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Adsa u4 ver 1.0


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Adsa u4 ver 1.0

  1. 1. Compiled ver 1.0 Dr. C.V. Suresh Babu 5/12/2013 Revision ver 1.1 Revision ver 1.2 Revision ver 1.3 Revision ver 1.4 Revision ver 1.5 Edited ver 2.0 Dr. C.V. Suresh Babu UNIT IV SHARED OBJECTS AND CONCURRENT OBJECTS 9 4.1. Shared Objects and Synchronization – 4.2. Properties of Mutual Exclusion4.2.1. The Moral – 4.2.2. The Producer–Consumer Problem – 4.2.3. The Readers–Writers Problem4.3. Realities of Parallelization4.3.1. Parallel Programming- Principles4.4. Mutual Exclusion4.4.1. Time4.4.2. Critical Sections— 4.5. Thread Solutions4.5.1. The Filter Lock4.5.2. Fairness4.5.2. Lamport’s Bakery Algorithm4.6. Bounded Timestamps4.6.1. Lower Bounds on the Number of Locations4.7. Concurrent Objects4.7.1. Concurrency and Correctness4.7.2. Sequential Objects4.7.3. Quiescent Consistency4.7.4. Sequential Consistency4.8. Linearizability4.8.1. Formal Definitions4.8.2. Progress Conditions4.9. The Java Memory Model
  2. 2. 4.1. Shared Objects and Synchronization When processes interact with one another two fundamental requirements must be satisfied:  Synchronization  Communication Balancing the work load using a shared counter. Each thread gets a dynamically determined number of numbers to test Counter counter = new Counter(1); // shared by all threads void primePrint { long i = 0; long limit = power(10, 10); while (i< limit) { // loop until all numbers taken i = counter.getAndIncrement(); // take next untaken number if (isPrime(i)) print(i); } }
  3. 3. An implementation of the shared counter public class Counter { private long value; // counter starts at one public Counter(inti) { // constructor initializes counter value = i; } public long getAndIncrement() { // increment, returning prior value return value++; } } 4.2. Properties of Mutual Exclusion First, if one pet wants to enter the yard, then it eventually succeeds. Second, if both pets want to enter the yard, then eventually at least one of them succeeds. We consider this deadlockfreedom property to be essential. Another property of compelling interest is starvation-freedom (sometimes called lockoutfreedom): if a pet wants to enter the yard, will it eventually succeed? Here, Alice and Bob‟s protocol performs poorly. Need for Mutual Exclusion  If there is no controlled access to shared data, processes or threads may get an inconsistent view of this data  The result of concurrent execution will depend on the order in which instructions are interleaved.  Errors are timing dependent and usually not reproducible.  Assume P1 and P2 are executing this code and share the variable a  Processes can be preempted at any time.  Assume P1 is preempted after the input statement, and P2 then executes entirely  The character echoed by P1 will be the one read by P2 !!
  4. 4. 4.2.1. The Moral First, we examine why shouting across the yard and placing cell phone calls did not work. Two kinds of communication occur naturally in concurrent systems: Transient communication requires both parties to participate at the same time. Shouting, gestures, or cell phone calls are examples of transient communication. Persistent communication allows the sender and receiver to participate at different times. Posting letters, sending email, or leaving notes under rocks are all examples of persistent communication. 2.2. PRODUCER-CONSUMER The producer – consumer problem is a common implementation pattern for cooperating processes or threads. Put simply, a producer produces information that is later consumed by a consumer. Traditionally, the problem is defined as follows: To allow the producer and consumer to run concurrently, the producer and consumer must share a common buffer. So that the consumer does not try to consume an item that has not yet been produced, the two processes must be synchronized. If the common data buffer is bounded, the consumer process must wait if the buffer is empty, and the producer process must wait if the buffer is full. There are many examples of this problem in “real world” applications.
