Difference between Dispatcher and Scheduler

In a multitasking operating system, multiple processes compete for CPU time. To manage this efficiently, the operating system uses two important components: the Scheduler and the Dispatcher.

Note: Both work together to decide which process will run and how it will be executed by the CPU. Although they are interrelated, they perform distinct functions to ensure optimal CPU utilization and smooth process management.

Scheduler

There are three main types of schedulers, each performing different tasks in process management:

1. Long-Term (Job) Scheduler

• Function: Moves processes from secondary memory (job pool) to main memory (ready queue).
• It decides which programs should be admitted into the system for processing.
• Controls the degree of multiprogramming (number of processes in memory).
• Example: Deciding whether a newly submitted process should be loaded into main memory or kept in secondary storage.

2. Medium-Term Scheduler

• Function: Suspends and resumes processes based on system status.
• Manages swapping - temporarily removes processes from main memory to reduce load.
• Reintroduces suspended processes back into memory when resources are available.
• Helps maintain a balanced level of multiprogramming.

3. Short-Term (CPU) Scheduler

1. Function: Dispatches ready processes to the CPU for execution.
2. Selects one of the ready processes in memory to execute next.
3. Runs most frequently and decides which process gets CPU next.
4. Works with algorithms such as FCFS, SJF, Round Robin and Priority Scheduling.

Advantages of Schedulers

• Optimized CPU Utilization: Ensures the CPU is always busy with process execution.
• Fair Process Handling: Provides equitable CPU time to each process.
• Better Process Management: Manages transitions among process states like ready, running or waiting.

Disadvantages of Schedulers

• Complexity: Can be difficult to design and tune for different workloads.
• Overhead: Frequent scheduling decisions may consume CPU cycles, slightly reducing performance

Dispatcher

Once the Scheduler selects the next process to execute, the Dispatcher takes over. The Dispatcher is a small, specialized program that gives control of the CPU to the process chosen by the short-term scheduler. It performs the low-level work needed to actually start executing the selected process..

Functions of Dispatcher

1. Context Switching: Saves the current process state and loads the next process’s state.
2. Switching to User Mode: Ensures that the process runs in user mode, maintaining security and isolation.
3. Jumping to the Correct Program Location: Resumes the process from the point where it was previously stopped.

Advantages of Dispatcher

• Fast Process Switching: Reduces delay while switching processes.
• Efficient CPU Allocation: Enables smooth multitasking by assigning CPU time quickly.

Disadvantages of Dispatcher

• Dispatch Latency: The small delay that occurs during context switching and control transfer.
• Scheduler Dependency: The dispatcher cannot work independently; it relies entirely on the scheduler’s selection

Difference Between Scheduler and Dispatcher

Note: The scheduler is the one that determines what processes are executed and when, while the dispatcher is the one that swaps out those processes into the CPU.