1. A system, comprising: one or more computers configured to implement a load testing system, wherein the load testing system comprises: one or more controllers configured to generate a plurality of test job descriptions based on a load step description, wherein the load step description specifies a total test duration for a load test of a service, wherein the load step description specifies a distribution of operation types that includes different types of operations for the load test of the service, wherein the load step description specifies a total transaction frequency or a total number of concurrent connections for the load test of the service, wherein individual test job descriptions of the plurality of test job descriptions specify respective portions of the total test duration, wherein the individual test job descriptions of the test job descriptions specify respective portions of the total transaction frequency or respective portions of the total number of concurrent connections, and wherein the individual test job descriptions of the test job descriptions specify respective portions of the distribution of operation types; a job queue configured to receive and queue the test job descriptions from the one or more controllers; and a plurality of workers, wherein individual workers of the plurality of workers are configured to: read one or more of the test job descriptions from the job queue; execute one or more jobs based on the test job descriptions read from the job queue, wherein the one or more jobs are executed for the respective portions of the total test duration in the corresponding test job descriptions, wherein the one or more jobs are executed according to the respective portions of the total transaction frequency or the respective portions of the total number of concurrent connections in the corresponding test job descriptions, and wherein the one or more jobs are executed according to the respective portions of the distribution of operation types; and monitor one or more of its own system metrics during execution of the one or more jobs and determine, based at least in part on its own system metrics, whether to read an additional test job description from the job queue.