SAP Memory Management

Functions of the SAP Memory Management System

An application runs in an SAP work process where an ABAP program is normally executed. The process requires memory to do this, which are allocated to it by the memory management system. The order in which the work process is assigned the memory type depends on the work process type, either dialog or non-dialog (see SAP Memory Types), and the underlying operating system. This is described in more detail in the documentation on the operating system.

The area of a user context that is directly accessible is now extended as needed, if the user context has expanded. For dialog work processes, the data of the user context, including internal tables is located in this expanded area. You can therefore access all the data in the user context. Only data of the types "extract" and "export to memory" stay in SAP Paging.

The SAP Roll Area is used for the initial memory assigned to a user context, and (if available) for additional memory if the expanded memory is full.

The following diagram displays the memory types that can be assigned to work processes on the SAP and operating system level. Here are the most important system profile parameters that control the availability of the memory types.

SAP Roll Area

The roll area is a memory area with a set (configurable) size that belongs to a work process. It is located in the heap of the virtual address space of the work process. When the context of a work process changes, the data is copied from the roll area to a common resource called the roll file. To prevent repeated copying, another roll buffer is located in between that is part of the shared memory. The roll area consists of 2 segments. The first segment, which can be set with the parameter ztta/roll_first, is assigned to the work process first as memory. If this is used up, the work process has more memory assigned to it. The amount of memory is determined by the difference between the parameters ztta/roll_area and ztta/roll_first.

SAP Extended Memory

SAP extended memory is the core of the SAP memory management system. Each SAP work process has a part reserved in its virtual address space for extended memory (see Virtual Address Space in a Work Process). You can set the size of extended memory using the profile parameter em/initial_size_MB: Extended Memory Pool Size. Under Windows NT, further memory is assigned dynamically as needed, and you can also set this amount. You can map the extended memory from the common resource onto any work process, and after onto another process (see graphic) on the same address in the virtual address space. This is important if you work with pointers in the ABAP program. Depending on the operating system, how you implement the extended memory is different. The SAP system builds a layer on to the operating system functions for the page management of this memory. This extended memory is implemented as an unnamed mapped file. This means the address space uses the paging file or uses the swap space of the operating system as background memory.

Private Memory

Other processes cannot use private (heap) memory. After releasing the assigned memory, the operating system still considers the (virtual) memory as being occupied by the allocating process. These characteristics of heap memory require that:

1. The work process can be run in PRIV mode (private) when the local memory is assigned. This means that the work process is reserved for processing the current user context until the context releases the work process again when the request has ended.

2. The work process, if it has used a lot of private memory, is restarted when the user context is terminated and the local memory is returned. The restart makes the local memory available again for other processes. A restart occurs if a work process uses more local memory than is defined in the parameter abap/heaplimit. The mechanism is displayed again there.

If a dialog work process has used up the roll area assigned to it and the extended memory, private memory is assigned to the work process. The work process goes into PRIV mode (private). In the parameter settings, ensure that not too many dialog work processes are simultaneously in the PRIV mode, since this lowers system performance. The other Work Process types (background, update, enqueue and spool work processes) are assigned heap memory after the roll area is used up. Allocating Extended Memory is not useful since no context change occurs for these processes.

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