[2019-1 OS] Pintos project1

Pintos is a simple operating system framework for the x86 architecture. It supports kernel threads, loading and running user programmes, and a file system, but it implements all of these in a very simple way.

In first project assignment, the goal is to reimplement functions related to threads and timer. The function I have to extend is the timer_sleep() defined in devices/timer.c.

The original version is working.

1void
2timer_sleep (int64_t ticks)
3{
4  int64_t start = timer_ticks ();
5
6  ASSERT (intr_get_level () == INTR_ON);
7  while (timer_elapsed (start) < ticks)
8    thread_yield ();
9}

 1void thread_yield(void) {
 2  struct thread *cur = thread_current ();
 3  enum intr_level old_level;
 4
 5  ASSERT (!intr_context ());
 6
 7  old_level = intr_disable ();
 8  if (cur != idle_thread)
 9    list_push_back (&ready_list, &cur->elem);
10  cur->status = THREAD_READY;
11  schedule ();
12  intr_set_level (old_level);
13}

But it keep checking the current time and calling thread_yield() because it implemented using while loop. This called “Busy waiting”. And also, thread_yield() place the current thread at the back of the ready queue, not the waiting queue.

To solve this problem, block the current thread with thread_block() funtion.

1void thread_block (void) {
2  ASSERT (!intr_context ());
3  ASSERT (intr_get_level () == INTR_OFF);
4
5  thread_current ()->status = THREAD_BLOCKED;
6  schedule ();
7}

This function makes the current thread to sleep. It means, the thread will not be scheduled again until awoken by thread_unblock().

We need to make a waiting queue since Pintos doesn’t have a waiting queue. And then put the current thread thread to the waiting queue. Next block that thread. It won’t be make ‘busy waiting’ after this three step process.