Considering Performance

Efficiency of exit routines is a prime concern for IMS performance. Most routines are called from the IMS control region and get control in key 7 supervisor state. Some routines might be called from mainline processing code running under the IMS Control Region task. The amount and type of processing that is done by those routines can directly contribute to the total path length and time required to complete a unit of work. Other units of work that must wait to run under a task currently in use by an exit routine can also be impacted. An abend in an exit routine that executes in the IMS control region can cause the IMS control region to abend.

Services such as z/OS WAIT calls, SVC calls, and I/O can all contribute to poor performance and should be used sparingly. When an IMS callable service exists, it is recommended that you use it, rather than the z/OS equivalent, because the callable service is usually optimized to perform more efficiently in an IMS subdispatching environment.

Recommendations:

• Code user-written routines in ways that minimize path length and processing time as much as possible.

• Code user exit routines only in assembler. Exit routines written in other languages might functionally work, but unacceptable performance can result. For example, using a high-level language runtime environment (such as Language Environment for z/OS) can result in unacceptable performance with the initialization and termination of the environment.