Using Automatic Memory Management

About Automatic Memory Management

The simplest way to manage instance memory is to allow the Oracle Database instance to automatically manage and tune it for you. To do so (on most platforms), you set only a target memory size initialization parameter (MEMORY_TARGET) and optionally a maximum memory size initialization parameter (MEMORY_MAX_TARGET). The instance then tunes to the target memory size, redistributing memory as needed between the system global area (SGA) and the instance program global area (instance PGA). Because the target memory initialization parameter is dynamic, you can change the target memory size at any time without restarting the database. The maximum memory size serves as an upper limit so that you cannot accidentally set the target memory size too high, and so that enough memory is set aside for the Oracle Database instance in case you do want to increase total instance memory in the future. Because certain SGA components either cannot easily shrink or must remain at a minimum size, the instance also prevents you from setting the target memory size too low.

If you create your database with Database Configuration Assistant (DBCA) and choose the basic installation option, automatic memory management is enabled. If you choose advanced installation, Database Configuration Assistant (DBCA) enables you to select automatic memory management.

Enabling Automatic Memory Management

If you did not enable automatic memory management upon database creation (either by selecting the proper options in DBCA or by setting the appropriate initialization parameters for the CREATE DATABASE SQL statement), you can enable it at a later time. Enabling automatic memory management involves a shutdown and restart of the database.

To enable automatic memory management

1. Start SQL*Plus and connect to the database as SYSDBA.

   See "Database Administrator Security and Privileges" and "Database Administrator Authentication" for instructions.

2. Calculate the minimum value for MEMORY_TARGET as follows:

   1. Determine the current sizes of SGA_TARGET and PGA_AGGREGATE_TARGET by entering the following SQL*Plus command:

      SHOW PARAMETER TARGET
      

      SQL*Plus displays the values of all initialization parameters with the string TARGET in the parameter name.

      NAME                          TYPE        VALUE
      ------------------------------ ----------- ----------------
      archive_lag_target             integer     0
      db_flashback_retention_target  integer     1440
      fast_start_io_target           integer     0
      fast_start_mttr_target         integer     0
      memory_max_target              big integer 0
      memory_target                  big integer 0
      pga_aggregate_target           big integer 90M
      sga_target                     big integer 272M
      

   2. Run the following query to determine the maximum instance PGA allocated since the database was started:

      select value from v$pgastat where name='maximum PGA allocated';
      

   3. Compute the maximum value between the query result from step 2b and PGA_AGGREGATE_TARGET. Add SGA_TARGET to this value.

      memory_target = sga_target + max(pga_aggregate_target, maximum PGA allocated)
      

   For example, if SGA_TARGET is 272M and PGA_AGGREGATE_TARGET is 90M as shown above, and if the maximum PGA allocated is determined to be 120M, then MEMORY_TARGET should be at least 392M (272M + 120M).

3. Choose the value for MEMORY_TARGET that you want to use.

   This can be the minimum value that you computed in step 2, or you can choose to use a larger value if you have enough physical memory available.

4. For the MEMORY_MAX_TARGET initialization parameter, decide on a maximum amount of memory that you would want to allocate to the database for the foreseeable future. That is, determine the maximum value for the sum of the SGA and instance PGA sizes. This number can be larger than or the same as the MEMORY_TARGET value that you chose in the previous step.

5. Do one of the following:

   • If you started your Oracle Database instance with a server parameter file, which is the default if you created the database with the Database Configuration Assistant (DBCA), enter the following command:

     ALTER SYSTEM SET MEMORY_MAX_TARGET = nM SCOPE = SPFILE;
     

     where n is the value that you computed in Step 4.

     The SCOPE = SPFILE clause sets the value only in the server parameter file, and not for the running instance. You must include this SCOPE clause because MEMORY_MAX_TARGET is not a dynamic initialization parameter.

   • If you started your instance with a text initialization parameter file, manually edit the file so that it contains the following statements:

     memory_max_target = nM
     memory_target = mM
     

     where n is the value that you determined in Step 4, and m is the value that you determined in step 3.

   Note:

   In a text initialization parameter file, if you omit the line for MEMORY_MAX_TARGET and include a value for MEMORY_TARGET, the database automatically sets MEMORY_MAX_TARGET to the value of MEMORY_TARGET. If you omit the line for MEMORY_TARGET and include a value for MEMORY_MAX_TARGET, the MEMORY_TARGET parameter defaults to zero. After startup, you can then dynamically change MEMORY_TARGET to a nonzero value, provided that it does not exceed the value of MEMORY_MAX_TARGET.

6. Shut down and restart the database.

   See Chapter 3, "Starting Up and Shutting Down" for instructions.

7. If you started your Oracle Database instance with a server parameter file, enter the following commands:

   ALTER SYSTEM SET MEMORY_TARGET = nM;
   ALTER SYSTEM SET SGA_TARGET = 0;
   ALTER SYSTEM SET PGA_AGGREGATE_TARGET = 0;
   

   where n is the value that you determined in step 3.

Monitoring and Tuning Automatic Memory Management

The dynamic performance view V$MEMORY_DYNAMIC_COMPONENTS shows the current sizes of all dynamically tuned memory components, including the total sizes of the SGA and instance PGA.

The view V$MEMORY_TARGET_ADVICE provides tuning advice for the MEMORY_TARGET initialization parameter.

SQL>  select * from v$memory_target_advice order by memory_size;
 
MEMORY_SIZE MEMORY_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR    VERSION
----------- ------------------ ------------ ------------------- ----------
        180                 .5          458               1.344          0
        270                .75          367              1.0761          0
        360                  1          341                   1          0
        450               1.25          335               .9817          0
        540                1.5          335               .9817          0
        630               1.75          335               .9817          0
        720                  2          335               .9817          0

The row with the MEMORY_SIZE_FACTOR of 1 shows the current size of memory, as set by the MEMORY_TARGET initialization parameter, and the amount of DB time required to complete the current workload. In previous and subsequent rows, the results show a number of alternative MEMORY_TARGET sizes. For each alternative size, the database shows the size factor (the multiple of the current size), and the estimated DB time to complete the current workload if the MEMORY_TARGET parameter were changed to the alternative size. Notice that for a total memory size smaller than the current MEMORY_TARGET size, estimated DB time increases. Notice also that in this example, there is nothing to be gained by increasing total memory size beyond 450MB. However, this situation might change if a complete workload has not yet been run.

Enterprise Manager provides an easy-to-use graphical memory advisor to help you select an optimal size for MEMORY_TARGET. See Oracle Database 2 Day DBA for details.