1 Concurrency Architecture Types Tasks Synchronization –Semaphores –Monitors –Message Passing Concurrency in Ada Java Threads

2 Motivations for Studying Concurrency Many real-world situations involve concurrency –~80% of software produced Multiple-core processors are now widely used –Even on PCs Very useful, but different way of programming

3 Multiprocessor Architecture Types Late 1950s –1 general-purpose processor, and –1+ special-purpose I/O processors Early 1960s –Multiple complete processors –Program-level concurrency Mid-1960s –Multiple partial processors –Instruction-level concurrency Architecture Types –SIMD (Single-Instruction Multiple-Data) –MIMD (Multiple-Instruction Multiple-Data) Synchronized Independent processors Unit-level concurrency

4 Categories of Concurrency A thread –Sequence of program points reached as program runs Physical concurrency –Multiple independent processors Multiple threads Logical concurrency, Quasi-concurrency –Time-sharing of one processor Software designed as if there were multiple threads –Coroutines Single thread

5 Tasks A task or process –A program unit that executes concurrently with other program units Task differs from a subprogram –May be started implicitly –The program unit that starts it may not be suspended –When a task ends, control may not return to the caller A heavyweight task –Runs in its own address space A lightweight task –Runs in the same address space as other tasks A disjoint task –Is completely independent from any other tasks

6 Task Synchronization Controls the order in which tasks execute Two kinds –Cooperation synchronization A task must wait for another task to complete some activity E.g., the producer-consumer problem –Competition synchronization Two or more tasks use shared resource Usually provided by mutually exclusive access Synchronized task communication is provided by –Shared nonlocal variables –Parameters –Message passing

7 Task Execution A scheduler controls task execution –Maps tasks onto available processors Task Execution States –New – created, but not yet started –Ready - ready to run, but currently not running No available processor –Running –Blocked – did run, but can't continue now Waiting for an event –Dead - no longer active

8 Liveness and Deadlock Liveness –A program unit will eventually continue its execution No problem in sequential code In a concurrent environment, liveness can be easily lost Deadlock –All tasks lose their liveness

9 Concurrency Design and Mechanisms Design Issues –Competition and cooperation synchronization –Controlling task scheduling –How and when tasks start and end –How and when are tasks created Mechanisms –Semaphores –Monitors –Message Passing

10 Semaphores Dijkstra A semaphore –Data structure with a counter and a queue of task descriptors –Two operations wait release –Guards access to shared data Incorrect use of semaphores can cause fatal errors –In cooperation synchronization E.g., buffer overflow –In competition synchronization E.g., deadlock

11 Monitors Ada, Java, C# The idea –Encapsulate the shared data and its operations to restrict access A monitor –An abstract data type for shared data –Shared data is not the client units –All accesses are managed by the monitor Only one task is allowed to access data at a time If the monitor is busy all other tasks requesting access are waiting in a queue

12 Cooperation Synchronization Cooperation between processes is still a programming task –Code must guarantee that a shared buffer does not underflow or overflow Process SUB1 Process SUB1 Process SUB1 Process SUB4 Insert Remove BUFFERBUFFER Monitor Program

13 Evaluation of Monitors A better way to provide competition synchronization than are semaphores Semaphores can be used to implement monitors Monitors can be used to implement semaphores Cooperation synchronization is similar to semaphores –It has the same problems

14 Message Passing The idea –task communication is like seeing a doctor most of the time she waits for you, or you wait for her only when you are both ready, you rendezvous A general concurrency model –Not just for competition synchronization –Can implement semaphores –Can implement monitors

15 Rendezvous Message passing must –Let a task indicate that it is accepting messages –Remember tasks waiting to have their message accepted –Provide a fair way of choosing the next message Rendezvous –Message transmission from the sender task to the receiver task

16 Message-Passing in Ada 83 Tasks have specification and body parts –Like packages The specification defines the entry points task Task_Example is entry entry_name (Item : in Integer); end Task_Example; The body describes the actions during a rendezvous – accept clauses correspond to the entry points in the spec accept entry_name (formal parameters) do … end entry_name

17 Message-Passing in Ada 83 (cont.) The task executes till the accept clause then waits for a message The sender is suspended during execution of the accept clause, The parameters of accept can transmit information in either or both directions Every accept clause has an associated queue to store waiting messages

18 Server and Actor Tasks A server task –Has only accept clauses An actor task –Has no accept clause An actor task can send messages to other tasks A sender must know the entry name of the receiver, but not vice versa –Asymmetric

19 Graphical Representation of a Rendezvous Accept clauses JOB2 Accept clauses JOB1 Body Accept clauses JOB4 Accept clauses JOB3 Body B. JOBS3 (Value) Task A Task B

20 Concurrency in Ada 95 Ada 95 has two additional features Protected objects –A more efficient way of accessing shared data without rendezvous Asynchronous communication

21 Protected Objects A protected object is like an abstract data type Access to a protected object –Either through message passing, –Or through protected subprograms A protected procedure –provides mutually exclusive read-write access A protected function –provides concurrent read-only access

22 Asynchronous Communication Asynchronous select structures Two triggering alternatives –The entry clause is triggered when sent a message –The delay clause is triggered when its time limit is reached

23 Evaluation of Ada Message passing is powerful and general Protected objects are a better way to provide synchronized shared data Message passing is a better concurrency model for truly distributed systems

24 Java Threads The concurrent unit are run() methods in a Thread – run() can be in concurrent execution with other threads – run() is called indirectly, through start() Class MyThread extends Thread public void run () {…} } … Thread thread = new MyThread (); thread.start();

25 Thread Control Methods in the Thread class control the execution of the thread – yield() Voluntarily surrenders the processor – sleep() Blocks the thread – join() Waits until another thread's run() is completed – setPriority() Changes the thread's priority

26 Competition Synchronization in Java Only one of the synchronized methods can run on the same object public synchronized void deposit(int i) {…} public synchronized int fetch() {…} If another synchronized method is called on the same object it has to wait in a queue –While obj.deposit() runs, obj.fetch() must wait A synchronized method has exclusive access to instance variables A synchronized statement synchronizes only a part of a method synchronized (expression) statement –The expression determines to which object is the statement synchronized

27 Cooperation Synchronization in Java Cooperation synchronization in Java is achieved using the methods wait(), notify(), and notifyAll() –Because the methods are defined in the root class Object, they are available for all objects wait() must be called in a loop notify() tells one waiting thread that the event it was waiting for has happened The notifyAll() method awakens all of the threads on the object’s wait list

28 Java’s Thread Evaluation Java’s support for concurrency is relatively simple but effective Not as powerful as Ada’s tasks