In parallel computing, a barrier is a type of synchronization method. A barrier for a group of threads or processes in the source code means any thread/process must stop at this point and cannot proceed until all other threads/processes reach this barrier.[1]
Many collective routines and directive-based parallel languages impose implicit barriers. For example, a parallel do loop in Fortran with OpenMP will not be allowed to continue on any thread until the last iteration is completed. This is in case the program relies on the result of the loop immediately after its completion. In message passing, any global communication (such as reduction or scatter) may imply a barrier.
In concurrent computing, a barrier may be in a raised or lowered state. The term latch is sometimes used to refer to a barrier that starts in the raised state and cannot be re-raised once it is in the lowered state. The term count-down latch is sometimes used to refer to a latch that is automatically lowered once a predetermined number of threads/processes have arrived.
Take an example for thread, known as the thread barrier. The thread barrier needs a variable to keep track of the total number of threads that have entered the barrier.[2] Whenever there are enough threads enter the barrier, it will be lifted. A synchronization primitive like mutex is also needed when implementing the thread barrier.
This thread barrier method is also known as Centralized Barrier as the threads have to wait in front of a "central barrier" until the expected number of threads have reached the barrier before it is lifted.
The following C code, which implemented thread barrier by using POSIX Threads will demonstrate this procedure:[3]
typedef struct _thread_barrier thread_barrier;
thread_barrier barrier;
void thread_barrier_init(thread_barrier *barrier, pthread_mutexattr_t *mutex_attr, int thread_barrier_number)
void thread_barrier_wait(thread_barrier *barrier)
void thread_barrier_destroy(thread_barrier *barrier)
void *thread_func(void *ptr)
int main
In this program, the thread barrier is defined as a struct, struct _thread_barrier, which include:
Based on the definition of barrier, we need to implement a function like thread_barrier_wait in this program which will "monitor" the total number of thread in the program in order to life the barrier.
In this program, every thread calls thread_barrier_wait will be blocked until THREAD_BARRIERS_NUMBER threads reach the thread barrier. The result of that program is:
Change TOTAL_THREADS to 3 and the thread barrier is lifted:
Beside decreasing the total thread number by one for every thread successfully passing the thread barrier, thread barrier can use opposite values to mark for every thread state as passing or stopping.[4] For example, thread 1 with state value is 0 means it's stopping at the barrier, thread 2 with state value is 1 means it has passed the barrier, thread 3's state value = 0 means it's stopping at the barrier and so on.[5] This is known as Sense-Reversal.
The following C code demonstrates this:[6]
typedef struct _thread_barrier thread_barrier;
thread_barrier barrier;
void thread_barrier_init(thread_barrier *barrier, pthread_mutexattr_t *mutex_attr, int thread_barrier_number)
void thread_barrier_wait(thread_barrier *barrier)
void thread_barrier_destroy(thread_barrier *barrier)
void *thread_func(void *ptr)
int mainThis program has all features similar to the previous Centralized Barrier source code. It just only implements in a different way by using 2 new variables:
When a thread stops at the barrier, local_sense'''local_sense''' variable.
When there are exactly THREAD_BARRIERS_NUMBER threads stopping at the thread barrier, the total thread number is reset to 0, and the flag is set to '''local_sense'''.
The potential problem with the Centralized Barrier is that due to all the threads repeatedly accessing the global variable for pass/stop, the communication traffic is rather high, which decreases the scalability.
This problem can be resolved by regrouping the threads and using multi-level barrier, e.g. Combining Tree Barrier. Also hardware implementations may have the advantage of higher scalability.
A Combining Tree Barrier is a hierarchical way of implementing barrier to resolve the scalability by avoiding the case that all threads are spinning at the same location.
In k-Tree Barrier, all threads are equally divided into subgroups of k threads and a first-round synchronizations are done within these subgroups. Once all subgroups have done their synchronizations, the first thread in each subgroup enters the second level for further synchronization. In the second level, like in the first level, the threads form new subgroups of k threads and synchronize within groups, sending out one thread in each subgroup to next level and so on. Eventually, in the final level there is only one subgroup to be synchronized. After the final-level synchronization, the releasing signal is transmitted to upper levels and all threads get past the barrier.[7]
The hardware barrier uses hardware to implement the above basic barrier model.
The simplest hardware implementation uses dedicated wires to transmit signal to implement barrier. This dedicated wire performs OR/AND operation to act as the pass/block flags and thread counter. For small systems, such a model works and communication speed is not a major concern. In large multiprocessor systems this hardware design can make barrier implementation have high latency. The network connection among processors is one implementation to lower the latency, which is analogous to Combining Tree Barrier.[8]
POSIX Threads standard directly supports thread barrier functions which can be used to block the specified threads or the whole process at the barrier until other threads to reach that barrier. 3 main API supports by POSIX to implement thread barriers are:
pthread_barrier_t barrier;
void *thread_func(void *ptr)
int main
Change TOTAL_THREADS to 3 and the thread barrier is lifted:
pthread_barrier_t barrier;
void *thread_func(void *ptr)
int mainThis example doesn't use to wait for 2 "newly created" threads to complete. It calls inside, in order to block the main thread, so that the process will be blocked until 2 threads finish its operation after 5 seconds wait (line 9 -).
Web site: Parallel Programming with Barrier Synchronization. sourceallies.com. March 2012.