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public class SharedQueue1 { class Node //inner class defines list nodes { Object task; Node next; Node(Object task) {thistask = task; next = null;} } private Node head = new Node(null); //dummy node private Node last = head; public synchronized void put(Object task) { assert task != null: "Cannot insert null task"; Node p = new Node(task); lastnext = p; last = p; } public synchronized Object take() { //returns first task in queue or null if queue is empty Object task = null; if (!isEmpty()) { Node first = headnext; task = firsttask; firsttask = null; head = first; } return task; } private boolean isEmpty(){return headnext == null;} }
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In general, to change a method that returns immediately if a condition is false to one that waits until the condition is true, two changes need to be made: First, we replace a statement of the form
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Figure 538 Queue that ensures at most one thread can access the data structure at one time Unlike the first shared queue example, if the queue is empty, the thread waits When used in a master/worker algorithm, a poison pill would be required to signal termination to a thread
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public class SharedQueue2 { class Node { Object task; Node next; Node(Object task) {thistask = task; next = null;}
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private Node head = new Node(null); private Node last = head; public synchronized void put(Object task) { assert task != null: "Cannot insert null task"; Node p = new Node(task); lastnext = p; last = p; notifyAll(); } public synchronized Object take() { //returns first task in queue, waits if queue is empty Object task = null; while (isEmpty()) {try{wait();}catch(InterruptedException ignore){}} { Node first = headnext; task = firsttask; firsttask = null; head = first; } return task; } private boolean isEmpty(){return headnext == null;} }
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The fact that wait can throw an InterruptedException must be dealt with; it is ignored here for clarity, but handled properly in the code examples
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Second, we examine the other operations on the shared queue and add a notifyAll to any operations that might establish condition The result is an instance of the basic idiom for using wait, described in more detail in the Java appendix, Appendix C Thus, two major changes are made in moving to the code in Fig 538 First, we replace the code
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Second, we note that the put method will make the queue not empty, so we add to it a call to notifyAll This implementation has a performance problem in that it will generate extraneous calls to notifyAll This does not affect the correctness, but it might degrade the performance One way this implementation could be optimized would be to minimize the number of invocations of notifyAll in put One way to do this is to keep track of the number of waiting threads and only perform a notifyAll when there are threads waiting We would have, for int w indicating the number of waiting threads:
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In this particular example, because only one waiting thread will be able to consume a task, notifyAll could be replaced by notify, which notifies only one waiting thread We show code for this refinement in a later example (Fig 540)
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Concurrency-control protocols for noninterfering operations
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If the performance of the shared queue is inadequate, we must look for more efficient concurrency
control protocols As discussed in the Shared Data pattern, we need to look for noninterfering sets of operations in our ADT Careful examination of the operations in our nonblocking shared queue (see Fig 537 and Fig 538) shows that the put and take are noninterfering because they do not access the same variables The put method modifies the reference last and the next member of the object referred to by last The take method modifies the value of the task member in the object referred to by headnext and the reference head Thus, put modifies last and the next member of some Node object The take method modifies head and the task member of some object These are noninterfering operations, so we can use one lock for put and a different lock for take This solution is shown in Fig 539
Figure 539 Shared queue that takes advantage of the fact that put and take are noninterfering and uses separate locks so they can proceed concurrently