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MainView for z/OS
MainView for z/OS CMF Monitor MainView SYSPROG Services
Why does the MainView SYSPROG Services AEW ENQ sampler produce different results to MainView for z/OS and CMF Monitor when monitoring ENQ contention?
The SYSPROG Services AEW ENQ sampler does not work the same way that MainView for z/OS and CMF Monitor do in monitoring ENQ contention.
The MainView for z/OS and the CMF MONITOR ENQ Sampler use the GRS ENF (IBM Event Notification Facility - 51) along with GQSCAN to track the start and end of all ENQ contention events. MainView for z/OS uses GQSCAN to establish a base list of contention at the beginning of the interval and then relies on the ENF 51 exit to be notified of new contentions and end of contention. One limitation is that the ENF routine does not provide any notification of Step Level Enqueues, so we only see the change in status of STEP level enqueues at the beginning of each interval.
The AEW ENQ sampler does not use ENF and only periodically issues GQSCAN to see what is in contention at that moment in time. When a resource is in contention, it adds it to a table or updates the time that contention has been "active". Therefore if a resource is constantly in and out of contention, there's a chance that the AEW ENQ sampler will not always see a "no contention" GQSCAN and the time in contention will continue to increase.