Lecture 13: Scheduling



Alex Melnik:

I worked all alone
Just me and my mac
and some bits of poems

uthread from last lecture

threads in xv6


Exercise: Barriers

In this assignment we will explore how to implement a barrier using condition variables provided by the pthread library. A barrier is a point in an application at which all threads must wait until all other threads reach that point too. Condition variables are a sequence coordination technique similar to xv6’s sleep and wakeup.

Download barrier.c and compile it on your laptop or attu:

$ gcc -g -O2 -pthread barrier.c
$ ./a.out 2
Assertion failed: (i == t), function thread, file barrier.c, line 55.

The 2 specifies the number of threads that synchronize on the barrier (nthread in barrier.c). Each thread sits in a tight loop. In each loop iteration a thread calls barrier() and then sleeps for some random number of microseconds. The assert triggers, because one thread leaves the barrier before the other thread has reached the barrier. The desired behavior is that all threads should block until nthreads have called barrier.

Your goal is to achieve the desired behavior. In addition to the lock primitives that you have seen before, you will need the following new pthread primitives (see man pthread for more detail):

pthread_cond_wait(&cond, &mutex);  /* go to sleep on cond, releasing lock mutex */
pthread_cond_broadcast(&cond);     /* wake up every thread sleeping on cond */

pthread_cond_wait releases the mutex when called, and re-acquires the mutex before returning.

We have given you barrier_init(). Your job is to implement barrier() so that the panic won’t occur. We’ve defined struct barrier for you; its fields are for your use.

There are two issues that complicate your task:

Test your code with one, two, and more than two threads.