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UTD CS 5348 - Section 5: Concurrency & Synchronization

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Section 5 Concurrency Synchronization Question 1 1 What is the definition of a Race Condition 2 From the bank account example describe the race condition that exists when two threads simultaneously post debits on the same bank account 3 Describe the race condition that exists when two threads simultaneously post debits on separate bank accounts 4 What solution was presented that ensures bank accounts are debited correctly Answer void postDebit double debit double balance readBalance accountID balance balance debit writeBalance balance accountID 1 A race condition occurs when multiple threads or processes read modify and write shared data items so that the final result depends on the order of execution of instructions in multiple threads Because the order of execution of two threads is non deterministic the results of the processing after all threads have completed is also unknown 2 The postDebit method will produce incorrect balances if the execution of two threads interleaves If the first thread to start execution times out after reading the balance and the second thread completes the operation thread one will calculate and write an incorrect balance 3 Trick Question If each thread operates on separate accounts there is no race condition and no chance of corrupting the account balances 4 A critical section is needed around the read modify write statements In the previous code segment a semaphore can be added with its acquire before the read and its release after the write operations Question 2 Note You will need to research this question 1 What are the two methods in Java for creating and launching a Thread 2 Which of these two methods is considered best i e a best practice Answer 1 The class which is to be executed in a thread implements the interface java lang Runnable This interface defines the method void run which is the new thread s entry point i e where the thread s execution begins The Thread is created by passing an instance of the runnable class into java lang Thread s constructor The new thread is created by calling Thread start See the class CriticalSectionBroken in the code samples for an example 2 The second method is to subclass Thread and override the run method The thread is started by executing the start method The Runnable approach is considered the best practice Question 3 1 What does a counting semaphore initialized to one with a value of zero signify 2 What is the value of a counting semaphore that has two blocked processes Answer 1 That a single process owns the semaphore and there are zero processes waiting for ownership 2 2 Question 4 Note You will need to research this question See the Java code in the eLearning folder Sample Synchronized Code 1 Why does the output of the sample code CriticalSectionBroken java fail to print correctly 2 Generally how do we correct this problem with a critical section Answer 1 The Runnable class StreamPrinter writes its message to the output stream System out one character at a time When there are multiple StreamPrinter threads executing output of each instance is interleaved with the outputs of other instances 2 The critical section in this code is the for loop in the run method that prints the message string Each thread runnable instance needs to execute their loop in a mutually exclusive manner one thread at a time to avoid interleaving with the print loops in other threads There are several examples of how the mutual exclusive execution can be accomplished in Java See CriticalSectionSynchronizedBlock CriticalSectionSemaphore CriticalSectionMonitor for three examples that the different approaches to critical sections we studied in this section Question 5 Note You will need to research this question See the Java code in the eLearning folder Sample Synchronized Code Why does the Java program CriticalSectionFailedSynchronize java fail to print messages correctly Answer This approach fails because the critical section created by the synchronized printMessage method is locking on the separate instances of StreamPrinter threads There is no mutual exclusive access to the for loop because each thread is locking on itself and not locking a single shared static object that will force the mutually exclusive access to the critical section See CriticalSectionSynchronizedBlock for a working example and notice that the synchronized block locks a static object lock Question 6 In the implementation of finite bounded buffer shown in Figure 5 16 1 What line blocks a producer thread when the buffer is full 2 What line release a blocked producer thread when space becomes available 3 What line blocks a consumer thread when the buffer is empty 4 What line release a blocked consumer thread when data becomes available Answer 1 If count N cwait notfull 2 csignal notfull 3 If count 0 cwait notempty 4 csighal notempty Question 7 What three advantages does semaphores offer over compare and swap instructions Answer 1 Semaphores cause the blocking of waiting processes threads eliminating the busy wait loop utilized by C S instructions 2 Semaphores offer the fair scheduling of blocked processes using FIFO queue of processes treads waiting on the semaphore i e no starvation of waiting processes threads 3 With C S if the process in the critical section dies before setting the shared variable to zero the waiting processes will never unblock Because a Semaphore is managed by the OS if the owning process dies while in a CS the OS will release the terminated process s ownership on any semaphores allowing waiting blocked processes to continue Question 8 What is the difference between the semaphore s sema wait operation and a monitor s wait operation Answer A semaphore s sema wait operation attempts to acquire the semaphore and blocks the calling thread only if the semaphore is already owned by a different thread process A thread that calls wait on an unowned semaphore will not block The monitor s wait operation always blocks the calling thread until a different thread in the system calls notify or notifyAll Question 9 In the slide Using a Semaphore to Implement Mutual Exclusion why is it critical that the argument semaphore be declared static Answer Synchronization between multiple threads processes requires that each thread operate on the same instance of a shared synchronization variable semaphore This sharing allows changes made to the variable by a single thread in the system visible to all the threads in the system If each tread operates on their own

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