Oracle Automatic Shared Memory Management by Donald Burleson

Notes: 

• Oracle ASMM and AMM are not the same and are associated with different releases of Oracle.  See my notes on the difference between ASMM and AMM.

• See these important notes scripts for detecting AMM resize operations.

• There is an advisory on using bitmap free lists (ASMM).  ASMM  (AMM) and dynamic memory management has measurable overhead for resize operations, and some shop may want to disable AMM.  See my important notes on Oracle dynamic memory management and how Oracle AMM resize operations can hurt performance.

• Automatic Memory Management has issues since its inception and by 11g release 2 it remains problematic, and in some cases ASMM should be disabled in 11g release 2.  See MOSC note 793845.1  titled: "High direct path read waits in 11g".

Oracle has always tried to provide advisory tools for the Oracle DBA who wants to monitor and re-size their data buffers (db_block_buffers, db_cache_size):

• Oracle7 - x$kcbcbh

• Oracle9i - v$db_cache_advice

• Oracle 10g - Oracle Automatic Shared Memory Management (ASMM)

• Oracle 11g - Oracle Automatic Memory Management (AMM)

Enabling Automatic Shared Memory Management (in 10g ONLY)

The Oracle Automatic Shared Memory Management is a feature that automatically readjusts the sizes of the main pools (db_cache_size, shared_pool_size, large_pool_size, java_pool_size) based on existing workloads. Oracle Automatic Shared Memory Management is enabled by setting:

• You must use an spfile for the init.ora values

• sga_target parameter is set to a non-zero value

• statistics_level parameter set to to TYPICAL (the default) or ALL

• shared_pool_size must be set to a non-zero value

Oracle10g has introduced special double underscore hidden parameter to control ASMM.  Once you see these, AMM is being used.:

• __db_cache_size

• __shared_pool_size

• __large_pool_size

Once enabled, Oracle Automatic Shared Memory Management will morph the pool areas within the confines of sga_max_size.

When using AMM you have to consider the interaction of these parameters:

• sga_target: (pre 11g):  If the sga_target is set to some value then the automatic shared memory management (ASMM) is enabled, the sga_target value can be adjusted up to the sga_max_size parameter, not more than sga_max_size parameter value.

  • sga_max_size:  The sga_max_size sets the overall amount of memory the SGA can consume but is not dynamic.  The sga_max_size parameter is the max allowable size to resize the SGA memory area parameters.

• memory_target (starting in 11g):  If memory_target  is set, then AMM is enabled:  If memory_target is set to non zero value and :

  • sga_target, sga_max_size and pga_aggregate_target are set to 0, then 60% of memory mentioned in memory_target is allocated to SGA and rest 40% is kept for PGA.

  • sga_target and pga_aggregate_target are set to non-zero values, then these values will be considered minimum values.

  • sga_target is set to non zero value and pga_aggregate_target is not set. still these values will be auto-tuned and pga_aggregate_target will be initialized with value of (memory_target-sga_target).

  • pga_aggregate_target is set and sga_target is not set. still both parameters will be auto-tuned. The sga_target will be initialized to a value of (memory_target-pga_aggregate_target).

AMM and the memory_target parameter

In Oracle 11g and beyond, automatic memory management is enabled by using the memory_target  and memory_max_target initialization parameters.  The memory_target parameter specifies the amount of shared memory available for Oracle to use when dynamically controlling the SGA and PGA. The memory_max_target AMM parameter specifies the max size that memory_target may take.  The Oracle documentation notes that the memory_target parameter specifies a target value for system-wide RAM memory, both PGA and SGA RAM:

 "MEMORY_TARGET specifies the Oracle system-wide usable memory.

The database tunes memory to the MEMORY_TARGET value, reducing or enlarging the SGA and PGA as needed."

Disabling Automatic Shared Memory Management

Simply re-set sga_target to zero.  Be aware that the data buffer cache (db_cache_size) will take-on the current value from the spfile.

Using Automatic Shared Memory Management

You need to note that Automatic Shared Memory Management does not change the multiple block sizes and the KEEP pool.  In an article titled Automatic Shared Memory Management we see:

Some pools in SGA are not subject to dynamic resizing, and must be specified explicitly. Notable among them are the buffer pools for nonstandard block sizes and the non-default ones for KEEP or RECYCLE. If your database has a block size of 8K, and you want to configure 2K, 4K, 16K, and 32K block-size pools, you must set them manually.

Their sizes will remain constant; they will not shrink or expand based on load. You should consider this factor when using multiple-size buffer, KEEP, and RECYCLE pools.

In addition, log buffer is not subject to the memory adjustment—the value set in the parameter log_buffer is constant, regardless of the workload. ( In 10g, a new type of pool can also be defined in the SGA: Streams pool, set with parameter streams_pool_size. This pool is also not subject to automatic memory tuning.)

From the bestselling book "Oracle 10g New Features", we see how Oracle monitors the pool workloads::

A new background process named Memory Manager (MMAN) manages the automatic shared memory. MMAN serves as the SGA Memory Broker and coordinates the sizing of the memory components. The SGA Memory Broker keeps track of the sizes of the components and pending resize operations.

