CSC/ECE 506 Spring 2013/3a bs

Overview

The main goal of this wiki is to explain which architectural mechanisms are used by library functions for DOALL, DOACROSS, and DOPIPE parallelism, reduction, and functional parallelism in various architectures.

Libraries

Semaphore.h

A semaphore is special variable that acts similar to a lock. For a process to enter into the critical section it must be able to acquire the semaphore. If the semaphore cannot be acquired, then the process is “put to sleep” and the processor is then used for another process. This means the processes cache is saved off in a place where it can be retrieved when the process is “woken up”. Once the semaphore is available the “sleeping” process is woken up and obtains the semaphore and proceeds in to the critical section.A simple way to execute a semaphore would be to use the following functions1;

Initializing a semaphore

int sem_init(sem_t *sem, int pshared, unsigned int value): This function initializes the unnamed semaphore at the address pointed to by sem. Value is the initial value of the semaphore. The variable pshared is indicative of whether the semaphore is shared between threads or processes. If its value is zero it is shared between threads else it is shared between processes.

Return Value: sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.

Locking the semaphore

1. int sem_wait(sem_t *sem): This function decrements (locks) the semaphore pointed to by sem. Decrement will only proceed if the value of the semaphore is greater than zero. If its value is zero, then the call blocks till the value of the semaphore becomes positive so that it can acquire it or a signal handler interrupts the call.

2. int sem_trywait(sem_t *sem): This function is the same as sem_wait(), except that if the decrement cannot be immediately performed, then call returns an error (errno set to EAGAIN) instead of blocking.

3. int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout): This function is also the same as sem_wait(), except that abs_timeout specifies a limit on the amount of time that the call should block if the decrement cannot be immediately performed. The abs_timeout argument points to a structure that specifies an absolute timeout in seconds and nanoseconds. This structure is defined as follows:

       struct timespec {
   	time_t tv_sec;      /* Seconds */
   	long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */};

If the timeout has already expired by the time of the call, and the semaphore could not be locked immediately, then sem_timedwait() fails with a timeout error (errno set toETIMEDOUT). If the operation can be performed immediately, then sem_timedwait() never fails with a timeout error, regardless of the value of abs_timeout. Furthermore, the validity ofabs_timeout is not checked in this case.

Return Value All of these functions return 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.

Releasing the semaphore

int sem_post(sem_t *sem):This function increments (unlocks) the semaphore pointed to by sem, thus making the value of the semaphore positive. Some other process can now acquire it.

Return value sem_post() returns 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.

Pseudo Code

     sem_t *sem; 
     int pshared;
     unsigned int value;
     int i = sem_init(sem, pshared, value); /*initialize the semaphore*/
     int wait=sem_wait(sem); /*will decrement the value of the semaphore i.e. acquire the lock */
     if(wait==-1)
     printf(“Error occurred, the value of the semaphore was not decremented”);
     /*critical section;*/
     int post=sem_post(sem); /*will increment the value of the semaphore i.e. release the lock*/
     if(post==-1)
     printf(“Error occurred, the value of the semaphore was not incremented”);

Pthread.h

The pthread library provides three synchronization mechanisms.

Mutexes

Mutual Exclusion Lock, mutex in short is another synchronization method and is used to avoid race conditions. Mutexes are used to prevent operations by multiple threads on the same memory location at the same time or when an order of operation is expected which would lead to data inconsistencies. It blocks access to variables by other threads. Mutexes are in particular used to protect a critical region (“a segment of memory”) from other threads. Mutexes work only between threads in a single process and donot work between processes as do semaphores.

The following are the functions for managing mutexes:

Initialising the mutex

pthread_mutex_init (pthread_mutex_t *restrict mutex,const pthread_mutexattr_t *restrict attr): This function is used to initialize the mutex referenced by the mutex with attributes specified by the attribute. The default mutex attributes are used if attribute passed is NULL which is same as passing the address of a default mutex. The state of the mutex is initialized and unlocked, upon successful initialization. A destroyed mutex object can be reinitialized using pthread_mutex_init().Attempting to initialize an already initialized mutex results in undefined behavior. In cases where default mutex attributes are appropriate, the macro PTHREAD_MUTEX_INITIALIZER can be used to initialize mutexes that are statically allocated. It is dynamic initialization by a call to pthread_mutex_init() with parameter attr specified as NULL, except that no error checks are performed.

