buffer cache (Only the default pool)

  • Shared pool

  • Large pool

  • Java pool

  • Streams pool

  • Fixed SGA and other internal allocations

Figure 2.3 shows an example of auto-tuned SGA components.

 

Figure 2.3:  SGA Memory Buffers Managed Dynamically Using ASMM

 

When using ASMM, configuration of the following buffers remains manual:

  • Log buffer

  • KEEP/RECYCLE buffer caches

  • Nonstandard block size buffer caches

When sga_target is set, the total size of manual SGA parameters are subtracted from the sga_target value and the balance is given to the auto-tuned SGA components.

 

Sga_target is also a dynamic parameter and can be changed through Enterprise Manager or with the ALTER SYSTEM command. However, the sga_target can be increased only up to the value of sga_max_size.

 

Important: statistics_level must be set to TYPICAL (default) or ALL to use Automatic Shared Memory Management.

Program Global Area (PGA)

A Program Global Area (PGA) is a memory region that stores the data and control information for the server processes. Each server process has a non-shared memory region created by Oracle when a server process is started. Access to the PGA is exclusive to that server process, and it is read and written only by Oracle code. Broadly speaking, PGA contains a private SQL area and a session memory area.

 

A private SQL area contains data such as bind information and runtime memory structures. Each session that issues a SQL statement has a private SQL area.  Session memory is the memory allocated to hold a session’s variables, logon information, and other information related to the session.

 

With the initialization parameter pga_aggregate_target, sizing of work areas for all dedicated sessions is made automatic, and all *_area_size parameters are ignored for these sessions.

How to disable AMM:  See these important notes on disabling AMM (Automatic Space Memory Management)

     

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