thread-pool v1.4

• Version 1.4 (2021-05-05)
  • Added three new public member functions to monitor the tasks submitted to the pool:
    • get_tasks_queued() gets the number of tasks currently waiting in the queue to be executed by the threads.
    • get_tasks_running() gets the number of tasks currently being executed by the threads.
    • get_tasks_total() gets the total number of unfinished tasks - either still in the queue, or running in a thread.
    • Note that get_tasks_running() == get_tasks_total() - get_tasks_queued().
    • Renamed the private member variable tasks_waiting to tasks_total to make its purpose clearer.
  • Added an option to temporarily pause the workers:
    • When public member variable paused is set to true, the workers temporarily stop popping new tasks out of the queue, although any tasks already executed will keep running until they are done. Set to false again to resume popping tasks.
    • While the workers are paused, wait_for_tasks() will wait for the running tasks instead of all tasks (otherwise it would wait forever).
    • By utilizing the new pausing mechanism, reset() can now change the number of threads on-the-fly while there are still tasks waiting in the queue. The new thread pool will resume executing tasks from the queue once it is created.
  • parallelize_loop() and wait_for_tasks() now have the same behavior as the worker function with regards to waiting for tasks to complete. If the relevant tasks are not yet complete, then before checking again, they will sleep for sleep_duration microseconds, unless that variable is set to zero, in which case they will call std::this_thread::yield(). This should improve performance and reduce CPU usage.
  • Merged this commit: Fixed weird error when using MSVC and including windows.h.
  • The README.md file has been reorganized and expanded.