Minimizing path length

The total number of machine instructions that are executed to process a transaction, including system services, IMS services, and the application program itself, has a direct bearing on throughput. The accumulation of executed instructions is termed the path length. Options chosen for performance affect path length.

The actions suggested in Choosing IMS options for performance all contribute to the minimization of path length. Avoid the regular use of traces such as the DL/I Call Image Capture and other traces invoked by the /TRACE command. These are specified as parameters on the OPTIONS statement in the DFSVSMxx member of IMS.PROCLIB.

Do not run the IMS Monitor (DFSMNTR0), except for during 10- to 20-minute preplanned intervals.

In a real storage-constrained system, the most effective way to reduce path length is to minimize paging. Minimal pools help to minimize paging by eliminating costly scanning of directories or buffers that might need to be paged in before they can be read. If virtual storage requirements are reduced:

• A minimal PSB pool minimizes buffer searching.
• A tuned database pool minimizes buffer searching; a larger database pool costs more in path length and might not reduce I/O.
• A tuned message queue pool minimizes buffer searching; a larger pool reduces IMS message queue I/O but does so at the expense of higher processor cycles per queue pool operation.
• The same applies to the message format buffer pool as to the message queue pool.