Implications of the multiple monitor environment

When multiple monitors are open concurrently, VTAM® maintains only one internal value for each resettable statistic. However, each monitor must be able to maintain the highwater and low water marks for their interval. To provide this capability, VTAM allows multiple active instances of the performance monitor exit routine. When the exit routine is invoked, all the instances are driven so that each open monitor can receive the data as it is reset.

Monitors might receive data for collections that they are not tracking. The data received for a given invocation, depends on the reason for data delivery:

For a Collect Performance Data CNM RU, the data delivered is the specified data for which the requesting monitor has collection enabled.
For SMF interval data, the data delivered is the union of all monitors' SMF-enabled collections.
For VTAM event notification, data for the affected resource is delivered if at least one monitor has appropriately enabled this notification.

For more information about multiple exit support, refer to z/OS Communications Server: SNA Customization.