Automate the Sizing of your SGA in Oracle 10g

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

SGACOMPONENT	SIZE CONTROLED BY	AREAS OF INFLUENCE	SIMPLE DESCRIPTONS
Shared Pool	SHARED_POOL_SIZE	Library Cache Shared SQL areas Private SQL areas PL/SQL procedures and packages Various control structures	Oracle needs to allocate & deallocate memory as SQL or procedural code is executed based on the individual needs of users' sessions and in accordance to the LRU algorithm.
		Dictionary Cache Row cache Library cache	Highly accessed memory structures that provide information on object structures to SQL statements being parsed.
Java Pool	JAVA_POOL_SIZE	Run state Methods Classes Session code Data in JVM	Memory available for the Java memory manager to use for all things Java.
Streams Pool	STREAMS_POOL_SIZE	Stream activity	New to Oracle 10g, memory available for stream processing.
Redo Log Buffer	LOG_BUFFER	Redo entries	Holds changes made to data and allows for the reconstruction of data in the case of failure.
Database Buffer Cache	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	Write list LRU list	Holds copies of data requested by SQL and reduces requests to disk by having data in memory. You may have many different buffer caches that help segregate on usage patterns.
Large Pool	LARGE_POOL_SIZE	Shared server Oracle XA I/O server processes Backup & restore	For large memory allocations.

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.

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=216m;
System altered.

3. Done

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.