In order to discuss scheduling algorithms for time-sharing computer systems, this paper analyzes the M/G/1 queue under the well known round-robin (RR) discipline. Three models are considered: the constant-quantum RR model, the processor-shared (or zero-quantum RR) model, and the variable-quantum RR model. The paper proposes an effective calculating method for obtaining the expected response time under an arbitrary processing-time distribution with overhead. By the theoretical analysis, one can show how the response time is affected by the scheduling and processing-time distributions, as is demonstrated by some typical examples.