Ivan Marsic Rutgers University

1. LECTURE 19: Concurrent Programming Ivan Marsic Rutgers University

2. Topics • Threads • Exclusive Resource Access • Exclusion Synchronization • Cooperation between Threads • Condition Synchronization • Concurrent Programming Example

3. Parallelism • What we want:Parallel instead of serial processing, to speed up service • What problems to solve:Concurrent access to a resource (or software object) can lead to conflict  ambiguous result or “frozen” program

4. Concurrency: Exclusion Synchronization

5. Thread vs. Process • Process roughly corresponds to a program (*program can “spawn” many processes) • Processes communicate using inter-process communication (pipes, sockets, shared memory, …) • An object from one process cannot directly call a method on an object shared by another process • Threads run in the same program • An object from one thread can directly call a method on an object shared by another thread

6. Concurrent Programming -- Threads Lifecycle of Java threads

7. Thread States: Alive • Alive: After a thread is start()-ed, it becomes alive: • Runnable: The thread can be run when the OS scheduler can arrange it (and nothing prevents it from being run) • Blocked: The thread could be run, but there is something that prevents it (e.g., another thread is holding the resource needed for this thread to do its work). While a thread is in the blocked state, the scheduler will simply skip over it and not give it any CPU time. A thread can become blocked for the following reasons: • Waiting for notification: Invoking the method wait() suspends the thread until the thread gets the notify() or notifyAll() message • Waiting for I/O or lock: The thread is waiting for an input or output operation to complete, or it is trying to call a synchronized method on a shared object, and that object’s lock is not available • Waiting for rendezvous: Invoking the method join(target) suspends the thread until the target thread returns from its run() method • Sleeping: Invoking the method sleep(milliseconds) suspends the thread for the specified time

8. Exclusion Synchronization

9. Example: Bank Account Accessby Two Users • Concurrent read/write of the same data by several threads “race condition” or “race hazard” • The outcome of the execution depends on the particular order in which the access takes place

10. Exclusion Synchronization in Java shared object public class AnyClass { ... public void method2( ... ) { ... synchronized (expression) { statement } ... } } public class SharedClass { ... public synchronized void method1( ... ) { ... } } acquire lock acquire lock release lock release lock shared object (a) (b) Synchronized Methods Synchronized Statements

11. Condition Synchronization suspend and wait … … resume the suspended work

12. Example: Safe Home Access Central Computer Backyard door: Access lock Front door: Access lock

13. Example of Concurrency Gains Single thread – sequential service (a) Multiple threads – parallel service (b)

14. [ keycode complete ] [ keycode valid ] Activate Devices Read Digits Check Validity Tenant [else] [else] Where Latency Matters? • While typing in the digits, the user does not notice latency • Latency becomes noticeable when waiting for validity check and device activation • Validity computation is quick; device activation may be somewhat slower • The longest latency is while the user is typing-in the keycode so we need to allow parallel entry of keycodes on both doors • But ...all communication goes over the same serial cable:

15. Hardware Implementation NOTE: Locks, lightbulb switch (with photosensor) and alarm bell are controlled by the central computer via the same serial port

16. Main Thread: interacts with serial I/O port Helper Thread: front door calculations and control Helper Thread: back door calculations and control HomeAccessControlSystem_2x + serialEvent(event : SerialPortEvent) contrlFront : ControllerThd contrlBack : ControllerThd keyFront : StringBuffer keyBack : StringBuffer Shared Object Shared Object Multithreaded Implementation ( See Listing 5-6 in the book for details )