CHANGELOG.md

BS::thread_pool: a fast, lightweight, and easy-to-use C++17 thread pool library

By Barak Shoshany
Email: [email protected]
Website: https://baraksh.com/
GitHub: https://github.com/bshoshany

• Version history
  • v4.1.0 (2024-03-22)
  • v4.0.1 (2023-12-28)
  • v4.0.0 (2023-12-27)
  • v3.5.0 (2023-05-25)
  • v3.4.0 (2023-05-12)
  • v3.3.0 (2022-08-03)
  • v3.2.0 (2022-07-28)
  • v3.1.0 (2022-07-13)
  • v3.0.0 (2022-05-30)
  • v2.0.0 (2021-08-14)
  • v1.9 (2021-07-29)
  • v1.8 (2021-07-28)
  • v1.7 (2021-06-02)
  • v1.6 (2021-05-26)
  • v1.5 (2021-05-07)
  • v1.4 (2021-05-05)
  • v1.3 (2021-05-03)
  • v1.2 (2021-04-29)
  • v1.1 (2021-04-24)
  • v1.0 (2021-01-15)

Version history

v4.1.0 (2024-03-22)

• This library is now published in SoftwareX! If you use it in published research, please cite it as follows: Barak Shoshany, "A C++17 Thread Pool for High-Performance Scientific Computing", doi:10.1016/j.softx.2024.101687, SoftwareX 26 (2024) 101687, arXiv:2105.00613
  • Updated the source files, as well as README.md, CITATION.bib, and CITATION.cff with the new citation.
• A new macro, BS_THREAD_POOL_DISABLE_EXCEPTION_HANDLING, allows the user to disable exception handling in submit_task() if it is not needed, or if exceptions are explicitly disabled in the codebase. See #139.
  • Note that this macro can be defined independently of BS_THREAD_POOL_ENABLE_WAIT_DEADLOCK_CHECK. Disabling exception handling removes the try-catch block from submit_task(), while enabling wait deadlock checks adds a throw expression to wait(), wait_for(), and wait_until().
  • If the feature-test macro __cpp_exceptions is undefined, BS_THREAD_POOL_DISABLE_EXCEPTION_HANDLING is automatically defined, and BS_THREAD_POOL_ENABLE_WAIT_DEADLOCK_CHECK is automatically undefined.
• Replaced #pragma once with old-school include guards using the macros BS_THREAD_POOL_HPP and BS_THREAD_POOL_UTILS_HPP. There are two main reasons for this:
  1. Even though #pragma once is supported by the vast majority of modern compilers, it is still a non-standard feature, so using it technically made the library not standards compliant.
  2. Include guards make it possible to include the library twice in the same project (for example, once with priority enabled and once without) by undefining the include guard and putting the second include in its own namespace.
• Included a description of the destructor behavior for the BS::thread_pool class in README.md, in the library reference section. See #143.
• Removed unnecessary locking in reset() if pausing is not enabled.

v4.0.1 (2023-12-28)

• Fixed linkage issue caused by the global variables BS::this_thread::get_index and BS::this_thread::get_pool not being defined as inline. See #134 and 137.
• Fixed redundant cast in the BS::thread_pool::blocks class, and added -Wuseless-cast to the GCC warning flags in BS_thread_pool_test.ps1 to catch similar issues in the future. See #133.
• Each of the three files BS_thread_pool_test.cpp, BS_thread_pool.hpp, and BS_thread_pool_utils.hpp now contains three macros indicating the major, minor, and patch version of the file. In addition, BS_thread_pool_test.cpp now checks whether the versions of all three files match, and aborts compilation if they do not.

v4.0.0 (2023-12-27)