  5. 5. e.g., a build system may run multiple processes concurrently. The compiler process will produce assembly code to be read by the assembler process. _ You would have had to worry about this problem in PA1 had you not used a socket as the data buffer between your client and server processes. Essentially, the operating system handled your synchronization issues PRODUCER PROBLEM Add last items to the queue. Set queue->empty = 0 i == WORK_MAX, producer exits. CONSUMER PROBLEM Remove an item from the queue. A check to see if the queue is empty shows that it is. Set queue->empty = 1. Set queue->full = 0 Start spinning. 4.2.3. Readers and Writers Problem W R R R Motivation: Consider a shared database – Two classes of users: » Readers – never modify database » Writers – read and modify database – Is using a single lock on the whole database sufficient? » Like to have many readers at the same time » Only one writer at a time Basic structure of a solution: – Reader() Wait until no writers Access data base Check out – wake up a waiting writer – Writer() Wait until no active readers or writers Access database Check out – wake up waiting readers or writer
  6. 6. – State variables (Protected by a lock called “lock”): » int AR: Number of active readers; initially = 0 » int WR: Number of waiting readers; initially = 0 » int AW: Number of active writers; initially = 0 » int WW: Number of waiting writers; initially = 0 » Condition okToRead = NIL » ConditioinokToWrite = NIL Transform an algorithm to a Java program The dining philosopher‟s problem is useful for modelling processes that are competing for exclusive access to a limited number of resources. Another famous problem is the readers and writers problem, which models access to a data base: it is acceptable to have multiple processes reading the data base at the same time, but if one process is writing the data base, no other process may have access to the data base, not even readers. A solution for this problem, as C fragment is shown below. typedefint semaphore; // use your imagination semaphoremutex = 1; // controls access to „rc‟ semaphore db = 1; // controls access to the data base intrc = 0; // # of processes reading or wanting to reader() { while (true) { // repeat forever down(&mutex); // get exclusive access to „rc‟ rc++; // one reader more now if (rc == 1) down(&db); // if this is the first reader … read_data_base(); // access the data rc--; // one reader fewer now up(&mutex); // release exclusive access to „rc‟ use_data_read(); // noncritical section } } writer() { while (true) { // repeat forever think_up_data(); down(&db); // get exclusive access write_data_base()(); // update the data up(&db); } // noncritical section // releases exclusive access
  7. 7. } PRODUCERS SIMPLE APPLICATION producer() { while(true) { ifi == N //full buffer turn = 1 while turn <> 0 { // nothing } produceitem(&item)//produce the item insertitem(item, buffer)//insert the item in the buffer turn = 1 //process zone } } consumer() { while(true) { ifi == 0 //empty buffer turn = 0 while turn <> 1 { // nothing } consumeitem(&item) deleteitem(buffer)//erase the item from buffer turn = 0 //process zone } } Readers and Writers Application A message distribution system is an example of Readers and Writers. We must keep track of how many messages are in the queue. If one notebook exists where writers may write information to, only one writer may write at a time. Confusion may arise if a reader is trying read at the same as a writer is writing. Since readers only look at the data, but do not modify the data, we can allow more than one reader to read at the same time. The main complexity with this problems stems from allowing more than one reader to access the data at the same time.
  8. 8. Writers have mutual exclusion, but multiple readers at the same time is allowed. Here is a basic Python class that implements a solution to Readers and Writers using simple locks: Importthreading classReader_writer(Object): def__init__(self): self.readers=0 self.writers=0 self.mutex1=threading.Lock() self.mutex2=threading.Lock() self.readPending=threading.Lock() self.writeBlock=threading.Lock() self.readBlock=threading.Lock() # This is kind of complicated locking stuff. Don't worry about why there # are so many locks, it is just part of the known solution to # the readers and writers problem. # # The basic idea of the readers and writers algorithm is to use locks to # quickly see how many other readers and writers there are. write Block # is the only lock held while reading the data. Thus the reader only # prevents writers from entering the critical section. Both write Block # and read Block are held while writing to the data. Thus the writer # blocks readers and other writers.
