1. A method for workload control, comprising: controlling workload of an externally observable computer system, said step of controlling comprising: using external performance monitors collecting data by observing only external performance measures of a plurality of requests in a common traffic class directed to the computer system, wherein said external performance measures are: a concurrency measure and a corresponding average response time of the plurality of requests in the common traffic class; using the collected data to build a black box queuing model of said computer system, said black box queuing model comprising only two resources: a virtual bottleneck resource; and a delay resource representing all non-bottleneck resources combined; operating the black box queuing model with only two parameters related to the common traffic class: a service time of the requests on a virtual bottleneck resource, and a delay accounting of the requests on non-bottleneck resources, wherein the two parameters are evaluated based on historical data collected by external performance monitors; using said evaluated two parameters for analyzing said black box queuing model to determine a desirable load capacity of said computer system, said desirable load capacity being the one that drives utilization of the virtual bottleneck resource to a prespecified and desirable target level; using the desirable load capacity to determine a concurrency limit that drives the utilization of the virtual bottleneck resource based on the computed load capacity; and employing said desirable load capacity as control parameters for controlling workload of said computer system.