Process States and State Transitions

Process States and State Transitions

Description
A process is the fundamental unit for resource allocation and scheduling in an operating system. Throughout its lifecycle, a process undergoes various state changes, such as creation, ready, running, blocked, and termination. Understanding these states and their transition conditions is essential for mastering process management. Common states include Ready, Running, Blocked (also known as Waiting), along with two special states: New (Creation) and Terminated.

Detailed States and Transition Process

  1. New State

    • Description: The process has just been created but has not yet been fully loaded into memory by the operating system. For example, after a user double-clicks a program icon, the system allocates a Process Control Block (PCB) for it, but resources are not yet ready.
    • Transition Conditions:
      • New → Ready: When the operating system completes resource allocation (e.g., memory space), the process enters the ready queue to wait for scheduling.

  2. Ready State

    • Description: The process has acquired all necessary resources except for the CPU. It can run immediately once selected by the scheduler. Multiple ready processes queue in the ready queue.
    • Transition Conditions:
      • Ready → Running: The scheduler allocates a CPU time slice according to an algorithm (e.g., round-robin), and the process begins execution.
      • Running → Ready: Common in two scenarios:
        • Time slice exhausted: The process has not finished in the Running state but its CPU time slice is used up, yielding the CPU to another process.
        • Higher-priority process becomes ready: If the system supports preemptive scheduling, a higher-priority process entering the Ready state may preempt the CPU from the currently running process.

  3. Running State

    • Description: The process is executing instructions and occupying CPU resources. Only one process can be in the Running state at any given moment on a single-core CPU.
    • Transition Conditions:
      • Running → Blocked: The process voluntarily relinquishes the CPU because it needs to wait for an external event, for example:
        • Initiating an I/O request (e.g., reading disk data);
        • Waiting for user input or a semaphore;
        • Requesting a resource that is not available (e.g., failing to allocate memory).
      • Running → Terminated: The process finishes execution or is forcibly terminated (e.g., triggering an exception or being killed).

  4. Blocked State

    • Description: The process is paused while waiting for an event (e.g., I/O completion). Even if the CPU were allocated, it could not run. Blocked processes are moved to corresponding wait queues.
    • Transition Conditions:
      • Blocked → Ready: The waited-for event has occurred (e.g., disk data is ready), and the operating system places it back into the ready queue.
        Note: A Blocked state cannot transition directly to Running; it must first return to the Ready state and wait for scheduling.

  5. Terminated State

    • Description: The process has finished execution or exited abnormally. The system reclaims its resources (e.g., memory, open files) but retains the PCB record with the exit status for the parent process to query.
    • Transition Conditions: No further transitions; the PCB is eventually cleared by the system.

Key Points to Emphasize

  • Activeness of State Transitions:
    • Running → Blocked is typically an active behavior of the process (e.g., calling sleep());
    • Running → Ready is typically a passive behavior (forced switch by the scheduler).
  • Unidirectionality: The Blocked state must pass through the Ready state to return to Running, preventing starvation (ensuring fair scheduling).
  • Extensions in Real Systems: For example, Linux might subdivide the Blocked state into interruptible/uninterruptible sleep, but the core logic remains unchanged.

Example Illustration
Assume process A needs to read a file while running:

  1. A is in the Running state and issues a read() system call;
  2. The kernel sets it to the Blocked state and adds it to the disk wait queue;
  3. The scheduler selects process B from the ready queue to run;
  4. After the disk I/O completes, an interrupt is triggered, and A is moved back to the Ready state;
  5. A may re-enter the Running state during the next scheduling, resuming execution.

By understanding state transitions, one can deeply analyze process concurrency behaviors, scheduling efficiency, deadlock prevention, and related issues.