• A major new release with numerous changes, additions, fixes, and improvements. Many frequently requested features have been added, and performance has been optimized. Please note that code written using previous releases will need to be modified to work with the new release. The changes needed to migrate to the new API are explicitly indicated below for your convenience.
• Highlights:
  • The light thread pool has been removed. However, by default, the thread pool is in "light mode". Optional features that may affect performance must be enabled by defining suitable macros.
  • This library now ships with two stand-alone header files:
    • BS_thread_pool.hpp contains the main BS::thread_pool class and the BS::multi_future helper classes, and is the only file needed to use the thread pool itself.
    • BS_thread_pool_utils.hpp contains the additional utility classes BS::signaller, BS::synced_stream, and BS::timer, which are fully independent of the thread pool itself and can be used either with or without it.
  • It is now possible to assign priorities to tasks. Tasks with higher priorities will be executed first.
  • Member functions for submitting tasks and loops have been renamed for consistency, e.g. detach_task() and submit_task(), where the prefix detach means no future will be returned and submit means a future or BS::multi_future will be returned.
  • There are now two ways to parallelize loops into blocks:
    • detach_blocks() and submit_blocks() behave the same as loop parallelization in previous releases, running the loop function once per block.
    • detach_loop() and submit_loop() have a simpler syntax, where the loop function is run once per index, so the user doesn't have to manually run the internal loop for each block.
  • The new member functions detach_sequence() and submit_sequence() allow submitting a sequence of tasks enumerated by indices.
  • It is now possible to run an initialization function in each thread before it starts to execute any submitted tasks.
  • Tasks submitted with detach_task() or submit_task() can no longer have arguments. Task with arguments must be enclosed inside lambda expressions. This simplifies the API and provides better readability. Tasks can still have return values.
  • Various ways to obtain information about the threads in the pool have been introduced:
    • The member function get_thread_ids() obtains the unique thread identifiers, and get_native_handles() obtains the underlying implementation-defined thread handles.
    • The new namespace BS::this_thread allows obtaining the thread's index in the pool using BS::this_thread::get_index() and a pointer to the pool that owns the thread using BS::this_thread::get_pool().
  • Member functions for waiting for tasks have been renamed for brevity: wait()/wait_for()/wait_until(). In addition, these functions can now optionally throw an exception if the user tries to call them from within a thread of the same pool, which would result in a deadlock.
  • The first index must now be specified explicitly when parallelizing blocks, loops, and sequences, and it must not be greater than the last index. Also, both indices must now have the same type, or the template parameter should be explicitly specified.
  • Optimized the way detach_blocks(), submit_blocks(), detach_loop(), and submit_loop() split the range of the loop into blocks.
  • Added a utility class BS::signaller to allow simple signalling between threads.
  • BS::multi_future<T> is now a specialization of std::vector<std::future<T>> with additional member functions.
• Breaking changes:
  • The light thread pool has been removed. The original idea was that the light thread pool will allow the user to sacrifice functionality for increased performance. However, in my testing I found that there was no actual performance benefit to the light thread pool. Therefore, there is no reason to keep it.
    • However, by default, the thread pool is in "light mode". Optional features that may affect performance due to additional checks or more complicated algorithms must be enabled by defining suitable macros before including the library:
      • BS_THREAD_POOL_ENABLE_PAUSE to enable pausing.
      • BS_THREAD_POOL_ENABLE_PRIORITY to enable task priority.
      • BS_THREAD_POOL_ENABLE_WAIT_DEADLOCK_CHECK to enable wait deadlock checks.
    • API migration:
      • If you previously used BS_thread_pool_light.hpp, simply use BS_thread_pool.hpp instead.
      • If you previously used the pausing feature, define the macro BS_THREAD_POOL_ENABLE_PAUSE before including BS_thread_pool.hpp to enable it.
  • Member functions have been renamed for better consistency. Each function has a detach variant which does not return a future, and a submit variant which does return a future (or a BS::multi_future):
    • detach_task() and submit_task() for single tasks.
    • detach_blocks() and submit_blocks() for loops to be split into blocks, where the loop function is executed once per block and must have an internal loop, as in previous releases.
    • detach_loop() and submit_loop() for loops to be split into blocks, where the loop function is executed once per index and the pool takes care of the internal loop.
    • detach_sequence() and submit_sequence() for sequences of enumerated tasks.
    • API migration: Use the new names of the functions:
      • push_task() -> detach_task()
      • submit() -> submit_task()
      • push_loop() -> detach_blocks()
      • parallelize_loop() -> submit_blocks()
  • wait_for_tasks(), wait_for_tasks_duration(), and wait_for_tasks_until() have been renamed to wait(), wait_for(), and wait_until() respectively.
    • API migration: Use the new names of the functions:
      • wait_for_tasks() -> wait()
      • wait_for_tasks_duration() -> wait_for()
      • wait_for_tasks_until() -> wait_until()
  • Functions for parallelizing loops no longer have dedicated overloads for the special case where the first index is 0. These overloads essentially amount to giving the first function argument a default value, which is not allowed in C++, and can be confusing. In addition, indicating the first index explicitly is better for readability.
    • API migration: Add the first index 0 manually as the first argument if it was omitted.
  • Functions for parallelizing loops no longer allow the last index to be smaller than the first index. Previously, e.g. detach_blocks(5, 0, ...) was equivalent to detach_blocks(0, 5, ...). However, this led to confusing results. Since the first argument is the first index and the second argument is the index after the last index (i.e. 0 to 5 actually means 0, 1, 2, 3, 4), the user might get the wrong impression that detach_blocks(5, 0, ...) will count 5, 4, 3, 2, 1 instead. This option was removed to avoid this confusion.
    • Sometimes the user might actually want to make a loop that counts down instead of up. This cannot be done by flipping the order of the arguments to e.g. detach_blocks() (nor could it be done in previous releases). However, it can be done by simply defining a suitable loop function. For example, if you call detach_blocks(0, 10, loop, 2) and define the loop function as for (T i = 9 - start; i > 9 - end; --i), then the first block will count 9, 8, 7, 6, 5 and the second block will count 4, 3, 2, 1, 0.
    • detach_loop(), submit_loop(), detach_sequence(), and submit_sequence() work the same way. The first index must be smaller than the last index, but you can count down by writing a suitable loop or sequence function.
    • API migration: Any loop parallelization that used a first index greater than the last index will work exactly the same after switching the first and second arguments so that the smaller index appears first.
  • Functions for parallelizing loops no longer accept first and last indices of different types. The reason for allowing this previously was that otherwise, writing something like detach_blocks(0, x, ...) where x is not an int would result in a compilation error, since 0 is by default an int and therefore the arguments 0 and x have different types. However, this behavior, which used std::common_type to determine the common type of the two indices, sometimes completely messed up the range of the loop. For example, the std::common_type of int and unsigned int is unsigned int, which means the loop will only use non-negative indices even if the int start index was negative, resulting in an integer overflow.
    • API migration: If you want to invoke e.g. detach_blocks(0, x, ...) where x is not an int, you can either:
      • Make the 0 have the desired type using a cast or a suffix. For example, if x is an unsigned int, write (unsigned int)0 or 0U instead of 0.
      • Specify the template parameter explicitly. For example, if x is a size_t, write detach_blocks<size_t>(0, x, ...).
  • detach_task() and submit_task() no longer accept arguments for the submitted task. Instead, you must enclose the function in a lambda expression. In other words, instead of detach_task(task, args...) you should write detach_task([] { task(args...); }), indicating in the capture list [] whether to capture the task itself, and each of the arguments, by value or reference. Please see README.md for examples. This was changed for the following reasons:
    1. Consistency with detach_blocks() and submit_blocks(), as well as the new detach_loop(), submit_loop(), detach_sequence(), and submit_sequence(), which do not accept function arguments either.
    2. In my own multithreaded projects, I find that I almost always need the task to have access to variables in the local scope. This is much simpler, easier, and more concise to do with a lambda capture list, especially an implicit capture [=] or [&], than by defining a function that takes arguments and then passing these arguments.
    3. Similarly, I find that I mostly submit tasks defined as a lambda on the spot, rather than creating them as separate functions, because it's faster to code and makes it clear exactly what the task does without having to look elsewhere.
    4. When users post issues to this repository asking for help with their own code that uses the thread pool, the solution often turns out to be "just wrap that in a lambda". Such issues can be avoided if lambdas must be used to begin with.
    5. Submitting member functions, which previously required the awkward syntax detach_task(&class::function, &object, args...), can now be achieved with the much simpler and more readable syntax detach_task([] { object.function(args...); }) with the appropriate captures.
    6. Passing arguments by reference, which previously required using std::ref, e.g. detach_task(task, std::ref(arg)), can now be achieved with the much simpler and more readable syntax detach_task([&arg] { task(arg); }).
    7. The new syntax allows specifying the priority of the task easily, as the second argument - otherwise, it would have been hard to distinguish the priority from a task argument, making the API more complicated and confusing. This syntax will also permit adding additional arguments to the member functions as needed in the future.
    • API migration: Enclose all tasks with arguments inside a lambda expression. All submitted tasks must have no arguments, but they can still have return values.
