Question

I’m having some trouble writing Pseduocode for a homework assignment in my operating systems class in which we are programming in C.

You will be implementing a Producer-Consumer program with a bounded buffer queue of N elements, P producer threads and C consumer threads
(N, P and C should be command line arguments to your program, along with three additional parameters, X, Ptime and Ctime, that are described below). Each
Producer thread should Enqueue X different numbers onto the queue (spin-waiting for Ptime*100,000 cycles in between each call to Enqueue). Each Consumer thread
should Dequeue P*X/C items from the queue (spin-waiting for Ctime*100,000 cycles
in between each call to Dequeue). The main program should create/initialize the
Bounded Buffer Queue, print a timestamp, spawn off C consumer threads & P
producer threads, wait for all of the threads to finish and then print off another
timestamp & the duration of execution.

My main difficulty is understanding what my professor means by spin-waiting for the variables times 100,000. I have bolded the section that is confusing me.

I understand a time stamp will be used to print the difference between each thread. We are using semaphores and implementing synchronization at the moment. Any suggestions on the above queries would be much appreciated.

Answer 1

I’m guessing it means busy-waiting; repeatedly checking the loop condition and consuming unnecessary CPU power in a tight loop:

while (current_time() <= wake_up_time);

One would ideally use something that suspends your thread until it’s woken up externally, by the scheduler (so resources such as the CPU can be diverted elsewhere):

sleep(2 * 60 * 1000 ms);

or at least give up some CPU (i.e. not be so tight):

while (current_time() <= wake_up_time)
    sleep(100 ms);

But I guess they don’t want you to manually invoke the scheduler, hinting the OS (or your threading library) that it’s a good time to make a context switch.

I’m not sure what cycles are; in assembly they might be CPU cycles but given that your question is tagged C, I’ll bet that they’re simply loop iterations:

for (int i=0; i<Ptime*100000; ++i); //spin-wait for Ptime*100,000 cycles

Though it’s always safest to ask whoever issued the homework.