Memory management is one of the factors that affect performance. The database administrator may configure the SGA optimally for morning work, but this configuration may not be suitable for evening hours when the nature of the work changes significantly. For example, backup operations with RMAN are typically run at night, and it is certain that RMAN backup operations require more space in the Large Pool to handle a large number of RMAN Channel Processes. Therefore, the optimal configuration for SGA should change based on the nature of the work, requiring the database administrator to monitor and reconfigure SGA as needed.

Starting with Oracle 10g, Oracle introduced automatic memory management (Automatic Shared Memory Management) for essential memory components in the SGA, easing the burden on the database administrator of manually managing and monitoring these critical memory components. The administrator only needs to define the Target_Size, which is the total memory size allocated to the SGA, as long as the value of the SGA_TARGET variable does not exceed the maximum allowed value of SGA_MAX_SIZE.

SGA_MAX_SIZE represents the maximum amount of memory the SGA can use. Once the database administrator configures the SGA_TARGET variable, Oracle will automatically manage and monitor the memory components in the SGA.

The key memory components automatically managed by Oracle under SGA are:

1. Database Buffer Cache Default Pool
2. Shared Pool
3. Large Pool
4. Java Pool

If these components are not configured, their values will be set to zero, and Oracle will manage them automatically according to the current workload needs. However, if they are configured, this configuration will serve as the minimum value that will not be reduced by Automatic Shared Memory Management.

In addition, there are other components in SGA that Oracle still manages manually:

1. Database Buffer Cache Keep Pool
2. Database Buffer Cache Recycle Pool
3. Database Buffer Cache Nonstandard Block Size Pools
4. Streams Pool
5. Log Buffer

To enable Automatic Shared Memory Management, configure the SGA_TARGET parameter, which defines the total memory allocated to the SGA (including both automatically and manually managed components). Upon starting the Instance, Oracle will configure the SGA based on the current values of the parameters. If the total SGA memory exceeds the value of SGA_TARGET, Automatic Shared Memory Management will be disabled, and manual adjustments will be required.

After that, the Oracle system will rely on the Parameter File. If the Instance is started using an SPFILE, Oracle will write the current memory configuration for the four memory components into the Parameter File, reducing the time required for configuration on subsequent startups.

If the Instance is started using a PFILE, the automatic memory management feature will continue to function, but the current memory configuration will be lost after the first Instance shutdown.

To enable Automatic Shared Memory Management, configure the SGA_TARGET parameter. When this parameter is configured, Oracle will add a new Background Process, the Memory Manager (MMAN), which will monitor the system and adjust memory resources according to the optimal settings.

The SGA_TARGET is an automatic parameter, and to enable Automatic Shared Memory Management, the STATISTICS_LEVEL parameter should be set to TYPICAL or ALL, which is the default value.