Skip to content

Process State & Process Queue 👯

Process State 🏃🏻

As process executes, it changes state. Each process may be in one of the following states:

  • New: OS is about to pick the program & convert it into process. Or, the process is being created.

  • Run: Instructions are being executed; CPU is allocated.

  • Waiting: Waiting for IO.

  • Ready: The process is in memory, waiting to be assigned to a processor.

  • Terminated: The process has finished execution. PCB entry removed from process table.

loading...

Process state

  • Long-term scheduler: Selects which processes from a pool of new processes should be brought into the ready queue. It is called long-term because it is invoked infrequently (let's say after 1 second).
  • When a process is in ready state, it is waiting to be assigned to a processor. Short-term scheduler selects a process from the ready queue and assigns it to the processor. It is called short-term because it is invoked very frequently (let's say after every 100 milliseconds).
  • Waiting state is a temporary state. A process is moved to the waiting state if it needs to wait for a resource (like I/O) to become available. For example, if a process is waiting for user input, it will be in the waiting state. When the input is available, the process will be moved to the ready state and the short-term scheduler will pick a process from the ready queue and assign it to the processor.
  • Terminated state is the final state. The process has finished its execution and is removed from the system.
  • Interrupt is a signal to the OS that an event has occurred and needs immediate attention. It can be generated by the hardware or software. For example, an interrupt can be generated by a clock to indicate that a fixed interval of time has passed and the running process needs to be moved to the ready queue so that another process can be assigned to the processor.
Remember
  • Short-term scheduler: CPU Scheduler.
  • Long-term scheduler: Job Scheduler.
What is Dispatcher?
  • It is the module that gives control of the CPU to the process selected by the short-term scheduler. It involves:
    • Switching context.
    • Switching to user mode.
    • Jumping to the proper location in the user program to restart that program.

Dispatch latency is the time taken to stop running process and run another process which was in the ready queue.

Process Queue 🚶🏻‍♂️

  1. Job Queue:

    • Processes in new state.
    • Present in secondary memory.
    • Job Schedular (Long term schedular (LTS)) picks process from the pool and loads them into memory for execution.

  2. Ready Queue:

    • Processes in Ready state.
    • Present in main memory.
    • CPU Schedular (Short-term schedular) picks process from ready queue and dispatch it to CPU.

  3. Waiting Queue:

    • Processes in Wait state.

Degree of multi-programming 📊

  • The number of processes in the memory.
  • The degree of multiprogramming describes the maximum number of processes that a single-processor system can accommodate efficiently.

Who controls degree of multi-programming?

LTS controls degree of multi-programming. Because it decides how many processes are to be loaded into the memory (ready queue) for execution.