Return Value The function returns zero on success; otherwise, an error number is returned to indicate the error.

Destroying the mutex

pthread_mutex_destroy (pthread_mutex_t *mutex) This function is used to destroy a mutex which is no longer needed. The function destroys the mutex object referenced by mutex and hence the mutex object becomes uninitialized. pthread_mutex_destroy() may set the object referenced by mutex to an invalid value. A destroyed mutex object can be reinitialized using pthread_mutex_init().A mutex cannot be referenced after it is destroyed. It shall be safe to destroy an initialized mutex that is unlocked. Attempting to destroy a locked mutex results in undefined behavior.

Only mutex itself may be used for performing synchronization. The result of referring to copies of mutex in calls to pthread_mutex_lock(), pthread_mutex_trylock(), pthread_mutex_unlock(), and pthread_mutex_destroy() is undefined.

Return Value The function returns zero on success; otherwise, an error number is returned to indicate the error.

Locking the mutex

pthread_mutex_lock (pthread_mutex_t *mutex): A mutex object referenced by the mutex can be locked using this function. If the mutex is already locked, the calling thread shall block until the mutex becomes available. This operation shall return with the mutex object referenced by mutex in the locked state with the calling thread as its owner.

If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall not be provided. Attempting to relock the mutex causes deadlock. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior results.

Error checking is provided if the mutex is of type PTHREAD_MUTEX_ERRORCHECK. Error shall be returned if a thread attempts to relock a mutex that it has already locked or if a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked.

If the mutex is of type PTHREAD_MUTEX_RECURSIVE, it shall maintain the concept of a lock count. The lock count shall be set to one when a thread successfully acquires a mutex for the first time. And the lock count shall be incremented by one every time a thread relocks this mutex. Each time the thread unlocks the mutex, the lock count shall be decremented by one. When the lock count reaches zero, the mutex shall become available for other threads to acquire. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error shall be returned.

If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively lock the mutex or attempting to unlock the mutex if it was not locked by the calling thread or attempting to unlock the mutex if it is not locked results in undefined behavior.

If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread shall resume waiting for the mutex as if it was not interrupted.

Return Value The functions return a zero on success; otherwise, an error number is returned to indicate the error.

Unlocking the mutex

pthread_mutex_unlock (pthread_mutex_t *mutex): This function is used to release a mutex that is previously locked.The function shall release the mutex object referenced by mutex. The manner in which a mutex is released is dependent upon the mutex's type attribute.If there are threads blocked on the mutex object referenced by mutex when pthread_mutex_unlock() is called, resulting in the mutex becoming available, the scheduling policy shall determine which thread shall acquire the mutex. An error is returned if mutex is already unlocked or owned by another thread.

Return Value The functions return a zero on success; otherwise, an error number is returned to indicate the error.

Pseudo code

    pthread_mutex_t *mutex,
    const pthread_mutexattr_t *attr;
    int p = pthread_mutex_init(mutex, attr);
    if(p!=0)
    printf(“Error occurred mutex was not created”);
    int pl = pthread_mutex_lock(mutex);
    if(pl!=0)
    printf(“Error occurred mutex was not locked”);
    //critical section
    int pu = pthread_mutex_unlock(mutex);
    if(pu!=0)
    printf(“Error occurred mutex was not unlocked”);
    int pd = pthread_mutex_destroy(mutex);
    if(pd!=0)
    printf(“Error occurred mutex was not destroyed”);

Joins

A join is performed when one wants to wait for a thread to finish. A thread calling routine may launch multiple threads and then wait for them to finish to get the results.

pthread_join(pthread_t thread,void **value_ptr): This function determines if a thread has completed before starting another task. The argument,thread refers to the thread id and value_ptr refers to the value passed from pthread_exit. This function suspends the execution of the calling thread until the target thread terminates, unless the target thread has already terminated.On return from a successful pthread_join() call with a non-NULL value_ptr argument, the value passed to pthread_exit() by the terminating thread shall be made available in the location referenced by value_ptr.When a pthread_join() returns successfully, the target thread has been terminated. The results of multiple simultaneous calls to pthread_join() specifying the same target thread are undefined. If the thread calling pthread_join() is canceled, then the target thread shall not be detached.

Return Value: The function returns zero on success; otherwise, the error number is returned to indicate the error.

