  Consider an unobservable $M|G|1$ queue with preemptive-resume scheduling and
two priority classes. Customers are strategic and may join the premium class
for a fee. We analyze the resulting equilibrium outcomes, equilibrium
stability, and social welfare. We find that for service distributions with
coefficient of variation greater than 1, there exists a unique and stable mixed
equilibrium at low loads. We also establish a tight bound on the price of
anarchy, which is $4/3$.
