Comparing Java and Ada Monitors queuing policies: a case study

Java deadlock free implementation

1. How to run the Java deadlock free implementation

The main program is diner.adb
Once compiled (with GNAT for example), just run ./diner

-- JAVA implementation simulated with an Ada protected object
-- simulates Java concurrency semantics
-- the Java wait operation is here an entry call with a barrier
-- one Java synchronized method simulated by Get_LR and requeue Get_R
-- allocates left stick first if it is available and not booked and then also right stick if available. If right is not available, it is booked. 
-- waits if one or both sticks have not been allocated

-- ********    reliable, deadlock free


2. A new solution for a well known paradigm, the dining philosophers

The dining philosophers, originally posed by Dijkstra [Dijkstra 7I], is a paradigm for concurrent resource allocation. Five philosophers spend their life alternately thinking and eating. To dine, each philosopher sits around a circular table at a fixed place. In front of each philosopher is a plate of food, and between each pair of philosophers is a chopstick. In order to eat, a philosopher needs two chopsticks, and they agree that each will use only the chopsticks immediately to the left and to the right of his place. The problem is to write a program simulating the philosopher’s behaviours and to devise a protocol that avoids two unfortunate conclusions. In the first one, all philosophers are hungry but none is able to acquire both chopsticks since each holds one chopstick and refuses to give it up. This is deadlock, a safety concern. In the second one, a hungry philosopher will always lack one of the two chopsticks which are alternately used by its neighbours. This is starvation, a liveness consideration.
This paradigm has two well known approaches for obtaining a solution. In the first one, the chopsticks are allocated one by one, and a reliable solution is obtained by adding one of the usual constraints for deadlock prevention: the chopsticks are allocated in fixed (e.g., increasing) order; a chopstick allocation is denied as soon as the requested allocation would lead to an unsafe state (seated dinner, with only 4 chairs). Ada implementation of this approach can be found [Burns 1995, Barkaoui 1997]. In the second one, the chopsticks are allocated globally only, which is a safe solution; when a fair solution is necessary, it is obtained by adding reservation constraints, care being taken that these constraints do not reintroduce deadlock. Ada implementation are given in [Brosgol 1996, Kaiser 1997]
Let us consider now another approach which does not seem to have been much experimented except in [Kaiser 1997]. The chopsticks are allocated as many as available and the allocation is completed as soon as the missing chopsticks are released. Let us observe the behaviour of this solution when implemented in Java and in Ada and from these experiments, let us determine the conditions of its correctness.


3. Java reliable implementation

3.1. The Java class

A modified Java implementation leads to the following Chop class with get_LR and release methods.
This implementation is reliable.

public final class Chop {

    private int N ;
    private boolean available [ ] ;
    private boolean booked [ ] ;

    Chop (int N) {
        this.N = N ;
        this.available = new boolean[N] ;
        for (int i =0 ; i < N ; i++) {
            available[i] = true ; // non allocated stick
            booked[i] = false;
        }
    }

    public synchronized void get_LR (int me) {
        int score = 0 ; // useful when simulating the Java solution in Ada
        while (!available [me]  ||  booked[me]) {
            try { wait() ; } catch (InterruptedException e) {}
        }
        available [me] = false ; score = 1; // left stick allocated
        booked[(me + 1)% N]  == true; // right stick booked
        // don’t release mutual exclusion lock and immediately requests second stick

        while (!available [(me + 1)% N]) {
            try { wait() ; } catch (InterruptedException e) {}
        }
        available [(me + 1)% N]  = false ; score = 2; // both sticks allocated
        booked[(me + 1)% N]  == false; // no more reason for booking right stick
    }

    public synchronized void release (int me) {
        available [me] = true ; available [(me + 1)% N]  = true ;
        notifyAll ;
    }
}

3.2. Ada simulation of the Java deadlock free implementation

    Transcribing this Java implementation in an Ada program with the eggshell semantics requires an entry for each waiting condition.


The solution is in chop.adb

The solution is  instrumented for measuring a concurrency denials ratio.


4. The full paper : Comparing Java and Ada Monitors queuing policies: a case study