Synchronization in Concurrent Programming. Amit Gupta

Transcription

1 Synchronization in Concurrent Programming Amit Gupta

2 Announcements Project 1 grades are out on blackboard. Detailed Grade sheets to be distributed after class. Project 2 grades should be out by next Thursday.

3 Introduction Concurrent programs allow multiple threads to be scheduled and executed with no user control. JVM and underlying OS responsible for scheduling threads onto processor cores. Program should work correctly despite the fact that different execution of same program lead to different sequences of thread execution.

4 Bank Account: Problem Classic Two people: One Account problem. Ryan and Monica share a common bank account. Both of them agree not to overdraw the account. So, the procedure is, whoever wants to withdraw money must check the balance in account before making withdrawal. Seems to be good solution with no problems.

5 Bank Account: Problem Ryan needs 50$, checks the account and saw it as 100$ and plans to withdraw money. But first he falls sleep. Monica comes up, checks the account and withdraws 100$ as Ryan has not made his withdrawal yet. Ryan wakes up and makes his withdrawal without checking account balance and we are overdrawn.

6 Bank Account: Problem Is there a solution?? We cannot stop Ryan from sleeping but can we make sure that Monica can t get her hands on bank account until Ryan wakes up?

7 Bank Account Problem in Code Bank Account class Ryan and Monica check balance before making any withdrawal. RyanAndMonicaJob class represents the behavior that Ryan and Monica both have checking the balance and making withdrawals.

8 Bank Account Problem in Code

9 Observations Both threads share same instance of Bank Account. In run() method, the thread loops through and tries to make a withdrawal with each iteration. makewithdrawal() method checks the account balance and if there is enough money, go to sleep, then wake up and complete the withdrawal.

10 Problem Ryan checks balance, sees enough money and falls asleep. Meanwhile, Monica comes in and checks the balance, sees enough money and falls asleep. Ryan wakes up and completes withdrawal. Monica wakes up and completes withdrawal.(problem) Monica s check of the account was not valid because Ryan was in the middle of making a withdrawal.

11 Lock for Account Access

12 Lock for Account Access We need the makewithdrawal() method to run as one atomic thing. In other words, we need to make sure once a thread has checked the account balance, that thread has a guarantee that it can wake up and finish the withdrawal before any other thread can check the account balance.

13 synchronized keyword synchronized keyword means that a thread needs a key in order to access the synchronized code. To protect data, synchronize the methods that act on data i.e we lock the method that does banking transaction.

14 Lost Update Problem Another classic concurrency problem. Two threads trying to increment balance. We increment the balance by 1 to whatever the value of balance was at the time we read it not the current value. Incrementing balance not an atomic operation.

15 Solution Make the increment() method atomic Once the thread enters the method, we have to make sure that all the steps in method complete before any other thread can enter the method.

16 Locks in Java Every object in Java has a lock and the lock has only one key. A thread can enter a synchronized method only if the thread can get the key to the object s lock. Locks are per object not per method. So, while the thread is holding the key, no other threads can enter any of that object s synchronized methods because the key has already been grabbed by the other thread.

17 synchronized on a code block Synchronized applied to a block of code gives more flexibility. First, we can control how much code needs to be synchronized. We can choose which object we want to synchronize on. public void danger(){ synchronized(this){ message = Danger + message } } Private static Object lock1 = new Object(); Private static Object lock1 = new Object(); synchronized(lock1){ // do dangerous thing 1 } synchronized(lock2){ // do dangerous thing 1 }

18 Why don t we synchronize every A synchronized method has a certain amount of overhead. method?? It restricts concurrency and make other threads to get in line and wait. It can lead to DEADLOCKS. Synchronize only the bare minimum that needs to be synchronized.

19 Busy Waiting Many times a thread waits for another thread to finish some task so that it can process the results. While(true){ synchronized(this){ if(votingcomplete) break; } } countvotes();

20 Busy Waiting On a uniprocessor machine, the counting thread is running through a while loop and slowing down the job of collecting thread. On a multiprocessor machine, the counting thread is still wasting CPU cycles that other thread could use. This is called as BUSY WAITING.

21 wait() and notify() When a thread calls wait(), it gives up the corresponding lock it is holding and runs again. Many other threads may be waiting on this particular lock. In order to notify a waiting thread, the other thread calls the notify() method. wait() and notify() must be called within a synchronized code. synchronized(this){ while(!votingcomplete) wait(); } countvotes(); synchronized(this){ votingcomplete = true; notifyall(); }

22 notify() and notifyall() notify() wakes up one thread waiting on the lock object notifyall() can be called to wake them all up.

23 Producer/Consumer Problem Classic Example in Concurrent programming. One thread produces some data and other thread consumes that data. Producer/Consumer problem use a bounded buffer which store items from the producer until the consumer can take them away.

24 Producer/Consumer Problem public class Buffer{ public final static int SIZE = 10; private Object[] objects = new Object[SIZE]; private int count = 0; public synchronized void additem(object object) throws InterruptedException{ while(count == SIZE) wait(); object[count++] = object; notifyall(); } public synchronized void removeitem() throws InterruptedException{ while(count == 0) wait(); object = object[count ]; notifyall(); return object; } }

25 Deadlock Deadlock happens when you have two threads, both of which holding the key of lock the other thread wants. Java has no mechanism to handle deadlock. So, we should write design our multithreaded code carefully.

26 A Simple Deadlock Scenario

27 Other Synchronization Challenges Starvation and LiveLock Sequential Execution Priority Inversion

28 Questions