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Difference Between Dispatcher and Scheduler

Dispatcher Vs. Scheduler: Know the Difference Between Dispatcher and Scheduler

Both of these are used in CPU process execution, but there is a fundamental difference between dispatcher and scheduler. A dispatcher refers to a module that provides the control of the CPU to that process that gets selected by the short term-scheduler. A scheduler is something that helps in selecting a process out of various available processes. Let us know a bit more about both of these.

What is a Dispatcher?

It is a special type of program that comes into play only after the scheduler. Once a scheduler completes selecting its processes, the dispatcher then takes that intended process to its desired queue/ state. A dispatcher is basically a module that provides a process with total control over the CPU (after the short-term scheduler finally selects it). This function requires the following:

  • Context switching
  • Switching to the user mode
  • Jumping into the user program’s proper location for restarting that given program.

What is a Scheduler?

A scheduler is basically a system of software. It is capable of handling the scheduling of processes in multiple ways. The main task of a Scheduler is to select the jobs that we need to submit into a system and then decide the process that we want to run. The Schedulers are mainly of three types:

  • Long-term Scheduler (Job) – Initially, we store all the programs in the secondary memory because the main memory is smaller in size. When these get loaded or stored in the main memory, then they are known as processes. Here, a long-term scheduler gets to decide how many processes must stay in the given ready queue. In simpler words, a long-term scheduler gets to decide the degree of multiprogramming available in a system.
  • Medium-term Scheduler – A running process, most of the time, requires an I/O operation that requires no CPU. Thus, when we require an I/O operation during the execution of a process, the OS (operating system) sends that intended process to the blocked queue out of the running queue. Once a process completes its first I/O operation, then we must shift it again to the ready queue. A medium-term scheduler takes care of all such kinds of decisions. The process of medium-term scheduling is basically a part of swapping.
  • Short-term Scheduler (CPU) – Initially, all the processes are present in the ready queue whenever there are plenty of them in the main memory. However, only a single process needs to be selected among all of these processes for execution. A short-term scheduler takes care of all such kinds of decisions.

Let us talk about the differences present between Dispatcher and Scheduler.

Difference Between Dispatcher and Scheduler

Let us consider a situation- all the processes reside in the ready queue, and they wait for execution. Here, the CPU cannot perform a simultaneous execution of all of these processes. Thus, the OS needs to choose one particular process (for execution) based on the scheduling algorithm used. In this case, the scheduler selects a process out of the various processes.

Once a scheduler selects a process out of the available queue, then the dispatcher comes into play. This dispatcher takes the process out of the ready queue and then finally moves it into the running state. In simpler words, a scheduler provides a dispatcher with an ordered list of various processes that the dispatcher moves to the CPU over time.

Parameters Dispatcher Scheduler
Meaning and Definition A dispatcher refers to a module that provides the control of the CPU to that process that gets selected by the short term-scheduler. A scheduler is something that helps in selecting a process out of various available processes.
Dependency The overall working of a dispatcher depends entirely on the scheduler. It means that the dispatcher has to wait unless and until a scheduler performs the selection of processes. The overall working of a scheduler occurs independently. It happens pretty immediately as and when required.
Types The dispatcher has no major types. It is a type of code segment. There are basically three major types of a scheduler- short-term, medium-term, and long-term.
Time Taken Dispatch latency refers to the time that is taken by a dispatcher. The overall time taken by a typical scheduler is very negligible. Thus, we can neglect it.
Algorithm There’s no specific algorithm for the implementation of a Dispatcher. A typical scheduler can work on various types of algorithms, like RR, SJF, FCFS, etc.
Functions A dispatcher is responsible for the Switching of Context, Switching to the user mode, Jumping into the user program’s proper location for restarting that given program. Scheduler’s only work is to perform the selection of various processes.

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