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This paper describes DGSchedSim, a trace driven simulator to evaluate scheduling algorithms focused on minimising turnaround time of applications executed in heterogeneous desktop grid systems. The simulator can be used to model task farming applications comprised of a set of independent and equal sized tasks similarly to numerous @Home public computing projects like the popular SETI@Home. DGSchedSim permits to assess scheduling policies under several scenarios allowing to control parameters such as the application's requirements (number of tasks, individual requirements of tasks like needed CPU time), the properties of the environment (machines computing capabilities and availabilities) and the characteristics of the execution (frequency and storage location of checkpoints, etc.). The simulations are driven by traces collected from real desktop grid systems. Besides DGSchedSim, the paper presents the Cluster Ideal Execution Time (CIET) algorithm that computes the ideal wall-clock time required by a fully dedicated and totally reliable cluster of M heterogeneous machines to process the T tasks of an application. As a test to the simulator capabilities, the paper analyses the suitability of two scheduling algorithms, FCFS and MinMax, for delivering fast turnaround time in desktop grids. Both algorithms, when combined with a centrally stored checkpoint policy, achieve efficiency close to 50% of CIET for certain scenarios.