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Process Control

Process life cycle

Generate, run, pause, terminate

Process control

The operating system manages the state transition of the process through process control primitives. Common process control operations include:

  1. Process creation:

    • Create a new process, allocate necessary resources to it, and initialize PCB.

    • After creation, the process enters the ready state, waiting for the scheduler to allocate CPU time.

  2. Process cancellation:

    • Cancel (terminate) the process, reclaim the resources occupied by the process, and remove its PCB from the system.

    • Termination reasons may include normal program termination, user active termination, abnormal errors, etc.

  3. Process blocking:

    • During the running process, if the process needs to wait for an event (such as I/O completion, signal arrival), it enters the blocking state.

    • After the process is blocked, the CPU will be released to allow other processes to execute.

  4. Process wakeup:

    • When the event that the blocked process is waiting for occurs, the process is awakened and enters the ready state.
    • The wake-up operation is usually triggered by the operating system or other processes.

  5. Change process priority number:

    • Adjust the priority of the process to change the priority of the process when scheduling.

    • A process with a higher priority will be scheduled for execution earlier than a process with a lower priority.

  6. Process scheduling:

    • Select a process from the ready queue, switch it to the running state, and start execution.

    • The scheduler selects the appropriate process based on the scheduling algorithm (such as first-come-first-served, priority scheduling, time slice rotation, etc.).

Four process control primitives

In the operating system, the four commonly used process control primitives include:

1. Process blocking (Block)

  • Function: Convert a process in the running state to the blocked state.

  • Trigger condition: When a process performs I/O operations and waits for an event (such as a semaphore), it needs to enter the blocked state.

2. Process wakeup (Wakeup)

  • Function: Convert a process in the blocked state to the ready state.
  • Trigger condition: When the waiting event occurs (such as I/O completion, resource availability), the process is awakened and re-enters the ready state.

3. Process creation (Create)

  • Function: Create a new process and set it to the ready state.

  • Trigger condition: When the system or user needs to create a new task, the process creation primitive is called.

4. Process scheduling (Dispatch/Schedule)

  • Function: Switch the process in the ready state to the running state.

  • Trigger condition: When the CPU is idle or the time slice ends, the scheduler selects a process from the ready queue to execute.