Lecture 15 Concurring Concurrently CS201j Engineering Software University of Virginia Computer Science David Evans http www cs virginia edu evans Our computer can only do one instruction at a time why would we want to program pretending it can do many things at once 23 October 2003 CS 201J Fall 2003 2 Concurrent Programming Some problems are clearer to program concurrently Modularity Don t have to explicitly interleave code for different abstractions especially user interfaces High level interactions synchronization communication Modeling Closer map to real world problems things in the real world aren t sequential 23 October 2003 CS 201J Fall 2003 3 Concurrency in Java public class Thread implements Runnable OVERVIEW A thread is a thread of execution in a program The Java Virtual Machine allows an application to have multiple threads of execution running concurrently public Thread Runnable target Creates a new Thread object that will run the target public void start Starts a new thread of execution many other methods 23 October 2003 CS 201J Fall 2003 4 Making a Thread from PS5 SimObject public class Thread implements Runnable public Thread Runnable target public void start many other methods class final public void init int x int y Grid grid REQUIRES init has not previously been called for this The cell is at row col on grid MODIFIES this EFFECTS Initializes the cell at row col on grid with isPaused true NOTE This method is final that means subtypes cannot override this method this mx x this my y this grid grid this isPaused true Thread thread new Thread this thread setPriority Thread MIN PRIORITY thread start set isInitialized true What do you know about SimObject type 23 October 2003 CS 201J Fall 2003 5 Runnable public interface Runnable public void run When an object implementing interface Runnable is used to create a thread starting the thread causes the object s run method to be called in that separately executing thread The general contract of the method run is that it may take any action whatsoever So to be a subtype of Runnable SimObject must have a method void run with no preconditions and any postconditions it wants 23 October 2003 CS 201J Fall 2003 6 Making a Runnable bstract public class SimObject implements Runnable public void run EFFECTS Executes one turn by calling the executeTurn method and sleeps for a time and repeats while true executeTurn delay TURN DELAY random nextInt TURN RANDOM 23 October 2003 CS 201J Fall 2003 7 Actually bstract public class SimObject implements Runnable public void run REQUIRES this has been initialized also requires isInitialized EFFECTS Executes one turn by calling the executeTurn method and sleeps for a time and repeats We are violating the substitution principle SimObject run has a stronger precondition than Runnable run 23 October 2003 CS 201J Fall 2003 8 Concurrency Making a concurrent Java program is easy Make a subtype R of Runnable new Thread new R start Making a concurrent Java program that behaves correctly is really really hard 23 October 2003 CS 201J Fall 2003 9 Scheduling Meetings Alice wants to schedule a meeting with Bob and Colleen Bob When can you meet Friday Alice When can you meet Friday Colleen 11am or 3pm 9am or 11am Reserves 11am for meeting 23 October 2003 Picks meeting time Let s meet at 11am Let s meet at 11am CS 201J Fall 2003 Reserves 11am for meeting 10 Partial Ordering of Events Sequential programs give use a total ordering of events everything happens in a determined order Concurrency gives us a partial ordering of events we know some things happen before other things but not total order Alice asks to schedule meeting before Bob replies Alice asks to schedule meeting before Colleen replies Bob and Colleen both reply before Alice picks meeting time Alice picks meeting time before Bob reserves time on calendar 23 October 2003 CS 201J Fall 2003 11 Race Condition Bob Doug When can you meet Friday When can you meet Friday Alice When can you meet Friday Colleen 9 11am or 3pm 9am or 11am 9 11am or 3pm Let s meet at 11am Reserves 11am for Doug Let s meet at 11am Picks meeting time Let s meet at 11am I m busy then 23 October 2003 CS 201J Fall 2003 12 Preventing Race Conditions Use locks to impose ordering constraints After responding to Alice Bob reserves all the times in his response until he hears back and then frees the other times 23 October 2003 CS 201J Fall 2003 13 Locking Doug When can you meet Friday Bob When can you meet Friday Alice When can you meet Friday Colleen 9 11am or 3pm 9am or 11am Locks calendar 3pm Let s meet at 11am Picks meeting time Let s meet at 11am Let s meet at 3 23 October 2003 CS 201J Fall 2003 14 Doug When can Bob you meet Friday When can you meet Friday Can t schedule meeting no response from Bob 23 October 2003 Deadlocks Alice When can you meet Friday Colleen When can you meet Friday Locks calendar for Doug can t respond to Alice 9 11am or 3pm Locks calendar for Alice can t respond to Doug CS 201J Fall 2003 Can t schedule meeting no response from Colleen 15 Why are threads hard Too few ordering constraints race conditions Too many ordering constraints deadlocks Hard impossible to reason modularly If an object is accessible to multiple threads need to think about what any of those threads could do at any time Testing is even more impossible than it is for sequential code Even if you test all the inputs don t know it will work if threads run in different order 23 October 2003 CS 201J Fall 2003 16 Charge Computers are single threaded machines that provide their owner the illusion of multiple threads Brains are multi threaded machines that provide their owner with the illusion of a single thread Friday section return exams practice with subtyping and concurrency 23 October 2003 CS 201J Fall 2003 17