Before revision:  Moreover, when the degree of the prolonged service time is pre-specified, we show that the pooling system with prolonged service time at non-dedicated servers is not preferred when the work load in the systemis greater than a certain threshold .
After revision:  Moreover, when the degree of the prolonged service time is pre-specified, we show that the pooling system with prolonged service time at non-dedicated servers is prolonged in an asymmetric system .