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VPE increases the amount of virtual storage used by your application. The amount of virtual storage used in a job step is one of the criteria that the MVS System Resource Manager uses to determine which job to swap out when there is too much contention for memory resources. This function is a very important part of MVS's ability to ensure optimum response for time-critical systems such as CICS, as it provides the means to ensure that priority systems obtain the memory resources they require for good response times.
Unfortunately, this same evaluation criteria works against optimum performance during the off-prime hours when online systems are largely idle and the CPU is doing more batch than interactive work. Under these circumstances, the bias in favor of online systems and lower multiprogramming levels can lead to bottlenecks and poor memory utilization. This typically results in high overall CPU utilization, long elapsed run times, high swap counts for large batch jobs, and relatively light I/O contention (because jobs spend a great deal of time swapped out). Should you find that VPE has relieved your I/O bottleneck only to create a memory problem, ask your MVS tuner about raising domain minimums for your performance group. This can be done by command to test the effect and made permanent using changes to IEAICSxx parameters. If you are still not satisfied, contact CA Technical Support for assistance.
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