Memory management is an important part of Oracle Database performance tuning. If the database does not have enough memory, queries may run slowly, sessions may consume more resources than expected, and the system may experience unnecessary disk I/O. On the other hand, allocating too much memory can affect the operating system and other applications running on the server.
Oracle provides different approaches to manage database memory. Two commonly discussed options are Automatic Memory Management (AMM) and Automatic Shared Memory Management (ASMM).
Both approaches help DBAs reduce manual memory tuning, but they work differently. Understanding these differences helps when configuring a new database, troubleshooting performance issues, or deciding whether an existing memory configuration needs to be changed.
1. What Is Automatic Memory Management (AMM)?
Automatic Memory Management allows Oracle to manage the main memory areas used by the database through a single memory allocation setting.
With AMM, the DBA configures the 'MEMORY_TARGET' parameter. Oracle then manages the allocation of memory between the System Global Area (SGA) and the Program Global Area (PGA), adjusting their sizes based on workload requirements. The SGA contains shared memory used by multiple database sessions, while the PGA contains private memory used by individual server processes.
For example, if a workload requires more memory for sorting or other operations that use the PGA, Oracle can adjust the memory distribution between the SGA and PGA within the configured limits.
The main parameters used for AMM are:
- 'MEMORY_TARGET' – Specifies the target amount of memory Oracle manages for the SGA and PGA combined.
- 'MEMORY_MAX_TARGET' – Specifies the maximum value to which 'MEMORY_TARGET' can be increased, subject to platform and configuration restrictions.
ALTER SYSTEM SET MEMORY_TARGET = 4G SCOPE=SPFILE;
ALTER SYSTEM SET MEMORY_MAX_TARGET = 4G SCOPE=SPFILE;
These settings configure a 4 GB memory target and maximum target. Because `SCOPE=SPFILE` is used, the changes take effect after the database is restarted.
The values are examples only. Actual settings should be based on available server memory, the database workload, and the memory requirements of the operating system.
Advantages of AMM
- Oracle manages memory distribution between the SGA and PGA automatically.
- Requires less manual tuning of individual memory areas.
- Can adapt memory allocation to changing workload requirements.
- Provides a straightforward configuration for environments where AMM is supported and appropriate.
Limitations of AMM
- It offers the DBA less direct control over the balance between SGA and PGA.
- Its behavior depends on workload characteristics and available memory.
- It may use operating-system shared-memory mechanisms that are unsuitable for certain configurations.- It is generally not the preferred choice for Oracle Database deployments using HugePages on Linux.
2. What Is Automatic Shared Memory Management (ASMM)?
Automatic Shared Memory Management takes a different approach. Instead of managing the entire memory allocation through one parameter, ASMM automatically manages the major components within the SGA.
The DBA configures the overall SGA size and sets a separate target for PGA memory management. Oracle then adjusts the sizes of eligible SGA components based on the workload.
These components can include:
- Database Buffer Cache
- Shared Pool
- Large Pool
- Java Pool
- Streams Pool
Oracle can increase or decrease the sizes of these components as needed, within the configured SGA limits.
However, ASMM does not automatically manage the total memory split between the SGA and PGA. The DBA is responsible for setting the SGA target and PGA target separately.
ALTER SYSTEM SET SGA_TARGET = 3G SCOPE=SPFILE;
ALTER SYSTEM SET SGA_MAX_SIZE = 3G SCOPE=SPFILE;
ALTER SYSTEM SET PGA_AGGREGATE_TARGET = 1G SCOPE=SPFILE;
In this example:
- SGA_TARGET = 3G sets the target SGA size.
- SGA_MAX_SIZE = 3G sets the maximum SGA size.
- PGA_AGGREGATE_TARGET = 1G sets the target for aggregate PGA memory usage.
These are illustrative values, not a recommended allocation for every database. The actual PGA usage can vary with workload, and `PGA_AGGREGATE_TARGET` is a target rather than a strict hard limit.
Advantages of ASMM
- Provides more direct control over the SGA and PGA allocation.
- Automatically adjusts eligible SGA components.
