Scheduler

Types of Queues for Process Scheduling (3 types)

• Job Queue

  • The set of all processes currently in the system

• Ready Queue

  • The set of processes currently in memory waiting to acquire the CPU for execution

• Device Queue

  • The set of processes waiting for Device I/O operations

Types of Schedulers

Long-Term Scheduler (Job scheduler)

Memory is limited, and when many processes try to load into memory at once, they are temporarily stored in large-capacity memory (typically disk)

→ Its role is to determine which processes from this pool should be allocated memory and sent to the ready queue

• Handles scheduling between memory and disk

• Allocates memory and various resources to processes (admit)

• Controls the number of running processes

• Changes a process's state from new → ready (in memory)

• Note)

  • Having too many or too few programs loaded in memory both result in poor performance.

  • For reference, in time sharing systems, there is no long-term scheduler

    • Processes are immediately loaded into memory and become ready

Short-Term Scheduler (CPU scheduler)

• Handles scheduling between CPU and memory

• Determines which process from the Ready Queue to run

• Allocates CPU to a process

  • scheduler dispatch

• Changes a process's state: ready → running → waiting → ready

Medium-Term Scheduler

Swapping!

• Evicts entire processes from memory to disk to free up space (swapping)

• Deallocates memory from processes

  • Releases memory allocation from a process!!

• A scheduler that controls too many programs being loaded into memory simultaneously in the current system

• Changes a process's state from ready → suspended

  • Process state - What is suspended?

    • A state where process execution is halted for external reasons, meaning the process has been removed from memory

    • The entire process is swapped out to disk

    • A Blocked state is waiting for other I/O operations, so it can return to ready on its own,

      • but this state is suspended for external reasons, so it cannot return on its own