Week 29

 CST-334 Week 5

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

Operating Systems: Three Easy Pieces

This week we read chapters 26-29 in our textbook.

Chapter 26: Concurrency and Threads

This chapter discusses concurrency and the basics of threads.

Key Takeaways:

  • A multi-threaded program has multiple points of execution, called threads.
  • Threads share the same code and heap, but have different stacks.
  • Threads are used to utilize parallelism or keep a program running during I/O requests.
  • Multi-threaded programs can be indeterminate.
  • Race conditions, also called a data races, are outputs or variables that change based on the timing of the code's execution.
  • Critical sections are pieces of code that access shared variables.
  • Atomic instructions are executed completely or not at all. 

Chapter 27: Thread API

This chapter discusses functions for creating and using threads.

Key Takeaways:

  • The function pthread_create(thread, attr, start_routine, arg) is used to create threads. Thread is a pointer to a pthread_t variable. Attr specifies attributes the thread might have. Start_routine is a pointer to a function. Arg is a pointer to arguments for the function pointed to by start_routine.
  • The function pthread_join(thread, value_ptr) returns when a thread completes. Thread is the thread to wait for. Value_ptr is the expected return value.
  • The function pthread_mutex_lock(mutex) acquires a lock, or waits for a lock to become available and then acquires it. Mutex is the lock to be acquired.
  • The function pthread_mutex_unlock(mutex) frees a lock. Mutex is the lock to be freed.
  • A lock can be initialized by:
    • Setting the lock to PTHREAD_MUTEX_INITIALIZER.
    • Calling pthread_mutex_init(mutex, NULL). Mutex is the lock to be initialized. NULL is any attributes the lock may have.
  • Condition variables are used to signal values between threads.
  • The function pthread_cond_wait(cond, mutex) puts the calling thread to sleep and waits for another thread to signal the condition. Cond is the condition to wait for.
  • The function pthread_cond_signal(cond) signals a condition. Cond is the condition to be signaled.
  • To use threads you have to include the pthread.h header and add -pthread when compiling. 

Chapter 28: Locks

This chapter discusses locks.

Key Takeaways:

  • Mutex is a common name for locks.
  • Mutual exclusion, fairness, and performance are used to judge the efficacy of a lock.

Chapter 29: Locked Data Structures

This chapter discusses how to make data structures thread safe.

Key Takeaways:

  • An approximate counter is sometimes used to improve multi-threaded performance. In an approximate counter, each core has a local version of a variable, and the local variables are occasionally synced with the global version of the variable.
  • An approximate counter becomes slower as it becomes more accurate, and vice-versa. 
  • Hand-over-hand locking, or lock coupling, uses multiple locks for different parts of a linked list. 

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