Conditional Variables

Condition variables are synchronization objects that allow threads to wait for certain events (conditions) to occur. Condition variables are slightly more complex than mutexes, and the correct use of condition variables requires the thread to co-operatively use a specific protocol in order to ensure safe and consistent serialization. The protocol for using condition variables includes a mutex, a boolean predicate (true/false expression) and the condition variable itself. The threads that are cooperating using condition variables can wait for a condition to occur, or can wake up other threads that are waiting for a condition.

The following are the functions used in conjunction with the conditional variable:

Creating/Destroying

int pthread_cond_init(pthread_cond_t *restrict cond, const pthread_condattr_t *restrict attr): This function is used to initialize the conditional variable referenced by 'cond' with attributes referenced by 'attr'. Default condition variable attributes shall be used if attribute is NULL.It is same as passing the address of a default condition variable attributes object.Upon successful initialization, the conditional variable state is initialized. Attempting to initialize an already initialized condition variable results in undefined behavior.

Return Value:If successful the function returns zero; otherwise, an error number is reurned to indicate the error.

pthread_cond_destroy(pthread_cond_t *cond):This function shall destroy the given condition variable specified by cond; the object becomes, in effect, uninitialized. An implementation may cause pthread_cond_destroy() to set the object referenced by cond to an invalid value. A destroyed condition variable object can be reinitialized using pthread_cond_init(); the results of otherwise referencing the object after it has been destroyed are undefined. It shall be safe to destroy an initialized condition variable upon which no threads are currently blocked. Attempting to destroy a condition variable upon which other threads are currently blocked results in undefined behavior.

Return Value:If successful the function returns zero; otherwise, an error number is reurned to indicate the error.

For example, consider the following code:

     struct list {
     pthread_mutex_t lm;
     ...
     }
     struct elt {
     key k;
     int busy;
     pthread_cond_t notbusy;
     ...
     }
     /* Find a list element and reserve it. */
     struct elt *
     list_find(struct list *lp, key k)
     {
     struct elt *ep;
     pthread_mutex_lock(&lp->lm);
     while ((ep = find_elt(l, k) != NULL) && ep->busy)
     pthread_cond_wait(&ep->notbusy, &lp->lm);
     if (ep != NULL)
     ep->busy = 1;
     pthread_mutex_unlock(&lp->lm);
     return(ep);
     }
     delete_elt(struct list *lp, struct elt *ep)
     {
     pthread_mutex_lock(&lp->lm);
     assert(ep->busy);
     ... remove ep from list ...
     ep->busy = 0;	 /* Paranoid. */
    (A) pthread_cond_broadcast(&ep->notbusy);
     pthread_mutex_unlock(&lp->lm);
    (B) pthread_cond_destroy(&rp->notbusy);
     free(ep);      }

In this example, the condition variable and its list element may be freed (line B) immediately after all threads waiting for it are awakened (line A), since the mutex and the code ensure that no other thread can touch the element to be deleted.

Waiting on condition

int pthread_cond_timedwait(pthread_cond_t *restrict cond,pthread_mutex_t *restrict mutex, const struct timespec *restrict abstime); and int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex): These functions are used to shall block on a condition variable. They shall be called with mutex locked by the calling thread or undefined behavior results.These functions atomically release mutex and cause the calling thread to block on the condition variable cond; atomically here means "atomi- cally with respect to access by another thread to the mutex and then the condition variable". That is, if another thread is able to acquire the mutex after the about-to-block thread has released it, then a subsequent call to pthread_cond_broadcast() or pthread_cond_signal() in that thread shall behave as if it were issued after the about-to-block thread has blocked.

Upon successful return, the mutex shall have been locked and shall be owned by the calling thread.

Waking thread based on condition

pthread_cond_broadcast(pthread_cond_t *cond); and pthread_cond_signal(pthread_cond_t *cond); These functions shall unblock threads blocked on a condition variable.The pthread_cond_broadcast() function shall unblock all threads currently blocked on the specified condition variable cond.

The pthread_cond_signal() function shall unblock at least one of the threads that are blocked on the specified condition variable cond (if any threads are blocked on cond).

If more than one thread is blocked on a condition variable, the scheduling policy shall determine the order in which threads are unblocked.When each thread unblocked as a result of a pthread_cond_broadcast() or pthread_cond_signal() returns from its call to pthread_cond_wait() or pthread_cond_timedwait(), the thread shall own the mutex with which it called pthread_cond_wait() or pthread_cond_timedwait().The thread(s) that are unblocked shall contend for the mutex according to the scheduling policy and as if each had called pthread_mutex_lock().