      • Alternatively, std::bind can also be used, if the old syntax is preferred to a lambda. Just wrap it around the task and its arguments: instead of detach_task(task, args...), write detach_task(std::bind(task, args...)). This achieves the same effect, and can be used to easily convert v3.x.x code to v4.0.0 using a simple regular expression search and replace:
        • push_task\((.*?)\) -> detach_task(std::bind($1))
        • submit\((.*?)\) -> submit_task(std::bind($1))
  • BS::synced_stream and BS::timer have been moved to BS_thread_pool_utils.hpp.
    • API migration: Include the new header file if either of these utility classes are used.
• BS_thread_pool.hpp new features:
  • A new optional feature, enabled by defining the macro BS_THREAD_POOL_ENABLE_PRIORITY, allows assigning priority to tasks. The priority is a number of type BS::priority_t, which is a signed 16-bit integer, so it can have any value between -32,768 and 32,767. The tasks will be executed in priority order from highest to lowest.
    • To assign a priority to a task, add the priority as the last argument to any of the detach or submit functions. If the priority is not specified, the default value will be 0.
    • The namespace BS::pr contains some pre-defined priorities for users who wish to avoid magic numbers and enjoy better future-proofing. In order of decreasing priority, the pre-defined priorities are: BS::pr::highest, BS::pr::high, BS::pr::normal, BS::pr::low, and BS::pr::lowest.
    • Please see README.md for more information, including performance considerations.
  • The new member functions detach_loop() and submit_loop() facilitate loop parallelization without having to worry about internal loops in the loop function. In previous releases, the loop function had to be of the form [](T start, T end) { for (T i = start; i < end; ++i) loop(i); }. This behavior has been preserved in detach_blocks() and submit_blocks(). However, the new detach_loop() and submit_loop() allow much simpler loop functions of the form [](T i) { loop(i) }, greatly simplifying the interface.
    • Performance-wise, due to fewer function calls, detach_blocks() and submit_blocks() are generally faster. However, the difference is usually not significant, and with compiler optimizations there may be no difference at all. In any case, detach_loop() and submit_loop() are provided as convenience functions, but performance-critical applications can stick with detach_blocks() and submit_blocks().
  • The new member functions detach_sequence() and submit_sequence() facilitate submitting a sequence of tasks enumerated by indices. This is a bit similar to detach_loop() and submit_loop(), except that the range of indices is not split into blocks with each block containing a smaller range of indices. Instead, there is exactly one task per index. This can be used, for example, to submit a sequence of tasks with each one independently processing a single array element. detach_sequence() does not return a future, while submit_sequence() returns a BS::multi_future.
  • It is now possible to run an initialization function in each thread before it starts to execute any submitted tasks. The function must take no arguments and have no return value. It will only be executed exactly once, when the thread is first constructed. It can be passed as an argument to the constructor or to reset(). See #104, #105, #113, and #119.
  • Added a member function get_thread_ids() which returns a vector containing the unique identifiers for each of the pool's threads, as obtained by std::thread::get_id(). See #126.
  • A new optional feature, enabled by defining the macro BS_THREAD_POOL_ENABLE_NATIVE_HANDLES, adds a member function get_native_handles() which returns a vector containing the underlying implementation-defined thread handles for each of the pool's threads. These can then be used in an implementation-specific way to manage the threads at the OS level; however, note that this will generally not be portable code. See #122.
    • This feature is disabled by default since it uses std::thread::native_handle(), which is in the C++ standard library, but is not guaranteed to be present on all systems.
  • A new namespace BS::this_thread was created to provide functionality similar to std::this_thread.
    • BS::this_thread::get_index() can be used to get the index of the current thread. If this thread belongs to a BS::thread_pool object, it will have an index from 0 to BS::thread_pool::get_thread_count() - 1. Otherwise, for example if this thread is the main thread or an independent std::thread, std::nullopt will be returned.
    • BS::this_thread::get_pool() can be used to get the pointer to the thread pool that owns the current thread. If this thread belongs to a BS::thread_pool object, a pointer to that object will be returned. Otherwise, std::nullopt will be returned.
    • Note that both functions return an std::optional object.
  • BS::multi_future<T> is now defined as a specialization of std::vector<std::future<T>>. This means that all of the member functions that can be used on an std::vector can also be used on a BS::multi_future. For example, it is now possible to use a range-based for loop with a BS::multi_future object, since it has iterators.
    • In addition to inherited member functions, BS::multi_future has the following specialized member functions, most of which are new in this release: get(), ready_count(), valid(), wait(), wait_for(), and wait_until(). Please see README.md for more information. See also #128.
  • A new optional feature, enabled by defining the macro BS_THREAD_POOL_ENABLE_WAIT_DEADLOCK_CHECK, allows wait(), wait_for(), and wait_until() to check whether the user tried to call them from within a thread of the same pool, which would result in a deadlock. If so, they will throw the exception BS::thread_pool::wait_deadlock instead of waiting.
