  We study a closed queueing system that offers two types of service. After
obtaining service the customer is active for a period of time whose random
duration depends on the service type: the longer service type enables longer
activity time. Users can choose a service type every time they are in service,
and we are interested in the system efficiency which is the average number of
active users. Applications include but are not limited to inventory control,
electrical vehicle charging, mobile robotics charging, and machine maintenance.
We prove that choosing the more efficient service is not always optimal. We
demonstrate that if we constrain the strategy to be a threshold strategy
depending on the number of inactive users, then the resulting maximal
efficiency is not significantly lower than the maximal efficiency. We also
study individuals' optimal selection of the service type, and observe that
individuals and the manager have opposite incentives: when the queue is long,
individuals choose the long service, while the manager prefers the short
service in this case.
