Dear Mooona, thanks for reply but my questions is that to which class i have to replace or update i.e. CloudletSheduler, VMAllocationPolicy or VmScheduler etc. ?
Take a look at Marc Bux's DynamicCloudSim. It adds a workflow scheduling layer to CloudSim and implements custom task graph schedulers. The core idea is indeed to extend the DatacenterBroker, as Saeed Ghadery pointed out. The schedulers are located here [1] and they all inherit from the AbstractWorkflowScheduler class. Take a look at the HEFT scheduler to see how to model machine-dependent task runtimes.