Provides control of the workload, flow control, and concurrency control of a computer system through the use of only external performance monitors. Data collected by external performance monitors are used to build a simple, black box model of the computer system, comprising two resources: a virtual bottleneck resource and a delay resource representing all non-bottleneck resources combined. The service times of the two resource types are two parameters of the black box model. The two parameters are evaluated based on historical data collected by the external performance monitors. The workload capacity that avoids saturation of the bottleneck resource is then determined and used as a control variable by a flow controller to limit the workload on the computer system. The workload may include a mix of traffic classes. In such a case, data is collected, parameters are evaluated and control variables are determined for each of the traffic classes.