SAP Parameter em/initial_size_MB - Initial size of extended memory pool

Parameter
em/initial_size_MB

Short text
Extended Memory Size

Parameter description
Note the operating system dependency for Microsoft Windows.
The value specifies the initial size of the memory area that SAP MemoryManagement manages as extended memory. The memory requirements forinteractive users' programs are met from this area. This memory type canbe assigned very quickly to changing work processes by mapping andtherefore allows very fast context switches.
Aim
Ideally, this pool should be large enough to contain the total of alluser contexts, so that context switching for all users could beperformed at optimal speed. You can restrict the maximum usage of anindividual user with the parameter
ztta/roll_extension. This prevents one user or a small number ofusers using all of the extended memory. If the extended memory isexhausted, additional memory requests are satisfied from the roll areaand then from from the local address area of a work process (heap,malloc). The data in the roll area must be copied during the contextswitch, while the data in the local process area leads to exclusivereservation of the entire work process (PRIV mode).

Parameter unit
1 MB

Default value
Platform-dependent, approximately 4 GB for 64-bit
Default Value
(other)
500 - 4096, in extreme cases much higher

  • Around 5 MB x number of active users on the application server

  • Additional tuning in accordance with the values from transaction ST02
    • (for individual application servers) or RZ03 (for all applicationservers)
    • From values > 500, you should expect operating system restrictions that
      • cannot be extended, particularly in 32 bit systems.
      Default Value
      (Microsoft Windows und Linux)
      Do not change the default = PHYS_MEMSIZE manually, since extended memoryincreases dynamically when necessary. No further calculation oradjustment is necessary.

      Who is allowed
      The customer. No manual maintenance is necessary for Microsoft Windowsor Linux.

      Other parameter
      The value must be smaller than or equal to (
      em/blocksize_KB * 16) and an integer multiple of (em/blocksize_KB /1024).

      Limitation for os
      The swap space must be large enough and should have a safety net ofseveral hundred megabytes at all times. Use transactions ST06 and RZ03(CCMS alerts) for monitoring. You may have to extend existing limits inthe operating system:

      • Maximum size of shared memory, mapped file, mprotect

      • Address space limits for each process or in total

      • And so on. You can use the test program 'memlimits' to determine some of
        • these limits.
        Microsoft Windows
        A dynamic extension of the extended memory is implemented underMicrosoft Windows and Linux. This means that if the current extendedmemory is insufficient, it is increased by the size [PHYS_MEMSIZE / 2].The default value is identical to PHYS_MEMSIZE. As a result of thismechanism, it is no longer necessary to maintain this profile parametermanually.
        Example
        Application server with 1 GB main memory. PHYS_MEMSIZE is not maintainedand its default value is the size of the main memory. This means that
        The initial size of the extended memory = 1 GB. If this size is notsufficient, the extended memory is increased in 500 MB steps until themaximum
        size em/max_size_MB (Default: 20 GB) is reached or until there is nomore address space in the NT page file.
        Central instance with 1 GB main memory. The database uses 400 MB and theSAP instance 600 MB of main memory. The value for PHYS_MEMSIZE is set to600 in the instance profile. This means that the extended memory has aninitial size of 600 MB, and if necessary extends itself in steps of 300MB.

        Limitation for db
        None

        Limitation for db
        None