• BS_thread_pool_utils.hpp:
  • The utility classes BS::synced_stream and BS::timer now reside in this header file instead of the main one.
  • BS::timer has a new member function, current_ms(), which can be used to obtain the number of milliseconds that have elapsed so far, but keep the timer ticking.
  • The new utility class BS::signaller allows simple signalling between threads. It can be used to make one or more threads wait, using the wait() member function. When another thread uses the ready() member function, all waiting threads stop waiting. This class is really just a convenient wrapper around std::promise, which contains both the promise and its future.
• BS_thread_pool.hpp bug fixes and minor changes:
  • Optimized locking in the worker function. This should result in increased performance.
  • Optimized the way detach_blocks(), submit_blocks(), detach_loop(), and submit_loop() split the range of the loop into blocks. All blocks are now guaranteed to have one of two sizes, differing by 1, with the larger blocks always first. See #96.
    • For example, in previous releases, 100 indices were split into 15 blocks as 14 blocks of size 6 and one additional block of size 16, which was suboptimal. Now they are split into 10 blocks of size 7 and 5 blocks of size 6, which means the tasks are as evenly distributed as possible.
  • Fixed a bug that caused paused pools to have high idle CPU usage if pausing was used. See #120.
  • The worker now destructs the task object as soon as it finishes executing. See #124 and #129.
  • Added Markdown inline code formatting in all comments whenever applicable, which makes the comments look nicer when displayed as a tooltip in Visual Studio Code or other supporting IDEs.
  • The BS::thread_pool::blocks helper class has been moved into the main thread pool class, and now returns a degenerate object (zero blocks) if index_after_last <= first_index.
• BS_thread_pool_test.cpp:
  • Removed tests for the light thread pool.
  • Added/modified tests for all new/changed features.
  • Many of the previous tests have been simplified and optimized.
  • The program now takes command line arguments:
    • help: Show a help message and exit.
    • log: Create a log file.
    • tests: Perform standard tests.
    • deadlock Perform long deadlock tests.
    • benchmarks: Perform benchmarks.
    • If no options are entered, the default is: log tests benchmarks.
  • By default, the test program enables all the optional features by defining the suitable macros, so it can test them. However, if the macro BS_THREAD_POOL_LIGHT_TEST is defined during compilation, the optional features will not be tested.
  • Instead of using a pre-defined list to specify the number of loop blocks to try in the benchmarks, the program now simply keeps increasing the number of blocks until it finds the optimal value. Often, the optimal number of blocks is much higher than the number of hardware threads, but if the number is too high it will result in diminishing returns.
  • check_loop_no_return() now checks that the loop modifies all the indices exactly once, to detect cases where an index has been modified more than once, e.g. if the same loop index was erroneously placed in more than one block.
  • Instead of defining _CRT_SECURE_NO_WARNINGS, the program now uses localtime_s instead of std::localtime if MSVC is detected to avoid generating a warning.
  • On macOS, the test program will exit with std::terminate() instead of std::quick_exit() if any tests failed. This is because macOS does not implement std::quick_exit() for some reason. Note that as a result, the number of failed tests cannot be returned by the program on macOS. Unfortunately, std::exit() cannot be used here, as it might get stuck if a deadlock occurs. See #106
  • The log file now uses the name of the executable file, followed by the date and time, so it's easy to distinguish between log files generated by different builds of the test (since the test script names them based on the compiler used). Also, the program now checks if the log file failed to open for some reason, and writes only to the standard output in that case.
  • The benchmarks now display a progress bar.
  • The test program will now detect the OS and compiler used.
• BS_thread_pool_test.ps1:
  • The script will compile and run a light version of the test, with no optional features enabled, in addition to the main test, for each compiler.
  • The source and build folders will now be determined relative to the script folder, to ensure that the script works no matter which folder it is executed from.
  • The script now checks that the include files BS_thread_pool.hpp and BS_thread_pool_utils.hpp are present before attempting to compile the test program.
• README.md:
  • Added/modified documentation for all new/changed features.
  • Revised many of the existing examples and explanations.
  • Added a complete library reference at the end of the documentation.
  • Added instructions for installing the package using Meson and CMake with CPM. The installation instructions with various package managers and build systems were moved to the end, before the reference.
• Miscellaneous:
  • A .clang-tidy file is now included, with all the checks that are enabled in this project. The pull request template has been updated to suggest that authors lint their code using this file before submitting the pull request.
• This release is dedicated to my wife (since December 1, 2023), Pauline. Her endless love, support, and encouragement have been a great source of motivation for working on this and other projects. I am so lucky and honored to my_future.share() with her ❤️

