SSC - Concurrency and Multi-threading Java multithreading programming - Synchronisation (II)

Transcription

1 SSC - Concurrency and Multi-threading Java multithreading programming - Synchronisation (II) Shan He School for Computational Science University of Birmingham Module : SSC

2 Outline Outline of Topics Review what we have learned More about Synchronized methods Liveness: deadlock

3 Review what we have learned High level synchronisation in Java High level concurrency tools Executors framework Fork/Join framework

4 Review what we have learned Low level synchronisation in Java Low level concurrency tools Volatile Variables Locks Atomic Operations Reentrant Locks Synchronized keyword Synchronized methods Synchronized statements

5 More about Synchronized methods How to use Java synchronized methods Simply add the synchronized keyword to method declaration: Example (See my Java source code for details): for a method public void add(long value){...} the synchronised method is: public synchronized void add(long value){...} The synchronized keyword automatically acquires the intrinsic lock for that method s object and releases it when the method finishes. A synchronized method: prevents invocations of synchronized methods on the same object; and automatically establishes a happens-before relationship

6 More about Synchronized methods Java synchronized methods: how do they work? Steps: Step 1: Thread A executes a synchronized method Step 2: Thread A automatically acquires the intrinsic lock for that method s object Step 3: Suspend execution of other threads that invoke synchronized methods for the same object since they don t have the intrinsic lock Step 4: Thread A releases the intrinsic lock when it is finished with the object. Step 5: Another thread scheduled by the scheduling algorithm acquires the intrinsic lock to subsequently execute its synchronised methods for the same object Note: Java guarantees that changes to the state of the object by a thread are visible to all threads.

7 More about Synchronized methods Java synchronized methods: drawbacks A synchronised method synchronises on the method s object instance or the class: cannot synchronise some parts of your code coarse grained, lack of granular control over lock, Thread synchronises on the whole object instead of a smaller critical section slow; You cannot set an intrinsic lock on an external object using synchronized methods.

8 Also called synchronized blocks, which can solve all the drawbacks of synchronized methods. Use to mark a block of code as synchronized The block is also called critical section where one and only one thread is executing Usage: Use the following statement to mark a synchronized block: synchronized(object){ synchronize operations on the object }

9 How to use Example 1 (See my Java source code for details): public void add(int value) { synchronized(this){ this.count += value;} } The above code is equivalent to: public synchronized void add(int value) { this.count += value;}

10 Synchronized statements must specify the object that provides the intrinsic lock. e.g., synchronized(object) In Example 1, we use synchronized(this){...} The object this is the instance the add method is called on The object specify by the synchronized statement is called a monitor object The code is said to be synchronized on the monitor object Only one thread can execute inside a Java code block synchronized on the same monitor object.

11 Question: synchronize operations on object Is the following code correct? long r1 = 1; synchronized(r1) { r1++; }

12 Answer: synchronize operations on object We can only synchronize on objects but long is a primitive value that doesn t have a lock We we can use Long which is an object: Long r1 = 1; synchronized(r1) { r1++; }

13 Question: synchronised on a static method If you have a static method, whether synchronized methods still work? Reminder 1: a static method belongs to the class, not to the specific objects of that class, that is, a static method has no associated object. Reminder 2: Synchronized methods prevent invocations of synchronized methods on the same object

14 Answer: synchronising a class For a class (static) method, the monitor associated with the Class object for the method s class is used. For example, synchronized static void foo() {... } is equivalent to static void foo() { synchronized(someclass.class) {... } } If one thread is executing a static synchronized method, any other threads executing any static synchronized methods in the same class will be blocked. However, non-static synchronized methods will not be blocked since they require the lock on the instance, i.e., synchronized(this){...}

15 Liveness: deadlock Liveness and Deadlock Liveness property in concurrent programming means something good eventually happens. More specifically, the ability of an application to execute in a timely manner. Liveness problems include deadlock, starvation and livelock Deadlock: a situation threads are blocked forever, waiting for each other: Can be two threads waiting for another to release a lock Can be more than two threads are waiting for resources in a circular chain

16 Liveness: deadlock 4 Necessary conditions for deadlock A deadlock situation will happen if all of the following conditions (called Coffman conditions) hold simultaneously: Mutual exclusion: at least one resource must be held in a non-sharable mode: Only one thread at a time can use the resource If another thread requests that resource, the requesting thread must wait until the resource has been released Hold and wait: Thread holds one resource while waits for another No preemption: Once a thread is holding a resource, then that resource cannot be taken away from that thread until the thread voluntarily releases it. Circular wait: A thread must be waiting for a resource which is being held by another thread, which in turn is waiting for the first thread to release the resource.

17 Liveness: deadlock Deadlock A Java example