  9. 9. defreader(self,lastread): "Reader of readers and writers algorithm" self.readPending.acquire() self.readBlock.acquire() self.mutex1.acquire() self.readers=self.readers+1 ifself.readers==1:self.writeBlock.acquire() self.mutex1.release() self.readBlock.release() self.readPending.release() #-------------------------# # critical section # #-------------------------self.mutex1.acquire() self.readers=self.readers-1 ifself.readers==0:self.writeBlock.release() self.mutex1.release() returnretVal defwriter(self,data): "Writer of readers and writers algorithm" self.mutex2.acquire() self.writers=self.writers+1 ifself.writers==1:self.readBlock.acquire() self.mutex2.release() self.writeBlock.acquire() #-------------------------# # critical section # #-------------------------self.writeBlock.release() self.mutex2.acquire() self.writers=self.writers-1 ifself.writers==0:self.readBlock.release() self.mutex2.release() 4.4.2. Critical Sections For brevity, we say a thread acquires (alternately locks) a lockwhen it executes alock() method call, and releases (alternately unlocks) the lock when it executes an 1 class Counter { 2 private intvalue; 3 public Counter(intc) { // constructor 4 value = c; 5} 6 // increment and return prior value 7 public intgetAndIncrement() {
  10. 10. 8 inttemp = value; // start of danger zone 9 value = temp + 1; // end of danger zone 10 return temp; 11 } 12 } Figure 2.1 The Counter class. 2.2 Critical Sections 23 1 public interface Lock { 2 public void lock(); // before entering critical section 3 public void unlock(); // before leaving critical section 4} Figure 2.2 The Lock interface. 1 public class Counter { 2 private long value; 3 private Lock lock; // to protect critical section 45 public long getAndIncrement() { 6 lock.lock(); // enter critical section 7 try { 8 long temp = value; // in critical section 9 value = temp + 1; // in critical section 10 }finally{ 11 lock.unlock(); // leave critical section 12 } 13 return temp; 14 } 15 } Figure 2.3 Using a lock object . Threads using the lock() and unlock()methods must follow a specific format. A thread is well formedif: 1. each critical section is associated with a unique Lock object, 2. the thread calls that object‟s lock() method when it is trying to enter the critical section, and 3. the thread calls the unlock() method when it leaves the critical section. Pragma2.2.1. In Java, these methods should be used in the following structured way. 1 mutex.lock(); 2 try { 3 ... // body 4 }finally{ 5 mutex.unlock(); 6} This idiom ensures that the lock is acquired before entering the try block, andthat the lock is released when control leaves the block, even if some statementin the block throws an unexpected exception.
  11. 11. Mutual Exclusion Critical sections of different threads do not overlap. ForThreadsA and B, and integers j and k, either CSk A →CSj B or CSj B →CSk A. Freedom from Deadlock If some thread attempts to acquire the lock, then some thread will succeed in acquiring the lock. If thread A calls lock() but never acquires the lock, then other threads must be completing an infinite number of critical sections. Freedom from Starvation Every thread that attempts to acquire the lock eventually succeeds. Every call to lock() eventually returns. This property is sometimes called lockout freedom. 4.5. Thread Solutions The LockOneClass The LockOnealgorithm satisfies mutual exclusion. Proof: Suppose not. Then there exist integers j and k such that CSj A _→ CSk B and CSk B _→ CSj A. Consider each thread‟s last execution of the lock() method before entering its k-th (j-th) critical section. Inspecting the code, we see that writeA(flag[A] = true)→readA(flag[B] == false)→CSA (2.3.1) writeB(flag[B] = true)→readB(flag[A] == false)→CSB (2.3.2) readA(flag[B] == false)→writeB(flag[B] = true) (2.3.3) Note that once flag[B] is set to true it remains true. It follows that Eq. 2.3.3 holds, since otherwise thread A could not have read flag[B] as false. Eq. 2.3.4 follows from Eqs. 2.3.1–2.3.3, and the transitivity of the precedence order. class LockOneimplementsLock { private boolean[] flag = new boolean[2]; // thread-local index, 0 or 1 public void lock() { inti = ThreadID.get(); intj = 1 - i; flag[i] = true; while (flag[j]) {} // wait } public void unlock() { inti = ThreadID.get(); flag[i] = false; } } Figure 2.4 TheLockOnealgorithm. writeA(flag[A] = true)→readA(flag[B] == false)→ (2.3.4) writeB(flag[B] = true)→readB(flag[A] == false) It follows that writeA(flag[A] = true)→readB(flag[A] == false) without an
