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We have two processes. One process produces F and the other process consumes F

Initialization:

Consuming = false
Produced = false
F = EMPTY

The first thread (we can think of it as some kind of producer):

P1:
loop_for_ever:
    Produce(F)
    Consuming = true
    Produced = true
    while(Consuming) {Busy Wait)

And here is P2:

P2:
loop_for_ever:
    while(NOT Produced) {Busy Wait}
    Consume(F)
    Produced = false
    Consuming = false

One problem with this solution is after P1 has produced F, there is no way for P2 to consume F until P1 unlocks the Produced variable. Any other problems with this solution with regards to mutual exclusion? Is there a scenario in which P1 and P2 try to produce and consume F at the same time?

EDIT:

After reading fade2black's answer, I think I first need to address another more basic question. So let's add these extra conditions.

Extra conditions:

1- No mutex or semaphore based solution (only busy wait).

2- The threads have no remainder section (in that case this approach would obviously not work). Basically, what I am trying to do is to take a sequential algorithm (such as the one @fade2black has given) and distribute it among two threads. I could have blindly used an algorithm such as Dekker's (with turn given to P1 at the beginning). So I think first I need to find an answer for this question : "Is there any point to try and use a concurrent algorithm for this problem"? (when there is only one consumer and one producer and there is only one shared space between the two).

jrook
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2 Answers2

2

Your approach works fine providing assignment operations are atomic. But I cannot see any concurrency in your approach. $P_2$ always waits until $P_1$ produces and reaches the upper limit, for example until it completely fills the buffer. Then it triggers $P_2$ to consume and waits until $P_2$ consumes all items, for example empties the buffer. So the algorithm can be rewritten as 

doStuff()
  loop-forever 
    produce(F) # N items
    consume(F) # N items
  end
end

In fact the consumer(s) consumes as soon as the producer(s) produces items. In this case you would have to use synchronizations techniques such as mutexes.

Also (from here)

The producer–consumer problem, particularly in the case of a single producer and single consumer, strongly relates to implementing a FIFO or a channel.

So there is no point to try and use a concurrent algorithm for this problem. But if your model is multi producer/consumer model then your approach may lead to the race condition.

fade2black
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0

A simpler solution to the original problem

Using Python style notation, here is the simplified and more correct solution.

Initialization:

consuming = False
init(F) //if needed

Code for P1:

while True:
    while consuming:
        pass
    produce(F)  //updates F
    consuming= True

Code for P2:

while True:
        while not consuming:
            pass
        consume(F)
        consuming= False

This is an improvement over the code posted in question. It only uses one variable and the busy wait is located before the body of the code for each thread (the code in the question could lead to trouble if P2 thread has other sections and they somehow manipulate the variables).

A note about fade2black's answer:

This original code is in fact concurrent. But it is not parallel.

As this source (p. 166) states:

A system is parallel if it can perform more than one task simultaneously. In contrast, a concurrent system supports more than one task by allowing all the tasks to make progress. Thus, it is possible to have concurrency without parallelism.

jrook
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