MASON GEORGE UNIVERSITY Computer Science Department CS 571 Operating Systems Fall 2003 Assignment 2 Multithreaded Programming DUE DATE Oct 6
Process Synchronization The goal of this exercise to give you some experience writing concurrent programs. You have been hired by MetroTrans to synchronize trafc over a narrow light-duty bridge on a public highway. Trafc may only cross the bridge in one direction at a time, and if there are ever more than 3 vehicles on the bridge at one time, it will collapse under their weight. In this system, each vehicle is represented by one thread, which executes the procedure OneVehicle when it arrives at the bridge: OneVehicle(int vehicle_id,int direc, int time_to_cross) { ArriveBridge(direc); CrossBridge(vehicle_id,direc,time_to_cross); ExitBridge(vehicle_id,direc); } In the code above, vehicle id is an integer which uniquely identies each vehicle (The vehicles arriving at the bridge should be assigned vehicle ids 1,2,3... and so on ). direc is either 0 or 1; it gives the direction in which the vehicle will cross the bridge. time to cross is the time it takes a vehicle to cross the bridge assume that every vehicle takes 4 seconds to cross the bridge. Implement a fair trafc control policy that imposes a limit (say 5) on the number of vehicles that can cross the bridge in one direction, while vehicles travelling in the opposite direction are waiting to get on the bridge. NOTE: if there are no vehicles waiting to get on the bridge in the opposite direction, your policy should not switch directions. In other words, your policy should be smart enough to provide fairness if there is trafc in both directions but should never force a vehicle to wait to get on the bridge if there is no trafc in the opposite direction. Write the procedures ArriveBridge,CrossBridge and ExitBridge, using mutex locks and condition variables for synchronization1 . The ArriveBridge procedure must not return until it is safe for the car to cross the bridge in the given direction (it must guarantee that there will be no head-on collisions or bridge collapses). The CrossBridge procedure should just delay for time to cross seconds and print out a debug message. ExitBridge is called to indicate that the caller has nished crossing the bridge; ExitBridge should take steps to let additional cars cross the bridge. In addition, ExitBridge should update a shared variable departure index, which keeps track of the order in which the cars leave the bridge, i.e., the rst car to leave the bridge has departure index 1, the second car has departure index 2, and so on. The ExitBridge procedure should also print out the departure index for that vehicle.
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If you use Java for the assignment, use the equivalent thread synchronization faciltities
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NOTES: Your solution must not employ busy waiting. Your solution must use two queues on either end of the bridge where vehicles wait until it is safe for them to enter the bridge. In terms of threads, this means that you need two condition variables (or Java objects) where a thread is suspended if it cannot enter the CrossBridge procedure. For full credit, your solution must not use the pthread cond broadcast() (or function if youre using Java, the notify all() method), i.e. your solution must only use pthread cond signal() or notify() for signalling any threads blocked on a condition variable. NOTE: invoking pthread cond signal() or notify() repeatedly in a loop is equivalent to pthread cond broadcast() and notify all() Occasionally, a vehicle (thread) may overtake another vehicle travelling in the same direction on the bridge. Do not worrry about this. You do not have to ensure that vehicles leave the bridge in the same order as they entered it. The debug message printed out in CrossBridge should provide a snapshot of the bridge and the queues on the two ends of the bridge. It is probably also a good idea to print out debug messages in ArriveBridge and ExitBridge. In this assignment, you have to run your program for the three vehicle arrival schedules given below: (i) 5 : DELAY(10) : 5 : DELAY(10) : 5 : DELAY(10) : 5: DELAY(10) : 5 : DELAY(10) : 5 (ii) 10 : DELAY(10) : 10 : DELAY(10): 10 (iii) 30 Here the numbers indicate the number of vehicles arriving simultaneously at the bridge, while the numbers in parentheses indicate the delay before the next arrival(s). For example, under schedule (i) 5 vehicles arrive simultaneously at the bridge at the start of the experiment, ve more vehicles arrive simultaneously 10 seconds after the arrival of the rst ve vehicles and so on. In each of the three schedules, thirty vehicles arrive at the bridge during the course of the experiment. Note that vehicles arriving simultaneously does not imply that they are all traveling in the same direction. Assume that the probability that a vehicle is traveling in direction 0 is 0.6
NOTES: 1. You can use either Java or the Pthreads multithreaded programming library for doing this assignment. Both Java and the Pthreads library are available on machines in the IT&E lab. 2. On Solaris, information on ho...