I am currently pursuing an undergraduate level course in Operating Systems. I'm somewhat confused about the functions of dispatcher and scheduler in process scheduling. Based on what I've learnt, the medium term scheduler selects the process for swapping out and in , and once the processes are selected, the actual swap operation is performed by Dispatcher by context switching. Also the short term scheduler is responsible for scheduling the processes and allocate them CPU time, based on the scheduling algorithm followed. Please correct me if I'm wrong. I'm really confused about the functions of medium term scheduler vs dispatcher, and differences between Swapping & context switching.
3 Answers
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You describing things in system specific terms.
The scheduler and the dispatcher could be all the same thing. However, the frequently are divided so that the scheduler maintains a queue of processes and the dispatcher handles the actual context switch.
If you divide the scheduler into long term, medium term, and short term, that division (if it exists at all) is specific to the operating system.
Swapping in the process of removing a process from memory. A process can be made non-executable through a context switch but may not be swapped out. Swapping is generally independent of scheduling. However, a process must be swapped in to run and the memory management will try to avoid swapping out executing processes.
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Basically,whatever you have known about scheduler and dispatcher is correct.Sometimes they are referred to as a same unit or scheduler(short time in this case) contains dispatcher as a single unit and together are responsible for allocating a process to CPU for execution.Sometimes they are referred as two separate units,the scheduler selects a process according to some algorithm and the dispatcher is a software that is responsible for actual context switching.
