Abstract: A test system includes a controller that creates test jobs to simulate a load for testing a target system or service. The jobs specify test data from a data store. The controller adds the test jobs to a job queue at a rate. Workers access jobs from the job queue and request data from the data store as specified in each job and send the request data to the service. Approximations (e.g., metric values indicating performance of the worker host resources, or performance of the system under test or performance of resources that implement the system under test, other feedback, etc.) of actual system characteristics such as the actual number of workers are obtained. Based on the metric values, the controller modulates various characteristics of the load generation system, such as a rate at which jobs are added to the job queue or the number of workers processing jobs.