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Process Synchronization

Definition

Process synchronization means that in concurrently running processes, when a process runs to a certain position (called synchronization point), the process can continue to run only after other processes have completed certain operations and met the conditions for continuing to run (called synchronization condition); otherwise, it must wait.

Key elements of process synchronization:

  1. Synchronization point: The process waits for certain operations of other processes to complete here to meet the conditions for continuing to execute.

  2. Synchronization condition: The condition that must be met so that the process can continue to execute at the synchronization point.

  3. Function: Through the synchronization mechanism, the execution order of multiple processes can be coordinated to ensure data consistency and correct use of resources, and prevent competition and conflict.

Process synchronization is often implemented through semaphore mechanism, event notification, conditional variables and other methods.

Mutual exclusion problem

The mutual exclusion problem mainly solves the access control problem of multiple concurrent processes to the critical section. In a concurrent system, multiple processes may access shared resources at the same time. If there is no appropriate control mechanism, it may cause race conditions, data inconsistency or resource conflicts.

  • Semaphore solves mutual exclusion problem: Semaphore can be used to solve mutual exclusion problem. The process access to shared resources is controlled by P (wait) and V (release) operations on the semaphore:
    • P operation: Request to enter the critical section. If the semaphore is 0, the process will be blocked until the semaphore is greater than 0.
    • V operation: Exit the critical section, release the semaphore, and notify other processes that they can enter the critical section.

Implementation of mutual exclusion: By setting a semaphore (such as semaphore mutex = 1), it is ensured that only one process can enter the critical section at any time. Other processes can only wait until the semaphore is restored, indicating that they can enter the critical section. This solves the problem of concurrent processes accessing the critical section mutually exclusive.

Process synchronization problem

The process synchronization problem focuses on how multiple concurrent processes coordinate their work to ensure that they are executed in a certain order or rule to achieve a common goal. Unlike mutual exclusion, process synchronization is more about collaboration and coordination between processes to solve the order and dependency between processes.

  • Process synchronization problem in asynchronous environment: In an asynchronous environment, each process is independent and advances at its own unpredictable speed. Coordination is required between processes to complete a common task. For example, the producer-consumer problem, read-write locks, semaphores and other mechanisms can solve the synchronization problem between processes.

  • Definition of process synchronization:

    • Process synchronization: A group of concurrent processes that cooperate with each other, each of which advances at its own independent speed, but they need to work closely together to complete a common task.
    • Between these processes, it is necessary to ensure that they "know" each other's existence and coordinate their work to ensure the correct execution order.

Solution: Use two semaphores to control synchronization:

  • Empty slot semaphore: Indicates the free space in the buffer, and the initial value is the buffer size.

  • Full slot semaphore: Indicates the number of items stored in the buffer, and the initial value is 0.