Process Management (computing) - Three-state Process Management Model

Three-state Process Management Model

Although the two-state process management model is a perfectly valid design for an operating system, the absence of a BLOCKED state means that the processor lies idle when the active process changes from CPU cycles to I/O cycles. This design does not make efficient use of the processor. The three-state process management model is designed to overcome this problem, by introducing a new state called the BLOCKED state. This state describes any process which is waiting for an I/O event to take place. In this case, an I/O event can mean the use of some device or a signal from another process. The three states in this model are:

• RUNNING: The process that is currently being executed.
• READY: A process that is queuing and prepared to execute when given the opportunity.
• BLOCKED: A process that cannot execute until some event occurs, such as the completion of an I/O operation.

At any instant, a process is in one and only one of the three states. For a single processor computer, only one process can be in the RUNNING state at any one instant. There can be many processes in the READY and BLOCKED states, and each of these states will have an associated queue for processes.

Processes entering the system must go initially into the READY state, processes can only enter the RUNNING state via the READY state. Processes normally leave the system from the RUNNING state. For each of the three states, the process occupies space in main memory. While the reason for most transitions from one state to another might be obvious, some may not be so clear.

• RUNNING → READY The most common reason for this transition is that the running process has reached the maximum allowable time for uninterrupted execution; i.e. time-out occurs. Other reasons can be the imposition of priority levels as determined by the scheduling policy used for the Low Level Scheduler, and the arrival of a higher priority process into the READY state.

• RUNNING → BLOCKED A process is put into the BLOCKED state if it requests something for which it must wait. A request to the OS is usually in the form of a system call, (i.e. a call from the running process to a function that is part of the OS code). For example, requesting a file from disk or a saving a section of code or data from memory to a file on disk.