  12. 12. intervening write to the flag[] array, a contradiction. _ The Lock One algorithm is inadequate because it deadlocks if thread executions are interleaved. If writeA(flag[A] = true) and writeB(flag[B] = true) events occur before readA(flag[B]) and readB(flag[A]) events, then both threads wait forever. Nevertheless, LockOnehas an interesting property: if one thread runs before the other, no deadlock occurs, and all is well. The LockTwoClass The LockTwoalgorithm satisfies mutual exclusion. Proof: Suppose not. Then there exist integers j and k such that CSj A _→ CSk B and CSk B _→ CSj A. Consider as before each thread‟s last execution of the lock() method before entering its k-th (j-th) critical section. Inspecting the code, we see that writeA(victim = A)→readA(victim == B)→CSA (2.3.5) writeB(victim = B)→readB(victim == A)→CSB (2.3.6) Thread B must assign B to the victim field between events writeA(victim = A) andreadA(victim = B) (see Eq. 2.3.5). Since this assignment is the last, we have writeA(victim = A)→writeB(victim = B)→readA(victim == B). (2.3.7) Once the victim field is set to B, it does not change, so any subsequent read returnsB, contradicting Eq. 2.3.6. _ 1 class LockTwoimplementsLock { 2 private volatile intvictim; 3 public void lock() { 4 inti = ThreadID.get(); 5 victim = i; // let the other go first 6 while (victim == i) {} // wait 7} 8 public void unlock() {} 9} Figure 2.5 TheLockTwoalgorithm. Example of lock algorithm The Peterson lock algorithm satisfies mutual exclusion. Proof: Suppose not. As before, consider the last executions of the lock() method by threads A and B. Inspecting the code, we see that writeA(flag[A] = true)→ (2.3.8) writeA(victim = A)→readA(flag[B])→readA(victim)→CSA writeB(flag[B] = true)→ (2.3.9) writeB(victim = B)→readB(flag[A])→readB(victim)→CSB Assume, without loss of generality, that A was the last thread to write to the victimfield. writeB(victim = B)→writeA(victim = A) (2.3.10) 1 class Peterson implements Lock { 2 // thread-local index, 0 or 1 3 private volatile boolean[] flag = new boolean[2];
  13. 13. 4 private volatile intvictim; 5 public void lock() { 6 inti = ThreadID.get(); 7 intj = 1 - i; 8 flag[i] = true; // I’m interested 9 victim = i; // you go first 10 while (flag[j] && victim == i) {}; // wait 11 } 12 public void unlock() { 13 inti = ThreadID.get(); 14 flag[i] = false; // I’m not interested 15 } 16 } Figure 2.6 The Peterson lock algorithm. 4.5.1. The Filter Lock The levels are depicted in Fig. 2.8. Levels satisfy two important properties: _ At least one thread trying to enter level _ succeeds. _ If more than one thread is trying to enter level _, then at least one is blocked (i.e., continues to wait at that level). 2.4 The Filter Lock 29 1 class Filter implements Lock { 2 int[] level; 3 int[] victim; 4 public Filter(intn) { 5 level = new int[n]; 6 victim = new int[n]; // use 1..n-1 7 for (inti = 0; i< n; i++) { 8 level[i] = 0; 9} 10 } 11 public void lock() { 12 intme = ThreadID.get(); 13 for (inti = 1; i< n; i++) { //attempt level 1 14 level[me] = i; 15 victim[i] = me; 16 // spin while conflicts exist 17 while ((∃k != me) (level[k] >= i&& victim[i] == me)) {}; 18 } 19 } 20 public void unlock() { 21 intme = ThreadID.get(); 22 level[me] = 0; 23 } 24 } Figure 2.7 The Filter lock algorithm. 4.5.2. Fairness A doorwaysection, whose execution intervalDAconsists of a bounded numberof steps, and 2. awaitingsection, whose execution intervalWAmay take an unbounded numberof steps
