Memory Management SAP BASIS

Starting with release 3.0 of R/3, SAP introduced a new concept in its use of main memory to improve the overall performance of the system, in respect to previous Web AS releases (2.2 and earlier). The main change was to make use of an extended memory management system to optimize the access to the user contexts and thus avoid the overhead caused by heavy roll-in/roll-out tasks of previous releases. User context is defined as the user data that are kept by the system between dialog steps to continue the processing of the transaction. The user context contains a user-specific area where user and authorization data are stored as well as a session context for each SAP session. These data contain such things as authorizations, field information, internal tables, runtime environment management information, and so on. Roll-in is the process of making the data available to the work processes when they need it. With the extended memory management functions, all user contexts of an application server are held in main memory and are shared by all work processes. In previous releases, the user contexts were in the roll files and had to be copied from one place to another when user sessions were handled by different work processes. With the new memory management, those previous copy functions (copying user contexts to roll areas) are handled by just reassigning pointers in main memory.

With this technique there is a significant improvement in performance, because context switching is a very frequent task when attending to interactive users. In order to observe the real improvements, the system needs more memory and swap space than previous versions. With SAP releases prior to 3.0, the user context area used by the dialog processes was limited to the size of the roll area. The new memory management allows the size of this area to be extended as the size of user contexts increases. All the data in the user context are directly accessible.

Work process memory allocation (Copyright by SAP AG)

Work process memory allocation (Copyright by SAP AG)

SAP, and not the operating system, takes care of page management for the user contexts held in shared memory because SAP's strategy is for openness and thus it is platform independent. This shared memory is technically implemented using an unnamed mapped file. In UNIX systems, this means that the address space is mapped onto the operating swap space. For more information on memory mapping, refer to the operating system manuals or online help. To tune and configure memory management, make sure that your system meets all requirements regarding memory and swap space. SAP automatically sets some default parameters depending on your particular configuration. There are some utilities available for monitoring the operation of the memory management to later fine-tune it. The configuration is accomplished setting parameter values in the system profile. SAP provides a comprehensive manual for administrators full of examples for setting different values for the required parameters that directly affect the mode of operation of the memory management system.

Refer to this manual, BCMemory Management, for a detailed description of particular configurations and a deeper description regarding the mode of operation. Setting the many profile parameters related to memory management can be a very complex job due to the many relationships of these parameters. Web AS notes 88416 (Zero administration memory management from 4.0A/NT) and 103747 (Performance 4.0/4.5: Parameter recommendations) are a good complement to the memory management online documentation.

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