tf::Semaphore class

class to create a semophore object for building a concurrency constraint

A semaphore creates a constraint that limits the maximum concurrency, i.e., the number of workers, in a set of tasks. You can let a task acquire/release one or multiple semaphores before/after executing its work. A task can acquire and release a semaphore, or just acquire or just release it. A tf::Semaphore object starts with an initial count. As long as that count is above 0, tasks can acquire the semaphore and do their work. If the count is 0 or less, a task trying to acquire the semaphore will not run but goes to a waiting list of that semaphore. When the semaphore is released by another task, it reschedules all tasks on that waiting list.

tf::Executor executor(8);   // create an executor of 8 workers
tf::Taskflow taskflow;

tf::Semaphore semaphore(1); // create a semaphore with initial count 1

std::vector<tf::Task> tasks {
  taskflow.emplace([](){ std::cout << "A" << std::endl; }),
  taskflow.emplace([](){ std::cout << "B" << std::endl; }),
  taskflow.emplace([](){ std::cout << "C" << std::endl; }),
  taskflow.emplace([](){ std::cout << "D" << std::endl; }),
  taskflow.emplace([](){ std::cout << "E" << std::endl; })
};

for(auto & task : tasks) {  // each task acquires and release the semaphore
  task.acquire(semaphore);
  task.release(semaphore);
}

executor.run(taskflow).wait();

The above example creates five tasks with no dependencies between them. Under normal circumstances, the five tasks would be executed concurrently. However, this example has a semaphore with initial count 1, and all tasks need to acquire that semaphore before running and release that semaphore after they are done. This arrangement limits the number of concurrently running tasks to only one.

tf::Semaphore semaphore(4);  // concurrency constraint of 4 workers