Barriers

A barrier is a type of synchronization method which is used for a group of threads or processes in the source code. It basically stops any thread/process at a certain point till all other threads/processes reach it. Only then are the processes allowed to proceed.

Barrier Wait

int pthread_barrier_wait(pthread_barrier_t *barrier): This function synchronizes participating threads at the barrier referenced by 'barrier'. The calling thread shall block until the required number of threads have called pthread_barrier_wait() specifying the barrier.

Once the required number of threads have referenced the function, the participating threads are allowed to proceed. To accomplish this task 'PTHREAD_BARRIER_SERIAL_THREAD' is returned to an unspecified thread and zero is returned to the remaining threads. The barrier is reset to the state it had as a result of the most recent pthread_barrier_init() function that referenced it.

The constant PTHREAD_BARRIER_SERIAL_THREAD is defined in <pthread.h> and its value shall be distinct from any other value returned by pthread_barrier_wait().The results are undefined if this function is called with an uninitialized barrier.

If a signal is delivered to a thread blocked on a barrier, upon return from the signal handler the thread shall resume waiting at the barrier if the barrier wait has not completed (that is, if the required number of threads have not arrived at the barrier during the execution of the signal handler); otherwise, the thread shall continue as normal from the completed barrier wait. Until the thread in the signal handler returns from it, it is unspecified whether other threads may proceed past the barrier once they have all reached it.

A thread that has blocked on a barrier shall not prevent any unblocked thread that is eligible to use the same processing resources from eventually making forward progress in its execution. Eligibility for processing resources shall be determined by the scheduling policy.

Return ValueUpon successful completion, the pthread_barrier_wait() function shall return PTHREAD_BARRIER_SERIAL_THREAD for a single (arbitrary) thread synchronized at the barrier and zero for each of the other threads. Otherwise, an error number shall be returned to indicate the error.

Destroying the Barrier

int pthread_barrier_destroy(pthread_barrier_t *barrier);The pthread_barrier_destroy() function shall destroy the barrier referenced by barrier and release any resources used by the barrier. The effect of subsequent use of the barrier is undefined until the barrier is reinitialized by another call to pthread_barrier_init(). An implementation may use this function to set barrier to an invalid value. The results are undefined if pthread_barrier_destroy() is called when any thread is blocked on the barrier, or if this function is called with an uninitialized barrier.

Initialising the Barrier

int pthread_barrier_init(pthread_barrier_t *restrict barrier, const pthread_barrierattr_t *restrict attr, unsigned count);The pthread_barrier_init() function shall allocate any resources required to use the barrier referenced by barrier and shall initialize the barrier with attributes referenced by attr. If attr is NULL, the default barrier attributes shall be used; the effect is the same as passing the address of a default barrier attributes object. The results are undefined if pthread_barrier_init() is called when any thread is blocked on the barrier (that is, has not returned from the pthread_barrier_wait() call). The results are undefined if a barrier is used without first being initialized. The results are undefined if pthread_barrier_init() is called specifying an already initialized barrier.

The count argument specifies the number of threads that must call pthread_barrier_wait() before any of them successfully return from the call. The value specified by count must be greater than zero.

If the pthread_barrier_init() function fails, the barrier shall not be initialized and the contents of barrier are undefined.

Only the object referenced by barrier may be used for performing synchronization. The result of referring to copies of that object in calls to pthread_barrier_destroy() or pthread_barrier_wait() is undefined.

Return ValueUpon successful completion, these functions shall return zero; otherwise, an error number shall be returned to indicate the error.

Pseudo code

   pthread_barrier_t *barrier;
   pthread_barrierattr_t *attr;
   unsigned int count;
   int i = pthread_barrier_init(barrier, attr, count); // initialize the barrier
   if(i!=0)
   printf(“Error occurred barrier was not initialized”):
   int b = pthread_barrier_wait(barrier); //synchronize participating threads
   if(b!=0)
   printf(“Error occurred in synchronizing threads”);
   // critical section
   int d = pthread_barrier_destroy(barrier); //destroy the barrier
   if(d!=0)
   printf(“Error occurred barrier was not destroyed”):

References:

[1] Linux Manual pages

[2]http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=%2Fapis%2Fusers_68.html

[3]http://maxim.int.ru/bookshelf/PthreadsProgram/toc.html

[4]http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html#BASICS