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Oracle Automatic Shared Memory Management
By Burleson Consulting
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Note: 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.
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):
Enabling Automatic Shared
Memory Management
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:
-
__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.
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
MetaLink 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$sga;
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select
sum(bytes) from v$sgastat;
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select
sum(current_size) from v$sga_dynamic_components;
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select *
from v$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.
MetaLink 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
Here we see the
double-underscore ASMM hidden parms:
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
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