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Easy Oracle Automation
Chapter 6 - Automatic System Resource Management

Automatic Shared Memory Management (ASMM)

When sga_target is set to a non-zero value to enable ASMM, the auto tuned SGA parameters are set to zero values. These components are then automatically sized by ASMM. If the four parameters are set to non-zero values along with ASMM, the specified values are considered as a lower limit by the auto-tuning algorithm. Whenever an auto tuned component is resized to a larger value, the memory is taken from another auto-tuned component to supplement the change, while the manually tuned components are left untouched.

The sga_target is a dynamic parameter and is set from OEM or with the ALTER SYSTEM command. The sga_target can be raised up to sga_max_size. It can be reduced until the auto-tuned components are reduced to their minimums. Any change in sga_target only affects the size of the auto-tuned components.

Manually sized SGA components are tuned by the user. The total size of manual SGA parameters is subtracted from the sga_target to be allotted to auto tuned components.

Some of the manually sized SGA components are: Keep/Recycle buffer which are controlled by db_keep_size and db_recycle_cache_size; multiple block size caches such as db_nk_cache_size where n=2, 4, 8, 16, 32; Streams Pool which is controlled by streams_pool_size; log_buffer, and etc. The memory consumed by these manually sized components reduces the amount of memory available for the automatic tuning of automatically sized components. 

The above text is an excerpt from:

Easy Oracle Automation
Oracle10g Automatic Storage, Memory and Diagnostic Features

ISBN 0-9745993-6-0

by Dr. Arun Kumar R.

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