  14. 14. 4.5.2. Lamport’s Bakery Algorithm 1 class Bakery implements Lock { 2 boolean[] flag; 3 Label[] label; 4 public Bakery (intn) { 5 flag = new boolean[n]; 6 label = new Label[n]; 7 for (inti = 0; i< n; i++) { 8 flag[i] = false; label[i] = 0; 9} 10 } 11 public void lock() { 12 inti = ThreadID.get(); 13 flag[i] = true; 14 label[i] = max(label[0], ...,label[n-1]) + 1; 15 while ((∃k != i)(flag[k] && (label[k],k) << (label[i],i))) {}; 16 } 17 public void unlock() { 18 flag[ThreadID.get()] = false; 19 } 20 } Figure 2.9 The Bakery lock algorithm. 4.6. Bounded Timestamps we see that a thread needs two abilities: _ to read the other threads‟ timestamps (scan), and _ to assign itself a later timestamp (label). 1 public interface Timestamp { 2 booleancompare(Timestamp); 3} 4 public interface TimestampSystem { 5 public Timestamp[] scan(); 6 public void label(Timestamp timestamp, inti); 7} Figure 2.10 Atimestamping system interface. 4.6.1. Lower Bounds on the Number of Locations Definition: A Lock object state sis inconsistent in any global state where some thread is in the critical section, but the lock state is compatible with a global state in which no thread is in the critical section or is trying to enter the critical section. Lemma2.8.1. No deadlock-free Lock algorithm can enter an inconsistent state. Proof: Suppose the Lock object is in an inconsistent state s, where no thread is in the critical section or trying to enter. If thread B tries to enter the critical section, it must eventually succeed, because the algorithm is deadlock-free. Suppose the Lock object is in an
  15. 15. inconsistent state s, where A is in the critical section. If thread B tries to enter the critical section, it must block until A leaves. We have a contradiction, because B cannot determine whether A is in the critical section. _ Any Lock algorithm that solves deadlock-free mutual exclusion must have Ndistinct locations. Here, we consider only the 3-thread case.Ndeadlock-freeLock algorithm accessed by three threads must use three distinctlocations. Definition 2.8.2.A covering state for a Lock object is one in which there is at leastone thread about to write to each shared location, but the Lock object‟s locations“look” like the critical section is empty (i.e., the locations‟ states appear as if thereis no thread either in the critical section or trying to enter the critical section). 4.7.1. Concurrency and Correctness Concurrent Objects 1 class LockBasedQueue<T> { 2 inthead, tail; 3 T[] items; 4 Lock lock; 5 public LockBasedQueue(intcapacity) { 6 head = 0; tail = 0; 7 lock = new ReentrantLock(); 8 items = (T[])new Object[capacity]; 9} 10 public void enq(T x) throws FullException { 11 lock.lock(); 12 try { 13 if (tail - head == items.length) 14 throw new FullException(); 15 items[tail % items.length] = x; 16 tail++; 17 }finally{ 18 lock.unlock(); 19 } 20 } 21 public T deq() throws EmptyException { 22 lock.lock(); 23 try { 24 if (tail == head) 25 throw new EmptyException(); 26 T x = items[head % items.length]; 27 head++; 28 return x; 29 }finally{ 30 lock.unlock(); 31 } 32 } 33 } Figure 3.1 A lock-based FIFO queue. The queue‟s items are kept in an array items, where headis the index of the next item to dequeue, and tail is the index of the first open array
  16. 16. slot (modulo the capacity). The lock field is a lock that ensures that methods are mutually exclusive. Initially head and tail are zero, and the queue is empty. If enq() finds the queue is full, i.e., head and tail differ by the queue size, then it throws an exception. Otherwise, there is room, so enq() stores the item at array entry tail, and then increments tail. The deq() method works in a symmetric way. A single-enqueuer/single-dequeuer FIFO queue. 1 class WaitFreeQueue<T> { 2 volatile inthead = 0, tail = 0; 3 T[] items; 4 public WaitFreeQueue(intcapacity) { 5 items = (T[])new Object[capacity]; 6 head = 0; tail = 0; 7} 8 public void enq(T x) throws FullException { 9 if (tail - head == items.length) 10 throw new FullException(); 11 items[tail % items.length] = x; 12 tail++; 13 } 14 public T deq() throws EmptyException { 15 if (tail - head == 0) 16 throw new EmptyException(); 17 T x = items[head % items.length]; 18 head++; 19 return x; 20 } 21 } 4.7.4. Sequential Consistency An execution is sequentially consistent if the method calls can be correctly arranged retaining the mutual order of method calls in eachthread.The calls in different threads can be re-arranged however you wish, regardless of when they start and when they end. An example of a sequentially consistent system is a naive notion of RAM in multi-core processor. When different threads read from the memory, they actually read from a cache or a register, which is synchronized with a common RAM at writes. While values read and written