Calculating message buffer space

To limit the number of message buffers in your system and maintain adequate signaling performance, you need to know how much buffer space is required to support a particular message length. (The message buffer space required for a particular message length can vary from release to release.) For this release of MVS™, compute the message buffer space required for a single message as follows: subtract 1K from the message length, round the result up to the nearest multiple of 4K, then add 2K. For example, if the message length is 40,000 bytes:

40,000 - 1,024 = 38,976
  (Rounding 38,976 up to the nearest multiple of 4K equals 40K)
40K + 2K = 42K

So, a MAXMSG value of 42K provides enough message buffer space to send a single message of 40,000 bytes. Because the transport class needs more than a single message buffer to have an adequate supply for signalling, the MAXMSG value, in this example, should be a multiple of 42K.

For details on deciding which MAXMSG value to use, see Planning signaling services in a sysplex.

Total Buffer Space: To compute the maximum message buffer space that MVS can allocate per system, use the following formula:

total = paths + classes + LOCALMSG value

Where:  paths          = the sum of all message buffer spaces
                         specified for all paths.
        classes        = the sum of all message buffer
                         spaces calculated for all transport classes.
        LOCALMSG value = the sum of local system message buffer
                         spaces for all transport classes

To calculate the buffer space for a transport class, multiply 64K or the specified message buffer space (whichever is greater) by the number of systems in the sysplex.

For local message buffer spaces, add the LOCALMSG value for each transport class specified for the system.

For example, in a sysplex of four systems, suppose COUPLxx is defined as follows for one of the systems:

COUPLE  SYSPLEX(PLEXOF4)   MAXMSG(16)

CLASSDEF CLASS(DEFAULT)    MAXMSG(96)
CLASSDEF CLASS(TCGRS)      MAXMSG(32)

PATHIN   DEVICE(110,111)
PATHIN   DEVICE(112,113)   MAXMSG(48)

PATHOUT  DEVICE(120,121)   CLASS(TCGRS)
PATHOUT  DEVICE(8C40)      MAXMSG(50)
PATHOUT  DEVICE(8C50)

LOCALMSG CLASS(DEFAULT)    MAXMSG(20)

For this system, the buffer space values for the paths and transport classes are:

Signal Path        Buffer Space
--------------     -----------------
 110                  16K         (default from COUPLE MAXMSG)
 111                  16K         (default from COUPLE MAXMSG)
 112                  48K         (specified in PATHIN statement)
 113                  48K         (specified in PATHIN statement)
 120                  32K         (default from class TCGRS)
 121                  32K         (default from class TCGRS)
 8C40                 50K         (specified in PATHOUT statement)
 8C50                 96K         (default from class DEFAULT)

Transport class    Buffer space
----------------   ----------------
  DEFAULT            404K (96K x 4 systems, + 20K local increment
                          specified for LOCALMSG)
  TCGRS              256K (Minimum 64K x 4 systems)

The total maximum buffer space for the system is:

Total = 16K + 16K + 48K + 48K + 32K + 32K + 50K + 96K + 404K + 256K
      = 998K