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Types of Processor Scheduling

Processor Scheduling

• The aim of processor scheduling is to assign processes to be executed by the processor (or processors) over time, in a way that meets system objectives, such as response time, throughput, and processor efficiency.

• Processor scheduling activity is broken down into three separate functions. The names suggest the relative time scales with which these functions are performed.

  • Long-term scheduling

    The decision to add to the pool of processes to be executed.

  • Medium-term scheduling

    The decision to add to the number of processes that are partially or fully in main memory.

  • Short-term scheduling

    The decision as to which available process will be executed by the processor.

  [!] Note: I/O scheduling - The decision as to which process's pending I/O request shall be handled by an available I/O devices.

Long-Term Scheduling

• Determines when and which programs are admitted to the system for processing.

  • When to create a new process is generally driven by the desired degree of multiprogramming.

  • Which job to admit next can be on a simple FCFS basis, or it can be a tool to manage system performance.

• Controls the degree of multiprogramming.

  • The more processes that are created, the smaller the percentage of time that each process can be executed.

  • May limit the degree of multiprogramming to provide satisfactory service to the current set of processes.

• Executes least frequently and makes the coarse-grained decision of whether or not to take on a new process.

Medium-Term Scheduling

• Part of the swapping (in/out of memory) function. (Suspend / Activate)

• Typically, the swapping-in decision is based on the need to manage the degree of multiprogramming.

  On a system that does not use virtual memory, memory management is also an issue. Thus, the swapping-in decision will consider the memory requirements of the swapped-out processes.

• Executes somewhat more frequently than the long-term scheduling to make a swapping decision.

Short-Term Scheduling

• Known as the dispatch carried out by the dispatcher.

• Invoked whenever an event occurs that may lead to the blocking of the current process, or that may provide an opportunity to preempt a currently running process in favor of another. Examples of such events include:

  • Clock interrupts

  • I/O interrupts

  • Operating system calls

  • Signals (e.g., semaphores)

• Executes most frequently and makes the fine-grained decision of which process to execute next. (It chooses from the ready queue.)

References

Stallings, W. (2018). Operating Systems: Internals and Design Principles (9th ed.). Pearson Education, Inc.