BS::thread_pool v3.5.0

v3.5.0 (2023-05-25)

• BS_thread_pool.hpp and BS_thread_pool_light.hpp:
  • Added a new member function, purge(), to the full (non-light) thread pool. This function purges all the tasks waiting in the queue. Tasks that are currently running will not be affected, but any tasks still waiting in the queue will be removed and will never be executed by the threads. Please note that there is no way to restore the purged tasks.
  • Fix a bug which caused wait_for_tasks() to only block the first thread that called it. Now it blocks every thread that calls it, which is the expected behavior. In addition, all related deadlock have now been completely resolved. This also applies to the variants wait_for_tasks_duration() and wait_for_tasks_until() in the non-light version. See #110.
    • Note: You should never call wait_for_tasks() from within a thread of the same thread pool, as that will cause it to wait forever! This fix is relevant for situations when wait_for_tasks() is called from an auxiliary std::thread or a separate thread pool.
  • push_task() and submit() now avoid creating unnecessary copies of the function object. This should improve performance, especially if large objects are involved. See #90.
  • Optimized the way condition variables are used by the thread pool class. Shared variables are now modified while owning the mutex, but condition variables are notified after the mutex is released, if possible. See #84.
  • Instead of a variable tasks_total to keep track of the total number of tasks (queued + running), the thread pool class now uses a variable tasks_running to keep track only of the number of running tasks, with the number of tasks in the queue obtained via tasks.size(). This makes more sense in terms of the internal logic of the class.
  • All atomic variables have been converted to non-atomic. They are now all governed by tasks_mutex, so they do not need to be atomic. This eliminates redundant locking, and may improve performance a bit.
  • running has been renamed to workers_running and task_done_cv has been renamed to tasks_done_cv.
  • The worker now only notifies thi condition variable tasks_done_cv if all the tasks are done, not just a single task. Checking if the tasks are done is cheaper than notifying the condition variable, so since the worker no longer notifies the condition variable every single time it finishes a task, this should improve performance a bit if wait_for_tasks() is used.
• BS_thread_pool_test.cpp:
  • Combined the tests for the full and light versions into one program. The file BS_thread_pool_light_test.cpp has been removed.
  • The tests for the light version are now much more comprehensive. The only features that are not tested in the light version are those that do not exist in it.
  • Added a test for the new purge() member function.
  • Added a test to ensure that push_task() and submit() do not create unnecessary copies of the function object.
  • Added a test to ensure that push_task() and submit() correctly accept arguments passed by value, reference, and constant reference.
  • Added a test to ensure that wait_for_tasks() blocks all external threads that call it.
  • _CRT_SECURE_NO_WARNINGS is now set only if it has not already been defined, to prevent errors in MSVC projects which already have it set as part of the default build settings. See #72.
• README.md:
  • Added documentation for the new purge() member function.
  • Added an explanation for how to pass arguments by reference or constant reference when submitting functions to the queue, using the wrappers std::ref() and std::cref() respectively. See #83.
  • Added a link to my lecture notes for a course taught at McMaster University, for the benefit of beginner C++ programmers who wish to learn some of the advanced techniques and programming practices used in developing this library.
  • Removed the sample test results, since the complete log file (including the deadlock tests) is now over 500 lines long.
• Other:
  • A .clang-format file with the project's formatting conventions is now included in the GitHub repository. The pull request template now asks to format any new code using this file, so that it is consistent with the rest of the library.
  • A PowerShell script, BS_thread_pool_test.ps1, is now provided in the GitHub repository to make running the test on multiple compilers and operating systems easier. Since it is written in PowerShell, it is fully portable and works on Windows, Linux, and macOS. The script will automatically detect if Clang, GCC, and/or MSVC are available, compile the test program using each available compiler, and then run each compiled test program 5 times and report on any errors. The pull request template now recommends using this script for testing.
  • Since the root folder has become a bit crowded, the header files BS_thread_pool.hpp and BS_thread_pool_light.hpp have been moved to the include subfolder, and the test file BS_thread_pool_test.cpp has been moved to the tests subfolder, which also contains the new test script BS_thread_pool_test.ps1.