Lab assignment 1: Synchronization Operating Systems ECE344 Lab assignment 1: Synchronization Question: java thread? Ding Yuan

Overview of the lab assignment Task 1: implementing synchronization primitives 1a: implement lock 1b: implement condition variable Task 2: use synchronization primitives to solve problems 2a: Mice and cats 2b: traffic lights Ding Yuan, ECE344 Operating System

Ding Yuan, ECE344 Operating System Lock and cond. var. P(sem) { Disable interrupts; while (sem->count == 0) { thread_sleep(sem); /* current thread will sleep on this sem */ } sem->count--; Enable interrupts; V(sem) { Disable interrupts; sem->count++; thread_wakeup (sem); /* this will wake up all the threads waiting on this sem. Why wake up all threads? */ Enable interrupts; } why wake up all threads: this is to make the threads runnable. Then it’s up to the scheduler to pick one. all these thread will “return” from thread_sleep. only one can pass P. --- this is also why we need a while (sem->count != 0) instead of “if (sem->count != 0)”. sleep with interrupts disabled: thread sleep will eventually call mips_switch to swtich to a new thread. when that thread is waken up, the first thing is to enable interrupt. Needs atomic region Atomic region can be done in a similar way to semaphore If you understand how semaphore is implemented, should be trivial! Cannot use semaphore to implement lock or cond. var. Ding Yuan, ECE344 Operating System

Synchronization problems How to start? First: write operation code Next: carefully reason about all the possible interleaving and timing scenarios Add synchronization Ding Yuan, ECE344 Operating System

Ding Yuan, ECE344 Operating System Mice and cats Two bowls, multiple cats and mice Safety criteria: If a cat is eating at either dishes, no mouse can eat If a mouse is eating at either dishes, no cat can eat Ding Yuan, ECE344 Operating System

Ding Yuan, ECE344 Operating System Operation code void sem_eat(const char *who, int num, int bowl, int iteration) { kprintf("%s: %d starts eating: bowl %d, iteration %d\n", who, num, bowl, iteration); clocksleep(1); kprintf("%s: %d ends eating: bowl %d, iteration %d\n", who, num, } void mousesem(void * p, unsigned long mousenumber) { int bowl, iteration; for (iteration = 0; iteration < 4; iteration++) { sem_eat (”mouse", mousenumber, bowl, iteration); int catmousesem(.. ..) { for (index = 0; index < NCATS; index++) thread_fork("catsem Thread”, NULL, index, catsem, NULL); for (index = 0; index < NMICE; index++) thread_fork(”mousesem Thread”, NULL, index, mousesem, NULL); Ding Yuan, ECE344 Operating System

Ding Yuan, ECE344 Operating System About starvation You do not need to consider priority or starvation e.g., mice can prevent cat from eating Since cats/mice will eventually finish eating, won’t starve forever Ding Yuan, ECE344 Operating System