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.
  • Fixed 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.

v3.4.0 (2023-05-12)

• BS_thread_pool.hpp and BS_thread_pool_light.hpp:
  • Resolved an issue which could have caused tasks_total to not be synchronized in some cases. See #70.
  • Resolved a deadlock which could rarely be caused when the pool was destructed or reset. See #93, #100, #107, and #108.
  • Resolved a deadlock which could be caused when wait_for_tasks() was called more than once.
  • Two new member functions have been added to the non-light version: wait_for_tasks_duration() and wait_for_tasks_until(). They allow waiting for the tasks to complete, but with a timeout. wait_for_tasks_duration() will stop waiting after the specified duration has passed, and wait_for_tasks_until() will stop waiting after the specified time point has been reached.
  • Renamed BS_THREAD_POOL_VERSION in BS_thread_pool_light.hpp to BS_THREAD_POOL_LIGHT_VERSION and removed the [light] tag. This allows including both header files in the same program in case we want to use both the light and non-light thread pools simultaneously.
• BS_thread_pool_test.cpp and BS_thread_pool_light_test.cpp:
  • Fixed an issue that caused a compilation error when using MSVC and including windows.h. See #72.
  • The number and size of the vectors in the performance test (BS_thread_pool_test.cpp only) are now guaranteed to be multiples of the number of threads, for optimal performance.
  • In count_unique_threads(), moved the condition variables and mutexes to the function scope to prevent cluttering the global scope.
  • Three new tests have been added to BS_thread_pool_test.cpp to check the deadlocks issue that were resolved in this release (see above). The tests rely on the new wait for tasks with timeout feature, so they are not available in the light version.
    • One test checks for deadlocks when calling wait_for_tasks() more than once.
    • Two tests check for deadlocks when destructing and resetting the pool respectively. They are turned off by default, since they take a long time to complete, but can be turned on by setting enable_long_deadlock_tests to true.
  • Two new tests have been added to the non-light version to check the new member functions wait_for_tasks_duration() and wait_for_tasks_until().
  • The test programs now return the number of failed tests upon exit, instead of just 1 if any number of tests failed, which was the case in previous versions. Also, if any tests failed, std::quick_exit() is invoked instead of return, to avoid getting stuck due to any lingering tasks or deadlocks.
• README.md:
  • Added documentation for the two new member functions, wait_for_tasks_duration() and wait_for_tasks_until().
  • Fixed Markdown rendering incorrectly on Visual Studio. See #77.
  • The sample performance tests are now taken from a 40-core / 80-thread dual-CPU computing node, which is a more typical use case for high-performance scientific software.

