For best performance, the initial stack segment should be large enough to satisfy all requests for stack storage. The Language Environment storage report generated by the RPTSTG(ON) option shows you how much stack storage is being used, the total number of segments allocated to the stack, and the recommended values for the STACK run-time option. An initial stack segment that is too large can waster storage and degrade overall system performance, especially under CICS where storage is limited.
You can tune stack storage by using the Language Environment STACK and THREADSTACK run-time options. RPTSTG(ON) can have a negative affect on the performance of your application, because as the application runs, statistics are kept on storage requests. Therefore, always use the IBM-supplied default setting RPTSTG(OFF) when running production jobs. Use RPTSTG(ON) only when debugging or tuning applications.