CSC/ECE 506 Spring 2013/3a bs

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Page started with: http://wiki.expertiza.ncsu.edu/index.php/CSC/ECE_506_Spring_2011/ch3_ab

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.

Synchronization

When using any parallel programming model, synchronization is needed to guarantee accuracy of the overall program. The following are a few example situations where synchronization will be needed.

• The code following the parallelized loop requires that all of the parallel processes be completed before advancing. It cannot be triggered simply by one of the processes completing.

• A portion of code in the middle of a parallelized section MUST be executed in a very particular order so that global variables used across processes get read and written in the proper order. This is known as the critical section.

• Multiple processes must update a global variable in such a way that one process does not overwrite the updates of a different process. (i.e. SUM = SUM + <process update>).

These are just a few examples. Every architecture implements synchronization in a unique way using different types of mechanisms. The subsequent section will highlight various libraries that are used to achieve synchronization.

Libraries<ref>http://linux.die.net/man/3/</ref>

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 functions;

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

• 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.

• 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.

• 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 to ETIMEDOUT). 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 of abs_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

POSIX threads<ref>http://maxim.int.ru/bookshelf/PthreadsProgram/toc.html</ref>, usually referred to as pthreads defines a set of programming language types,functions and constants.It is implemented with a pthread.h header file and a thread library. The pthread library provides the following synchronization mechanisms:

1. Mutexes

2. Joins

3. Conditional Variables

4. Barriers

Mutexes

Mutual Exclusion Lock, mutex in short is another synchronization method and is used to avoid race conditions. In cases leading to data inconsistencies,like when multiple threads are to be prevented from operating on the same memory location simultaneously or when a specific order of operation is expected, mutexes are used.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): The mutex referenced by the mutex is initialised with the attributes specified by attr using this function. The default mutex attributes are used if attribute passed is NULL.Passing a NULL attribute implies passing the address of a default mutex. The state of the mutex is initialized and unlocked,upon successful initialization.

The pthread_mutex_init can be used to reinitialize an already destroyed mutex.But attempting to initialize an already initialized mutex results in undefined behavior. The macro PTHREAD_MUTEX_INITIALIZER is used to initialize mutexes that are statically allocated in cases where default mutex attributes are appropriate. It is dynamic initialization by a call with parameter attr specified as NULL to pthread_mutex_init()but in this case 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 passed by mutex attribute and hence the mutex object becomes uninitialized. pthread_mutex_destroy() sets the object referenced 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 is advised to destroy an initialized mutex that is unlocked. Attempting to destroy a locked mutex results in undefined behavior.

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): This function is used to lock the mutex passed. If the mutex is already locked by another process, the calling thread blocks until the mutex is unlocked by the other process.This operation returns the mutex object referenced by mutex in the locked state with the calling thread as its owner.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.

Mutexes of type,PTHREAD_MUTEX_RECURSIVE maintain a lock count. The lock count is set to one when a mutex is acquired by a thread succesfully for the first time. And the lock count is incremented in units of one every time a thread relocks this mutex. Each time the thread unlocks the mutex, the lock count is decremented by one. When the lock count reaches zero, the mutex becomes available for other threads to acquire.An error is returned if a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked.

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”);

Avoiding Deadlock

Deadlocks occur when the program hold more than one mutex. A classic example<ref>http://www2.chrishardick.com:1099/Notes/Computing/C/pthreads/mutexes.html</ref> of deadlock is;

Thread 1:

   lock mutex_a
   |
   lock mutex_b
   -blocked forever waiting for mutex_b

Thread 2:

   lock mutex_b
   |
   lock mutex_a
   -blocked forever waiting for mutex a

Few common techniques exist which can be used to avoid deadlocks, which are;

1. Establish a locking hierarchy.

2. Spin lock.

3. Chaining.

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

Conditional variables<ref>http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp</ref> are a kind of synchronization objects that are used to allow threads to wait for certain events to occur and are slightly more complex than mutexes. In order to ensure a safe and consistent serialization, usage of condition variables requires the thread to co-operatively use a specific protocol which includes a mutex, a boolean predicate 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 destroys 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.An already 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 condition variable upon which other threads are currently blocked results in an undefined behavior.

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

For example<ref>http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html#BASICS</ref>, 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 block a condition variable. They are 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 atomically 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.The mutex would be locked and be owned by the calling thread upon successful return.

Waking thread based on condition

• pthread_cond_broadcast(pthread_cond_t *cond): and pthread_cond_signal(pthread_cond_t *cond): These functions unblock threads blocked on a condition variable.The pthread_cond_broadcast() function unblocks all threads currently blocked on the specified condition variable cond.The pthread_cond_signal() function unblocks 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 determines 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 owns the mutex with which it called pthread_cond_wait() or pthread_cond_timedwait().The thread(s) that are unblocked 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 are allowed to proceed.

Initialising the Barrier

• int pthread_barrier_init(pthread_barrier_t *restrict barrier, const pthread_barrierattr_t *restrict attr, unsigned count): The init function initializes the barrier with the specified attributes and reserves any resources required to use the barrier. Attempting to initialise an already initialized barrier or initializing a barrier when any thread is blocked on the barrier or using an uninitialized barrier would lead to undefined results.

The count argument specified the number of threads that must call before any of the them succesfully return from the call and hence it should be a positive number greater than zero.Failure of the init function results in non initialization of the barrier 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 Value: Upon successful completion, these functions shall return zero; otherwise, an error number shall be returned to indicate the error.

Barrier Wait

• int pthread_barrier_wait(pthread_barrier_t *barrier): The wait function is used to synchronize parallel threads.Until a required number of threads call pthread_barrier_wait() referring the barrier, the calling thread blocks.When the required number of threads call the barrier referenced, a zero value is returned to all the threads, except for one.The constant PTHREAD_BARRIER_SERIAL_THREAD is returned to one unspecified thread.And then it is sent to the state it has as a result of the most recent init function.

When the required number of threads have arrived at the barrier during the execution of a signal handler, it marks the completion of barrier wait.If a signal is delivered to a thread blocked on a barrier, upon return from the signal handler the thread resumes waiting at the barrier if the barrier wait has not completed; otherwise, the thread continues 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 does 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 is determined by the scheduling policy.

Return Value: Upon successful completion, the function shall return PTHREAD_BARRIER_SERIAL_THREAD for an 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):This function is used to destroy the barrier passed by the barrier attribute and also releases any resources used by the barrier. A destroyed barrier can be reused when reinitialized by another call to pthread_barrier_init().The destroyed barrier is set 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.

Return Value: Upon successful completion, the functions returns zero; otherwise, an error number is 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:

<References>http://linux.die.net/man/3/

http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp

http://maxim.int.ru/bookshelf/PthreadsProgram/toc.html

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

</References>

Quiz

1. Which of the following is true about mutexes and semaphores?

  1.Both semaphore and mutexes are used to prevent multiple threads from operating on a single memory location.
  2.Mutexes work between processes and semaphores work only between threads of a single process. 
  3.Mutexes work only between threads in a process and semaphore work between processes.
  4.None of the above.

2. Error returned when failed?

3. int pthread_barrier_wait(pthread_barrier_t *barrier) function returns ??

4. Why do we get race conditions?//

5. What parameters does the method pthread_join() take?

6. What happens if a semaphore cannot be acquired?(Choose one or more than one options)

  1. The process is woken up.
  2. The process cache is saved off and can be retrieved when the process is woken up.
  3. The process is put to sleep.
  4. The process proceeds into the critical section.