on different cores may not be immediately synchronized (cores may diverge in the notion of what value a cell contains), the values read and written on the same core are always consistent with each other. This is naive, though: even this guarantee is relaxed by modern compilers, and accesses to different variables within one thread can be reordered. You might notice that the definition of sequential consistency does not directly prevents methods to return values from the future (i.e. to read what will be written later in a different thread). Yet I claim that RAM is sequentially consistent; how's that? The thing is that each execution should be sequentially consistent. If read and write do not overlap, and write
  17. 17. follows the read, we may just stop the program between them. Since each execution should be consistent, the value read should be written before in some thread. Therefore, this "future prediction" is not possible in consistent data structures (not only those that are "sequentially" consistent) even that the definition doesn't directly prevent it. However, if we combine two sequentially consistent executions, the result won't be sequentially consistent. But there is another model, weaker than linearizability, that makes it possible. 4.7.3. Quiescent Consistency An execution is quiescently consistent if the method calls can be correctly arranged retaining the mutual order of calls separated by quiescence, a period of time where no method is being called in any thread. This consistency condition is stronger than sequential consistency, but is still not strong enough that we usually expect from a multithreaded system. The reason to use this model is to attain a higher degree of concurrency and better response time in return of weakening consistency a bit. Quiescent consistency is not as esoteric as you might think, especially since you barely heard about it. Here's an easy way to imagine a quiescently consistent system: assume that all writes are cached, and the cache is flushed to a persistent storage when there are no ongoing writes (or after it reaches certain size). There are specialdatastructures that are quiescently consistent.Quiescent consistency components can be combined to form quiescently consistent components again. Note also, that strong consistency implies quiescent consistency, hence you can combine these two kinds of systems, and the result will be quiescently consistent. If you're a system developer, you should note that you are not bound to choose between linearizability and "thread-unsafely" when designing a library. The models presented above may be viewed as patterns that enumerate what behaviors in multithreaded context are considered universally useful. Just consider carefully what your users need, and do not forget to specify which model your system follows. It will also help to link to something that explains what lies behind the words "something consistent". 4.8. LINEARIZABILITY A concurrent object is a data object shared by concurrent processes. Linearizability is a correctness condition for concurrent objects that exploits the semantics of abstract data types. It permits a high degree of concurrency, yet it permits programmers to specify and reason about concurrent objects using known techniques from the sequential domain. Linearizability provides the illusion that each operation applied by concurrent processes takes effect instantaneously at some point between its invocation and its response, implying that the meaning of a concurrent object‟s operations can be given by pre- and post-conditions. 4.9. JAVA MEMORY MODEL: The Java memory model describes how threads in the Java programming language interact through memory. Together with the description of single-threaded execution of code,
