Week 30

 CST-334 Week 6

This is our sixth week in CST-334, also known as Operating Systems.

Operating Systems: Three Easy Pieces

This week we read chapters 30-32 of our textbook.

Chapter 30: Condition Variables

Chapter 30 discusses condition variables.

Key Takeaways:

  • Threads can use condition variables to check if a condition is true before executing.
  • Condition variables are queues that threads can wait on. Waiting threads can then be woken when the condition variable changes.
  • The function pthread_cond_wait(cond, mutex) adds a thread to the queue, releases the lock, and puts it to sleep. Cond is the condition variable. Mutex is the lock.
  • The function pthread_cond_signal(cond) wakes the next thread in the queue and gives it the lock. Cond is the condition variable.
  • You should use while loops instead of if statements when checking a condition variable. 

Chapter 31: Semaphores

Chapter 31 discusses semaphores.

Key Takeaways:

  • A semaphore is an object that contains an integer. It is used as both a lock and condition variable.
  • The function sem_init(m, 0, x) initializes a semaphore. M is the semaphore. 0 indicates that the semaphore will be shared between threads. X is the starting value of the semaphore.
  • The function sem_wait(m) decrements the semaphore and either returns if the semaphore was 1 or higher, or puts the thread to sleep. M is the semaphore.
  • The function sem_post(m) increments the semaphore and wakes one of the threads waiting on the semaphore. M is the semaphore.
  • When a semaphore is negative, the value is equal to the number of threads waiting on said semaphore.

Chapter 32: Concurrency Bugs

Chapter 32 discusses common concurrency bugs in concurrent programs.

Key Takeaways:

  •  Atomicity-violation bugs happen when a section of code is intended to be atomic is run un-atomically. Locks can be used to prevent them.
  • Order-violation bugs happen when code that is intended to run in a certain order, such as one thread 1 running before thread 2, runs in an intended order, such as thread 2 running before thread 1. Condition variables can be used to prevent them. 
  • A large portion of non-deadlock bugs are atomicity or order violations.
  • Deadlock occurs when threads are waiting for other threads to release locks while the threads themselves are holding the locks being waited on.
  • Four conditions need to be true for Deadlock to occur:
    • Mutual exclusion
    • Hold-and-wait
    • No preemption
    • Circular wait
  • To prevent deadlocks one can:
    • Order lock acquisition to prevent circular waiting.
    • Acquire all locks at once to prevent hold-and-wait.
    • Preempt locks.
    • Eliminate the need for mutual exclusion.
  • Deadlocks can be avoided by scheduling threads so that no threads require the same locks at the same time. 

 

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