Thread Scheduler - SO Homework 2

Copyright 2022 Toaca Alexandra Simona

Implementation

• The basic idea behind the scheduler is that every thread gets its semaphore and it is initially in a waiting state (before the routine is even called).
• This is done by using an auxiliary function (start_thread) which is used in pthread_create. When the thread ends, the next one is started

void *start_thread(void *info)
{
	thread_t *thread = (thread_t *)info;

	plan_thread(thread);

	sem_wait(&thread->run);

	thread->start_routine(thread->priority);

	thread->status = TERMINATED;
	run_next_thread();
	return NULL;
}

• The scheduler plans every thread that enters the program (decides whether to enqueue it or start it immediately) - function plan_thread
• plan_thread signals to a semaphore (placed before the next action is taken) that the thread has been planned
• To start the thread immediately, the one that is currently running is preempted (the semaphore is put again in a waiting state - sem_wait) and the new thread is signaled to start (sem_post)
• Preemption is also done when the time quantum of the current thread expires, in which case the next thread is run (next in the queue)- run_next_thread
• There is a structure which holds queues for the threads that are waiting for a certain event. Every queue coresponds to the id of the event. io_devices[id].waiting_list
• a third type of semaphore is the one that waits for all threads to finish their job before calling pthread_join in so_end()

What I have learned

• Working with threads. New concept, new way of thinking, hard to debug. Before introducing the semaphore which waits for all threads to terminate, pthread join in so_end was ending threads prematurely.
• I read extra on mutexes, condition variables and semaphores. Then decided that semaphores where the right fit for this homework.

Comments

• Debugging when multiple threads are runnning is tedious and difficult
• I believe this homework is good is terms of understading how a scheduler works, but it is hard to do when one has never worked with threads before

How to use

• the makefile generates the dynamic library so_scheduler.so which is then linked with the test program.