Comparing Java and Ada Monitors
queuing policies: a case study
Ada
implementation of the global allocation solution
1. How to run the Ada implementation of the
global allocation solution
The main program is diner.adb
Once compiled (with GNAT for example), just run ./diner
-- dining philosophers paradigm.
-- Solution implementing
global allocation of sticks
-- The chopsticks are allocated only when both are available
-- ADA solution using the
eggshell semantic of Ada protected
object
--an entry families
-- *************** reliable, no deadlock, not fair, starvation
prone
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. Ada global allocation
Ada program with the eggshell semantic.
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