  18. 18. the memory model provides the semantics of the Java programming language. The Java programming language and platform provide thread capabilities. Synchronization between threads is notoriously difficult for developers; this difficulty is compounded because Java applications can run on a wide range of processors and operating systems. To be able to draw conclusions about a program's behavior, Java's designers decided they had to clearly define possible behaviors of all Java programs. On modern platforms, code is frequently not executed in the order it was written. It is reordered by the compiler, the processor and the memory subsystem to achieve maximum performance. On multiprocessor architectures, individual processors may have their own local caches that are out of sync with main memory. It is generally undesirable to require threads to remain perfectly in sync with one another because this would be too costly from a performance point of view. This means that at any given time, different threads may see different values for the same shared data. In a single-threaded environment, it is easy to reason about code execution. The typical approach requires the system to implement as-if-serial semantics for individual threads in isolation. When an individual thread executes, it will appear as if all of the actions taken by that thread occur in the order they appear in the program, even if the actions themselves occur out of order. If one thread executes its instructions out of order, then another thread might see the fact that those instructions were executed out of order, even if that did not affect the semantics of the first thread. For example, consider two threads with the following instructions, executing concurrently, where the variables x and y are both initialized to 0: Thread 1 Thread 2 x = 1; int r1 = y; y = 2; int r2 = x; If no reorderings are performed, and the read of y in Thread 2 returns the value 2, then the subsequent read of x should return the value 1, because the write to x was performed before the write to y. However, if the two writes are reordered, then the read of y can return the value 2, and the read of x can return the value 0. The Java Memory Model (JMM) defines the allowable behavior of multithreaded programs, and therefore describes when such reorderings are possible. It places execution-time constraints on the relationship between threads and main memory in order to achieve consistent and reliable Java applications. By doing this, it makes it possible to reason about code execution in a multithreaded environment, even in the face of optimizations performed by the dynamic compiler, the processor(s) and the caches. For execution of a single thread, the rules are simple. The Java Language Specification requires a Java Virtual Machine to observe within-thread as-if-serial semantics. The runtime (which, in this case, usually refers to the dynamic compiler, the processor and the memory subsystem) is free to introduce any useful execution optimizations as long as the result of the thread in isolation is guaranteed to be exactly the same as it would have been had all the
  19. 19. statements been executed in the order the statements occurred in the program (also called program order). The major caveat of this is that as-if-serial semantics do not prevent different threads from having different views of the data. The memory model provides clear guidance about what values are allowed to be returned when the data are read. The basic rules imply that individual actions can be reordered, as long as the as-if-serial semantics of the thread are not violated, and actions that imply communication between threads, such as the acquisition or release of a lock, ensure that actions that happen prior to them are seen by other threads that see their effects. For example, everything that happens before the release of a lock will be seen to be ordered before and visible to everything that happens after a subsequent acquisition of that same lock. Mathematically, there is a partial order called the happens-before order over all actions performed by the program. The happens-before order subsumes the program order; if one action occurs before another in the program order, it will occur before the other in the happens-before order. In addition, releases and subsequent acquires of locks form edges in the happens-before graph. A read is allowed to return the value of a write if that write is the last write to that variable before the read along some path in the happens-before order, or if the write is not ordered with respect to that read in the happens-before order. The Java memory model was the first attempt to provide a comprehensive memory model for a popular programming language.