Monitoring Automatic Shared Memory Management

MOSC Note:295626.1, How To Use Automatic Shared Memory Management (ASMM) In Oracle10g, has some script for monitoring Oracle Automatic Shared Memory Management:
 

select
   component,
   oper_type,
   oper_mode,
   initial_size/1024/1024 "Initial",
   TARGET_SIZE/1024/1024  "Target",
   FINAL_SIZE/1024/1024   "Final",
   status
from
   v$sga_resize_ops;

COMPONENT                      OPER_TYPE     OPER_MODE INITIAL    TARGET     FINAL      STATUS
------------------------------ ------------- --------- ---------- ---------- ---------- ---------
DEFAULT buffer cache           SHRINK        MANUAL    160         148       148        COMPLETE
streams pool                   GROW          MANUAL      0          12        12        COMPLETE

select
   component,
   current_size/1024/1024 "CURRENT_SIZE",
   min_size/1024/1024 "MIN_SIZE",
   user_specified_size/1024/1024 "USER_SPECIFIED_SIZE",
   last_oper_type "TYPE"
from
   v$sga_dynamic_components;

COMPONENT                      CURRENT_SIZE MIN_SIZE   USER_SPECIFIED_SIZE TYPE
------------------------------ ------------ ---------- ------------------- -------------
shared pool                    80           80         80                  STATIC
large pool                      8            8          8                  STATIC
java pool 4                     8           48         48                  STATIC
streams pool                   12            0         12                  GROW
DEFAULT buffer cache           48           24         24                  SHRINK
KEEP buffer cache               0            0          0                  STATIC
RECYCLE buffer cache            0            0          0                  STATIC
DEFAULT 2K buffer cache         0            0          0                  STATIC
DEFAULT 4K buffer cache         0            0          0                  STATIC
DEFAULT 8K buffer cache         0            0          0                  STATIC
DEFAULT 16K buffer cache        0            0          0                  STATIC
DEFAULT 32K buffer cache        0            0          0                  STATIC
OSM Buffer Cache                0            0         24                  STATIC
 

Click here to see scripts for monitoring the shared pool for free RAM chunks.  You can also issue these queries to see free RAM in Oracle10g:

• select sum(value) from v$sga;

• select sum(bytes) from v$sgastat;

• select sum(current_size) from v$sga_dynamic_components;

• select * from v$sga_dynamic_free_memory;
   

Automatic Shared Memory Management Internals

With the advent of the advisory utilities in Oracle9i (v$db_cache_advice, v$shared_pool_advice, and v$pga_target_advice, we see how Oracle plots marginal benefits from different pool sizes:

Robert Freeman notes the default behavior with a blank init.ora file:

I did shutdown my local 10g database and brought it up with a blank init.ora (only had the db_name parameter in it). The actual default in XP 10g is db_cache_size=48m, shared_pool_size=32m.

MOSC Note:257643.1, Oracle Database 10g Automated SGA Memory Tuning gives hints about how MMAN determines current workloads.  The documentation shows that Oracle uses the memory advisories from Oracle9i and applies heuristics (rules of thumb) to determine the best shift in RAM pool sizes.  These heuristics consist of hypothesis testing with "what if" scenarios, computing the ratio of the marginal reduction in physical disk reads, and choosing the size with the greatest overall marginal benefit:

The SGA Memory Broker observes the system and workload in order to determine the ideal distribution of memory. It is never complacent and performs this check every few minutes so that memory can always be present where needed.

Based on workload information, automatic shared memory tuning:

• Captures statistics periodically in the background

• Uses the different memory advisories

• Performs ?what-if? analyses to determine best distribution of memory

• Moves memory to where it is most needed

• Has no need to configure parameters for the worst-case scenario

• Resurrects component sizes from last shutdown if SPFILE is used

Automatic Shared Memory Management Errors

ASMM can precipitate these errors:

ORA-00093: _shared_pool_reserved_min_alloc must be between 4000 and 0

alter system set sga_target = 330M
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00827: could not shrink sga_target to specified value

alter system set sga_target=160M;
alter system set sga_target=160M

* ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00823: Specified value of sga_target greater than sga_max_size

SQL> startup

ORA-00824: cannot set sga_target due to existing internal settings

Cause:  If you enable automatic SGA Management by setting SGA_Target >0 and
if you are using db_block_buffers(Obsolete parameter) in your init.ora.
Startup of Database fails with ORA-00824 Error

Potential issues with ASMM

The Oracle 10g Automatic Storage Memory Management (ASMM) feature (the default on Oracle10g) should anticipate high updates and allocate additional data buffers during high update periods. 

For example, here is an actual output from an Oracle10g database where it appears that ASMM is not allocating enough free blocks to accommodate concurrent updates:

STATUS  NUMBER_BUFFERS

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

cr                 616

free                 1 

xcur             14790
 

 

 316     6 __db_cache_size                       130,023,424

  96     6 __java_pool_size                       12,582,912

  94     6 __large_pool_size                       4,194,304

  92     6 __shared_pool_size                    142,606,336

 

How to disable AMM:  See these important notes on disabling AMM (Automatic Space Memory Management)