Saturday, February 22, 2014

Automatic Shared Memory Management

Automatic Shared Memory Management: Overview
Automatic Shared Memory Management (ASMM) simplifies the configuration of the System Global Area (SGA). ASMM uses memory advisor data to evaluate the best memory configuration, and then resizes the amount of memory to be allocated for the database buffer cache, shared pool, Java pool, large pool, and Streams pool by using the dynamic SGA feature. 

ASMM makes more effective use of available memory and thereby reduces the cost incurred for acquiring additional hardware memory resources, and significantly simplifies Oracle database administration with a more dynamic, flexible, and adaptive memory management scheme.

ASMM enables you to specify a total memory amount to be used for all SGA components. The Oracle database server periodically redistributes memory between the components given in the slide according to workload requirements. 

For example, in a system that runs high-concurrency OLTP workload during the day, which requires a large buffer cache, you would have to configure both the buffer cache and the large pool to accommodate your peak requirements. With ASMM, when the OLTP workload runs, the buffer cache is given the required memory to optimize buffer cache access. When the decision support system (DSS) batch job starts up later, the memory is automatically migrated to the large pool so that it can be used by parallel query operations without producing memory overflow errors.

Note: With ASMM, component sizes are saved across a shutdown if an SPFILE is used. The sizes are resurrected from before the last shutdown to avoid relearning.

Automatic Memory Management: Overview

With Automatic Memory Management (AMM), the system causes an indirect transfer of memory from SGA to PGA (and vice versa). It automates the sizing of PGA and SGA according to your workload.

This indirect memory transfer relies on the OS mechanism of freeing shared memory. After memory is released to the OS, the other components can allocate memory by requesting memory from the OS.

Currently, this is implemented on Linux, Solaris, HP-UX, AIX, and Windows. Set your memory target for the database instance and the system then tunes to the target memory size, redistributing memory as needed between the system global area (SGA) and the aggregate program global area (PGA).

The slide displays the differences between the Automatic Shared Memory Management mechanism and the Automatic Memory Management.

The simplest way to manage memory is to allow the database to automatically manage and tune it for you. To do so (on most platforms), you have to set only a target memory size initialization parameter (MEMORY_TARGET) and a maximum memory size initialization parameter (MEMORY_MAX_TARGET). 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 do not accidentally set the target memory size too high. Because certain SGA components either cannot easily shrink or must remain at a minimum size, the database also prevents you from setting the target memory size too low.

Oracle Database Memory Parameters

The slide displays the memory initialization parameters hierarchy. Although you have to set only MEMORY_TARGET to trigger Automatic Memory Management, you still have the ability to set lower-bound values for various caches. So if the child parameters are set by the user, they will be the minimum values below which that component is not auto-tuned.

The SGA is made up of several components, some are auto-tuned when ASMM is enabled, some are dynamic, and some are static. When Automatic Memory management is enabled the Automatic Shared Memory Management is also enabled.

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