Allocating Message Format Service buffer pool space

The FBP and FRE keywords in the DFSPBxxx member of the IMS PROCLIB data set or the JCL override allow you to reserve storage for efficient processing of communications activity. The amount of storage that is required depends primarily on how much concurrent use of message format blocks is expected, and how many communication lines are active.

You can control the size of the message format buffer pool and the number of fetch request elements (FREs) using the FBP and FRE keywords in the DFSPBxxx member of the IMS PROCLIB data set or the JCL override. One FRE is required to control each active block. Without a FRE, space cannot be assigned from the message format buffer pool. When estimating the number of FREs, allow at least 10 additional FREs to handle system activity. This eliminates the possibility of delayed response to other terminals. If preallocated FREs are not available, a dynamic FRE is allocated from the general area of the pool. However, dynamic FREs tend to fragment the pool, so avoid using them.

To determine an adequate size for the message format buffer pool:

Procedure

  1. Compile a list of the Message Format Service (MFS) blocks, arranging them in DIF/MID and DOF/MOD pairs.
  2. Record their sizes and some indication of their frequency (for example, whether they are used by priority or by quick response transactions).
  3. Allow space for input/output pairs to be in the pool. Watch for large messages or those with multiple segments, because these might pre-empt small, frequent messages from finding space for their format blocks in the pool, and this would directly affect their response time.

If you use the MFS Service utility to build an index of the DASD addresses that reside in the message format buffer pool during online execution, the index entries also reside in extended private storage.