Automatic Memory Management

posted Sep 10, 2010, 5:46 AM by Sachchida Ojha [ updated Sep 26, 2010, 7:14 AM by Sachchida Ojha ]

Oracle Database 10g automates the management of shared memory used by an instance and liberates administrators from having to manually configure the sizes of shared memory components. Automatic Shared Memory Tuning significantly simplifies Oracle database administration by introducing a more dynamic, flexible and adaptive memory management scheme. It makes more effective use of available memory therefore reducing the cost incurred on acquiring additional hardware.

SQL to find the size of the SGA

select name, value
from v$parameter
where name in ('shared_pool_size', 'java_pool_size', 'streams_pool_size',
'log_buffer', 'db_cache_size', 'db_2k_cache_size', 'db_4k_cache_size',
'db_8k_cache_size', 'db_16k_cache_size', 'db_32k_cache_size',
'db_keep_cache_size', 'db_recycle_cache_size', 'large_pool_size');

Using Oracle's Automatic Shared Memory Tuning, you can instruct Oracle to manage a subset of the components that make up the SGA by merely telling the instance the target size of the SGA through the new SGA_TARGET parameter.Oracle will then pool from this value and dynamically distribute memory across selected components of the SGA.

You now need not set values for SHARED_POOL_SIZE, JAVA_POOL_SIZE, LARGE_POOL_SIZE, or DB_CACHE_SIZE as Oracle will automatically size them for you.Once you set the SGA_TARGET parameter to a desirable size for your SGA these parameters will take on a value of zero and new parameters will be created designated by

__SHARED_POOL_SIZE,__JAVA_POOL_SIZE,LARGE_POOL_SIZE, and __DB_CACHE_SIZE.

As workloads go through the system and memory is needed in these areas, Oracle will allocate more memory based on internal statistics trends.

Oracle will not manage the DB_KEEP_CACHE_SIZE, DB_RECYCLE_CACHE_SIZE, DBnK_CACHE_SIZE, or the STREAMS_POOL_SIZE and you must still determine the value for these parameters.

In order for this all to take place, you must be using an SPFILE as this is the only way Oracle can dynamically make all these changes happen. Also, note that SGA_TARGET is the sum of all parameters that make up the SGA, not just the parameters it controls, so you must take those components it does not control into consideration when you give a value for SGA_TARGET.
Stepping through Letting Oracle Take Control

There is really nothing to switching into automatic shared memory tuning. You only need to set the SGA_TARGET parameter.

1. Take a look and see if you are already in automated sizing of SGA

SQL> show parameter sga_target
NAME TYPE VALUE
------------------------------------ ----------- --------
sga_target big integer 0

2. Alter system to begin automated sizing of SGA

SQL> alter system set sga_target=2G;
System altered.

Switching over to the Automatic Shared Memory Tuning is as easy as setting an initialization parameter. How this will behave under load is yet to be determined but since these numbers are driven by the various advisories and I am mostly happy with them as individual components, I see no reason not to venture down the path of having Oracle automatically size my SGA. Of course, as always, in a test environment first. I would suggest you take a snapshot of your initialization parameters before letting Oracle take control and then compare the end settings that Oracle has implemented. It is always easy to switch back, just reset the SGA_TARGET parameter and set the individual components back to their original values.

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