OS 6.01 - Process Synchronization

Process synchronization refers to various mechanisms used to ensure the orderly execution of cooperating processes that share a logical address space, ensuring data consistency is maintained.

6.1 Background

• Concurrent vs. Parallel Execution:

  • Concurrent execution occurs when processes share CPU time and are executed in overlapping time periods. While a process may not be executing at every moment, it appears to be progressing due to rapid switching between processes by the CPU scheduler.
  • Parallel execution, on the other hand, involves two or more processes executing simultaneously on separate processing cores.

• Context of Process Execution:

  • When processes execute concurrently or in parallel, one process may only partially complete execution before another process is scheduled.
  • In concurrent execution, a process can be interrupted at any point in its instruction stream, and another process might begin execution immediately, potentially causing issues when both processes manipulate shared data.
  • Parallel execution involves multiple processes running at the same time on different processors or cores, which may lead to similar data integrity issues when accessing shared data.

Problems Arising from Concurrent Execution

• Race Condition:

  • A race condition occurs when multiple processes or threads concurrently access and manipulate shared data, and the outcome depends on the order or timing of the execution of these processes.
  • For example, if two processes simultaneously attempt to increment a shared counter variable, the outcome can vary based on the order in which the increments occur, leading to inconsistent or incorrect results.

  Example:

  • Consider two processes, P1 and P2, both incrementing the shared variable counter:
    • P1 reads the value of counter.
    • P2 also reads the same value of counter before P1 has a chance to update it.
    • P1 increments the counter and writes back the value.
    • P2 increments the old value and writes it back, overwriting P1’s update.

  This results in lost updates or incorrect values, which is a classic race condition.

Importance of Process Synchronization

• Guarding Against Race Conditions:

  • To prevent issues like race conditions, it is necessary to ensure that only one process can manipulate shared data at a time. Process synchronization mechanisms are required to control access to shared resources.

• Goal:

  • Synchronization guarantees that processes are executed in such a way that shared data is protected from conflicting updates, ensuring correctness and data consistency.

The Impact of Multicore Systems

• Multithreading and Data Sharing:

  • The rise of multicore systems has led to an increase in multithreaded applications, where multiple threads run in parallel on different processing cores.
  • Shared Data: In multithreaded programs, multiple threads may access and modify the same shared data, further emphasizing the need for synchronization.
  • Potential for Conflict: Without synchronization, concurrent updates to shared data could result in race conditions, even in parallel execution environments.

• Challenges:

  • Synchronizing processes across multiple cores can be more complex due to the simultaneous execution of threads.
  • Effective synchronization ensures that updates from threads running on different cores do not interfere with each other and guarantees data integrity.