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Oracle10g Automatic Shared Memory Management

The system global area (SGA) consists of memory components. A component represents a pool of memory used to satisfy a particular class of memory allocation requests. The most commonly configured memory components include the database buffer cache, shared pool, large pool, and java pool. Since we fix the values for these components at instance start time, we are constrained to use them as they are during the instance runtime (with some exceptions).

Often it happens that a certain component’s memory pool is never used but the pool is not available for another component, which is in need of extra memory. Under-sizing can lead to poor performance and out-of-memory errors (ORA-4031), while over-sizing can waste memory.

With the Database 10g, we can employ the Automatic Shared Memory Management feature. This feature enables the Oracle database to automatically determine the size of each of these memory components within the limits of the total SGA size. This solves the allocation issues that we normally face in a manual method.

This feature enables us to specify a total memory amount to be used for all SGA components. The Oracle Database periodically redistributes memory between the components above according to workload requirements.

Using the sga_target initialization parameter configures automatic Shared Memory Management. If you specify a non-zero value for sga_target, the following four memory pools are automatically sized:

  • Database Buffer cache (The Default pool)
  • Shared pool
  • Large pool
  • Java pool

If you set sga_target to 0, the Automatic Shared Memory Management is disabled. The default value of sga_target is 0. When sga_target is not set, or equal to zero, auto-tuned SGA parameters behave as in previous releases of the Oracle database.

In 10g, the initialization parameters such as db_cache_size, shared_pool_size, large_pool_size, and java_pool_size are referred to as 'autotuned sga’ parameters.

Configuration of the following buffers still remains manual and they are now referred to as manually sized components:

  • Log Buffer
  • Other Buffer Caches (KEEP/RECYCLE, other block sizes)
  • Streams Pool (new in Oracle Database 10g)
  • Fixed SGA and other internal allocations

The user specifies manual SGA parameters, and parameter sizes precisely control the sizes of their corresponding components. 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. Figure 2.3 shows an example of SGA components.

Are you wondering how this auto management is possible? A new background process named Memory Manager (MMAN) manages the automatic shared memory. 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.

Get the complete Oracle10g story:

The above text is an excerpt from "Oracle Database 10g New Features: Oracle10g Reference for Advanced Tuning and Administration", by Rampant TechPress.  Written by top Oracle experts, this book has a complete online code deport with ready to use scripts. 

To get the code instantly, click here:

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