How much memory does each of the individual components of the SGA need? Oracle now has methods to determine most of the major parameters all on its own.

What is the SGA

Simply stated, the system global area (SGA) is just shared memory structures that are created at instance startup, hold information about the instance and control its behavior. The following table gives a brief synopsis of the particular components of the SGA, the variables that control the size of memory allocated, some of the areas of the Oracle server the particular component has an influence on, and then a very brief description. What can be seen from this simple list is that there are plenty of options available for us to tweak the SGA and without a complete understanding of what our applications are doing in the background, our ability to guess the appropriate amount of memory to give each of these individual components is not always optimal. What we do not want to have happen in this process of allocation of memory is to waste it.

Components of the SGA

You can look at the size of your SGA by looking at the initialization parameters that control its size. Here is a simple query and its output.

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');

NAME                      VALUE

------------------------- ---------

shared_pool_size          83886080

large_pool_size           8388608

java_pool_size            50331648

streams_pool_size         54525952

db_cache_size             25165824

db_2k_cache_size          0

db_4k_cache_size          0

db_8k_cache_size          0

db_16k_cache_size         0

db_32k_cache_size         0

db_keep_cache_size        0

db_recycle_cache_size     0

log_buffer                262144

13 rows selected.

Letting Oracle Take Control

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.

SQL> show parameter sga_target

NAME                                 TYPE        VALUE

------------------------------------ ----------- --------

sga_target                           big integer 0

SQL> alter system set sga_target=216m;

System altered.

What happens when you switch to Automatic Shared Memory Tuning is a bit interesting. After you alter SGA_TARGET parameter, your SPFILE will undergo a change and now have the following parameters defined. Note that k101 is my instance name and will take on whatever the instance name is.

k101.__db_cache_size=25165824

k101.__java_pool_size=50331648

k101.__large_pool_size=8388608

k101.__shared_pool_size=83886080

In addition, when you issue the previously given SQL to show the parameter settings for the individual components of the SGA you will notice that they now have a value of zero.

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');

NAME                      VALUE

------------------------- ---------

shared_pool_size          0

large_pool_size           0

java_pool_size            0

streams_pool_size         0

db_cache_size             0

db_2k_cache_size          0

db_4k_cache_size          0

db_8k_cache_size          0

db_16k_cache_size         0

db_32k_cache_size         0

db_keep_cache_size        0

db_recycle_cache_size     0

log_buffer                262144

13 rows selected.

If you truly want to see the parameters after setting the SGA_TARGET you will need to modify the query to include the newly created underscore variables. This can be of some concern if you are relying on the "normal" parameters for any database monitoring scripts.

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');

NAME                      VALUE

------------------------- ---------

__shared_pool_size        67108864

__large_pool_size         4194304

__java_pool_size          8388608

streams_pool_size         0

__db_cache_size           142606336

db_2k_cache_size          0

db_4k_cache_size          0

db_8k_cache_size          0

db_16k_cache_size         0

db_32k_cache_size         0

db_keep_cache_size        0

db_recycle_cache_size     0

log_buffer                262144

13 rows selected.

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.