Concurrency

1. Pizza Group Problem

A group of students are studying for CS 4532 exam. The students can study only while eating pizza. Each student executes the following loop:

while (true) {
    pick up a slice of pizza; 
    study while eating the pizza
}

If a student finds that the pizza is gone, the student goes to sleep until another pizza arrives. The first student to discover that the group is out of pizza calls Kamal’s Pizza to order another pizza before going to sleep. Each pizza has s slices. Once Kamal delivers pizza, he wake up all the students in the group. Then the students pick up a slice of pizza and go back to studying, and the process continues. Write code to synchronize the student threads and the Kamal’s pizza delivery thread. Your solution should avoid deadlock and call Kamal’s Pizza (i.e., wake up the delivery thread) exactly once each time a pizza is exhausted. No slice/piece of pizza may be consumed by more than one student. Comment your code.

Solution Using mutex, semaphore and conditional variables

see pizza.js

Technique	Reason
Set initial value of Semaphore kamal to 0, kamal.down() at the top of pizzaKamal()	Kamal will not deliver pizza unless there's an order
Mutex for the m, m.lock() before accessing variable 'slices'	For mutual Exclusion
Kamal.up only if slices == 0	call Kamal's Pizza exactly once each time a pizza is exhausted
Check for number of slices and call Kamal before waiting	To avoid deadlock (the last the student to discover that the group is out of pizza sleeps without calling Kamal)
Use broadcast instead of singnal() s number of students.	To avoid starvation (if number of students in the group > s)

2. Kottu Problem

Kamal and Rani work in a restaurant that specialises in a made-to-order Koththu Roti and Curry combination. Kamal makes 2 rounds of Koththu Roti while, at the same time, Rani makes 2 bowls of Curry. When the Koththu Roti and Curry are ready Kamal and Rani each, at the same time, serve a Koththu Roti and Curry combination to a customer(s). These actions are repeated throughout the day. Note that Kamal cannot serve until Rani has made the curry and Rani cannot serve until Kamal has made the Koththu Roti. Write a concurrent program that demonstrates the behavior of Kamal and Rani while satisfying the given constraints.

Solution Using semaphores

see kottu.js

2. Tickets Purchase Problem

Tickets for a concert are to be sold through a web site. Only a limited number of tickets are available and multiple fans may try to purchase them at the same time. Each fan can purchase up to 4 tickets at a time. Write a server-side script to keep track of the remaining number of tickets and income earned from tickets. No need to keep track of who purchased tickets.

Solution Using Mutex

see tickets.js