- Works well in environments where the DBA wants to manage shared and process memory separately.
- Is commonly preferred in Linux environments configured to use HugePages for Oracle SGA memory.
Limitations of ASMM
- The DBA must size the SGA and PGA targets separately.
- It does not automatically rebalance memory between the SGA and PGA.
- Incorrect target values can still cause memory pressure or inefficient resource utilization.
- Some SGA components may require explicit sizing, depending on the configuration and Oracle features in use.
3. AMM vs ASMM: Key Differences
One important distinction is that ASMM and automatic PGA management can work together. Using ASMM does not mean the DBA must manually manage every PGA allocation.
4. How to Check the Current Memory Management Configuration
Before changing memory parameters, a DBA should first identify how the database is currently configured.
The following query displays the main memory-related parameters:
SHOW PARAMETER memory_target;
SHOW PARAMETER memory_max_target;
SHOW PARAMETER sga_target;
SHOW PARAMETER sga_max_size;
SHOW PARAMETER pga_aggregate_target;
You can also check them together using a SQL query:
SELECT name, value, display_value
FROM v$parameter
WHERE name IN (
'memory_target',
'memory_max_target',
'sga_target',
'sga_max_size',
'pga_aggregate_target'
)
ORDER BY name;
If 'MEMORY_TARGET' is nonzero, AMM is enabled. If 'MEMORY_TARGET' is zero and 'SGA_TARGET' is nonzero, ASMM is generally being used, provided the configuration is not manually managing the SGA.
A DBA should also check the values of 'SGA_TARGET', 'PGA_AGGREGATE_TARGET', and other relevant parameters before making any changes.
5. How to Monitor Memory Usage
Setting the memory parameters is only the first step. DBAs should also monitor actual memory usage to determine whether the configuration is working well under the application's workload.
Check SGA component allocation
SELECT component,
current_size / 1024 / 1024 AS current_size_mb,
min_size / 1024 / 1024 AS min_size_mb,
max_size / 1024 / 1024 AS max_size_mb
FROM v$sga_dynamic_components
ORDER BY current_size DESC;
This query helps identify how much memory is allocated to individual SGA components and how their sizes have changed.
Check PGA usage
SELECT name, value, unit
FROM v$pgastat
WHERE name IN (
'total PGA allocated',
'maximum PGA allocated',
'total PGA inuse',
'over allocation count'
);
Pay particular attention to PGA over-allocation and peak usage. These statistics can help identify whether the current PGA target is adequate for the workload.
A high PGA allocation alone does not necessarily indicate a problem. It should be evaluated alongside workload patterns, available physical memory, and operating-system memory pressure.
6. Which Is Better: AMM or ASMM?
There is no single answer that applies to every Oracle Database environment. The right choice depends on the operating system, database version, workload, and memory configuration.
AMM may be suitable when:
- You want Oracle to manage the overall SGA/PGA memory distribution.
- The platform supports AMM and its memory allocation mechanisms fit your environment.
- You prefer a simpler memory configuration.
ASMM may be suitable when:
- You want to configure SGA and PGA targets independently.
- You are running Oracle Database on Linux and plan to use HugePages.
- You need greater control over memory allocation for a production workload.
For many production environments on Linux, ASMM combined with automatic PGA management is a common choice, particularly when HugePages are part of the server configuration.
Before choosing either option, review the Oracle Database version-specific documentation and platform requirements. Also, avoid switching memory management modes without checking parameter dependencies, available memory, and restart requirements.
Automatic Memory Management and Automatic Shared Memory Management both reduce the amount of manual memory tuning required in Oracle Database, but they differ in how memory is allocated.
AMM manages the overall distribution between the SGA and PGA, while ASMM focuses on adjusting the components within the SGA and leaves the overall SGA/PGA targets under the DBA's control.
From a DBA perspective, the decision should not be based only on which approach requires less configuration. The operating system, HugePages requirements, workload behaviour, and memory statistics should all be considered before selecting a memory management strategy. Regular monitoring is essential to ensure the database has enough memory to perform efficiently without putting unnecessary pressure on the server.
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