CS170 P2: Pizza Synchronization
Intro
For our pizza sharing implementation, we implemented a FIFO-like scheduling algorithm to ensure no starvation. Because the "eating" action is expected to last approximately the same time for each student, we are able to use FIFO, expecting a maximum delay of EAT_TIME each round (not including semaphore delays).
Algorithm
This implementation essentially uses a producer-consumer model to replicate FIFO behavior. To do this, we used the 5 semaphores described in Figure 1 below:
| sems index | description |
|---|---|
| 0 | Mutex for table 1, initialized to 1 |
| 1 | Mutex for table 2, initialized to 1 |
| 2 | Semaphore for undergrad student 1, initialized to 0 |
| 3 | Semaphore for undergrad student 2, initialized to 0 |
| 4 | Semaphore for graduate students, initialized to 6 |
Figure 1: Semaphores in sems array |
At the beginning, the table mutexes are initialized to 1, allowing for one student at each table. Each student is assigned to a table based on his/her student number. The grad students get to eat first.
First, the graduate student decrements the graduate student semaphore by 1 and waits to be seated at a table. Every time a grad student eats, he/she increments both ugrad students' semaphores by 1 before releasing the table mutex. Finally, the grad student goes back to waiting for the grad semaphore to be incremented.
Meanwhile, the ugrad students decrement their respective semaphores once for each grad student (6 times). Now it is the ugrads' turn for pizza. Once they finish eating, they restore the grad student semaphore to 6. They synchronize by incrementing each others' semaphore and decrementing their own to ensure both are finished eating. Finally, they release their tables and go back to waiting.
While there are opportunities for graduate students to steal each others' food, the semaphore waiting queue ensures that those who miss out get pizza first the next round.
Results
In our tests, we found that the students were allocated pizza quite fairly. In a couple unscientific test runs, the all students hovered around 1800-1900 slices of pizza in 10 seconds.