A stochastic model of a multiserver queueing system with simultaneous service of a customer by a random number of servers and threshold-based (hysteretic) control of the service rate is presented. The performance and quality-of-service measures of the model are defined. Numerical results of experiments studying the dependence of performance/energy measures on the service rate switching threshold are presented. The model allows studying the energy efficiency of a high-performance cluster under quality-of-service and performance constraints.