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Automatic Memory Management (AMM) in Oracle Database 11g Release 1

Oracle has made great strides in simplifying memory management over the last few versions of the database. Oracle 9i automated PGA management by introducing PGA_AGGREGATE_TARGET parameter. Oracle 10g continued this trend by automating SGA management using the SGA_TARGET parameter. Oracle 11g takes this one step further by allowing you to allocate one chunk of memory, which Oracle uses to dynamically manage both the SGA and PGA.

At the time of writing, Automatic Memory Management (AMM) is only supported on the major platforms (Linux, Solaris, Windows, HP-UX, AIX).

AMM Parameters

Automatic memory management is configured using two new initialization parameters:

When using automatic memory management, the SGA_TARGET and PGA_AGGREGATE_TARGET act as minimum size settings for their respective memory areas. To allow Oracle to take full control of the memory management, these parameters should be set to zero.

If you are using UNIX/Linux, before you consider using AMM you should check the current size of your shared memory file system. On Linux you do this by issuing the following command.

# df -k /dev/shm
Filesystem           1K-blocks      Used Available Use% Mounted on
tmpfs                  1029884    350916    678968  35% /dev/shm
#

The shared memory file system should be big enough to accommodate the MEMORY_TARGET and MEMORY_MAX_TARGET values, or Oracle will throw the following error.

ORA-00845: MEMORY_TARGET not supported on this system

To adjust the shared memory file system size issue the following commands, specifying the required size of shared memory.

# umount tmpfs
# mount -t tmpfs shmfs -o size=1200m /dev/shm

Make the setting permanent by amending the "tmpfs" setting of the "/etc/fstab" file to look like this.

tmpfs                   /dev/shm                tmpfs   size=1200m      0 0

AMM Configuration

The Database Configuration Assistant (DBCA) allows you to configure automatic memory management during database creation.

Automatic Memory Management

When creating the database manually, simply set the appropriate MEMORY_TARGET and MEMORY_MAX_TARGET initialization parameters before creating the database.

Enabling automatic memory management on a system that didn't previously use it is a simple task. Assuming you want to use a similar amount of memory to your current settings you will need to use the following calculation.

MEMORY_TARGET = SGA_TARGET + GREATEST(PGA_AGGREGATE_TARGET, "maximum PGA allocated")

The following queries show you how to display the relevant information and how to combine it in a single statement to calculate the required value.

-- Individual values.
COLUMN name FORMAT A30
COLUMN value FORMAT A10

SELECT name, value
FROM   v$parameter
WHERE  name IN ('pga_aggregate_target', 'sga_target')
UNION
SELECT 'maximum PGA allocated' AS name, TO_CHAR(value) AS value
FROM   v$pgastat
WHERE  name = 'maximum PGA allocated';

-- Calculate MEMORY_TARGET
SELECT sga.value + GREATEST(pga.value, max_pga.value) AS memory_target
FROM (SELECT TO_NUMBER(value) AS value FROM v$parameter WHERE name = 'sga_target') sga,
     (SELECT TO_NUMBER(value) AS value FROM v$parameter WHERE name = 'pga_aggregate_target') pga,
     (SELECT value FROM v$pgastat WHERE name = 'maximum PGA allocated') max_pga;

Assuming our required setting was 5G, we might issue the following statements.

CONN / AS SYSDBA
-- Set the static parameter. Leave some room for possible future growth without restart.
ALTER SYSTEM SET MEMORY_MAX_TARGET=6G SCOPE=SPFILE;

-- Set the dynamic parameters. Assuming Oracle has full control.
ALTER SYSTEM SET MEMORY_TARGET=5G SCOPE=SPFILE;
ALTER SYSTEM SET PGA_AGGREGATE_TARGET=0 SCOPE=SPFILE;
ALTER SYSTEM SET SGA_TARGET=0 SCOPE=SPFILE;

-- Restart instance.
SHUTDOWN IMMEDIATE;
STARTUP;

Once the database is restarted the MEMORY_TARGET parameter can be amended as required without an instance restart.

ALTER SYSTEM SET MEMORY_TARGET=4G SCOPE=SPFILE;

AMM Tuning

In addition to the existing memory management V$ views, Oracle 11g includes four new V$ views to support automatic memory management:

The amount of memory allocated to each dynamic component is displayed using the V$MEMORY_DYNAMIC_COMPONENTS view.

COLUMN component FORMAT A30

SELECT  component, current_size, min_size, max_size
FROM    v$memory_dynamic_components
WHERE   current_size != 0;


COMPONENT                      CURRENT_SIZE   MIN_SIZE   MAX_SIZE
------------------------------ ------------ ---------- ----------
shared pool                       197132288  192937984  197132288
large pool                          4194304    4194304    4194304
java pool                          41943040   41943040   41943040
SGA Target                        318767104  285212672  318767104
DEFAULT buffer cache               71303168   41943040   75497472
PGA Target                        104857600  104857600  138412032

6 rows selected.

SQL>

The V$MEMORY_CURRENT_RESIZE_OPS and V$MEMORY_RESIZE_OPS views provide information on current and previous component resize operations.

The V$MEMORY_TARGET_ADVICE view provides information to help tune the MEMORY_TARGET parameter. It displays a range of possible MEMORY_TARGET settings, as factors of the current setting, and estimates the potential DB Time to complete the current workload based on these memory sizes.

SELECT * FROM v$memory_target_advice ORDER BY memory_size;

MEMORY_SIZE MEMORY_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR    VERSION
----------- ------------------ ------------ ------------------- ----------
        303                .75         3068              1.0038          2
        404                  1         3056                   1          2
        505               1.25         3056                   1          2
        606                1.5         3056                   1          2
        707               1.75         3056                   1          2
        808                  2         3056                   1          2

6 rows selected.

SQL>

Enterprise Manager includes the memory management configuration and advisor functionality in the "Memory Advisors" screen (Advisor Central > Memory Advisors).

Memory Advisors

Clicking the "Advice" button displays the "Memory Size Advice" screen, which contains a graphical representation of the information from the V$MEMORY_TARGET_ADVICE view.

Memory Size Advice

Considerations Before Using AMM

When you have large SGA sizes you can get considerable benefits from using HugePages. Automatic Memory Management and HugePages on Linux are not compatible, which means AMM is probably not a sensible option for any large systems. Instead, Automatic Shared Memory Management and Automatic PGA Management should be used as they are compatible with HugePages.

Even so, AMM is the default for all ASM instances and should be left that way. From a database perspective, it still may be relevant for smaller, less important databases.

For more information see:

Hope this helps. Regards Tim...

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