Basic Memory Structures

Refer: http://docs.oracle.com/cd/E11882_01/server.112/e40540/memory.htm#CNCPT7778

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. Examples of data stored in the SGA include cached data blocks and shared SQL areas.
• Program global area (PGA)
  A PGA is a nonshared memory region that contains data and control information exclusively for use by an Oracle process. The PGA is created by Oracle Database when an Oracle process is started.
  One PGA exists for each server process and background process. The collection of individual PGAs is the total instance PGA, or instance PGA. Database initialization parameters set the size of the instance PGA, not individual PGAs.
• User Global Area (UGA)
  The UGA is memory associated with a user session.
• Software code areas
  Software code areas are portions of memory used to store code that is being run or can be run. Oracle Database code is stored in a software area that is typically at a different location from user programs—a more exclusive or protected location.

Figure 14-1 illustrates the relationships among these memory structures.

Figure 14-1 Oracle Database Memory Structures

Description of "Figure 14-1 Oracle Database Memory Structures"

Oracle Database Memory Management

Memory management involves maintaining optimal sizes for the Oracle instance memory structures as demands on the database change. Oracle Database manages memory based on the settings of memory-related initialization parameters. The basic options for memory management are as follows:

• Automatic memory management
  You specify the target size for instance memory. The database instance automatically tunes to the target memory size, redistributing memory as needed between the SGA and the instance PGA.
• Automatic shared memory management
  This management mode is partially automated. You set a target size for the SGA and then have the option of setting an aggregate target size for the PGA or managing PGA work areas individually.
• Manual memory management
  Instead of setting the total memory size, you set many initialization parameters to manage components of the SGA and instance PGA individually.