v3.3.0 (2022-08-03)

• BS_thread_pool.hpp:
  • The public member variable paused of BS::thread_pool has been made private for future-proofing (in case future versions implement a more involved pausing mechanism) and better encapsulation. It is now accessible only via the pause(), unpause(), and is_paused() member functions. In other words:
    • Replace pool.paused = true with pool.pause().
    • Replace pool.paused = false with pool.unpause().
    • Replace if (pool.paused) (or similar) with if (pool.is_paused()).
  • The public member variable f of BS::multi_future has been renamed to futures for clarity, and has been made private for encapsulation and simplification purposes. Instead of operating on the vector futures itself, you can now use the [] operator of the BS::multi_future to access the future at a specific index directly, or the push_back() member function to append a new future to the list. The size() member function tells you how many futures are currently stored in the object.
  • The explicit casts of std::endl and std::flush, added in v3.2.0 to enable flushing a BS::synced_stream, caused ODR (One Definition Rule) violations if BS_thread_pool.hpp was included in two different translation units, since they were mistakenly not defined as inline. To fix this, I decided to make them static members of BS::synced_stream instead of global variables, which also makes the code better organized in my opinion. These objects can now be accessed as BS::synced_stream::endl and BS::synced_stream::flush. I also added an example for how to use them in README.md. See #64.
• BS_thread_pool_light.hpp:
  • This package started out as a very lightweight thread pool, but over time has expanded to include many additional features, and at the time of writing it has a total of 340 lines of code, including all the helper classes. Therefore, I have decided to bundle a light version of the thread pool in a separate and stand-alone header file, BS_thread_pool_light.hpp, with only 170 lines of code (half the size of the full package). This file does not contain any of the helper classes, only a new BS::thread_pool_light class, which is a minimal thread pool with only the 5 most basic member functions:
    • get_thread_count()
    • push_loop()
    • push_task()
    • submit()
    • wait_for_tasks()
  • A separate test program BS_thread_pool_light_test.cpp tests only the features of the lightweight BS::thread_pool_light class. In the spirit of minimalism, it does not generate a log file and does not do any benchmarks.
  • To be perfectly clear, each header file is 100% stand-alone. If you wish to use the full package, you only need BS_thread_pool.hpp, and if you wish to use the light version, you only need BS_thread_pool_light.hpp. Only a single header file needs to be included in your project.

v3.2.0 (2022-07-28)

• BS_thread_pool.hpp:
  • Main BS::thread_pool class:
    • Added a new member function, push_loop(), which does the same thing as parallelize_loop(), except that it does not return a BS::multi_future with the futures for each block. Just like push_task() vs. submit(), this avoids the overhead of creating the futures, but the user must use wait_for_tasks() or some other method to ensure that the loop finishes executing, otherwise bad things will happen.
    • push_task() and submit() now utilize perfect forwarding in order to support more types of tasks - in particular member functions, which in previous versions could not be submitted unless wrapped in a lambda. To submit a member function, use the syntax submit(&class::function, &object, args). More information can be found in README.md. See #9.
    • push_loop() and parallelize_loop() now have overloads where the first argument (the first index in the loop) is omitted, in which case it is assumed to be 0. This is for convenience, as the case where the first index is 0 is very common.
  • Helper classes:
    • BS::synced_stream now utilizes perfect forwarding in the member functions print() and println().
    • Previously, it was impossible to pass the flushing manipulators std::endl and std::flush to print() and println(), since the compiler could not figure out which template specializations to use. The new objects BS::endl and BS::flush are explicit casts of these manipulators, whose sole purpose is to enable passing them to print() and println().
    • BS::multi_future::get() now rethrows exceptions generated by the futures, even if the futures return void. See #62.
    • Added a new helper class, BS::blocks, which is used by parallelize_loop() and push_loop() to divide a range into blocks. This class is not documented in README.md, as it most likely will not be of interest to most users, but it is still publicly available, in case you want to parallelize something manually but still benefit from the built-in algorithm for splitting a range into blocks.
• BS_thread_pool_test.cpp:
  • Added plenty of new tests for the new features described above.
  • Fixed a bug in count_unique_threads() that caused it to get stuck on certain systems.
  • dual_println() now also flushes the stream using BS::endl, so that if the test gets stuck, the log file will still contain everything up to that point. (Note: It is a common misconception that std::endl and '\n' are interchangeable. std::endl not only prints a newline character, it also flushes the stream, which is not always desirable, as it may reduce performance.)
  • The performance test has been modified as follows:
    • Instead of generating random vectors using std::mersenne_twister_engine, which proved to be inconsistent across different compilers and systems, the test now generates each element via an arbitrarily-chosen numerical operation. In my testing, this provided much more consistent results.
    • Instead of using a hard-coded vector size, a suitable vector size is now determined dynamically at runtime.
    • Instead of using parallelize_loop(), the test now uses the new push_loop() function to squeeze out a bit more performance.
    • Instead of setting the test parameters to achieve a fixed single-threaded mean execution time of 300 ms, the test now aims to achieve a fixed multithreaded mean execution time of 50 ms when the number of blocks is equal to the number of threads. This allows for more reliable results on very fast CPUs with a very large number of threads, where the mean execution time when using all the threads could previously be below a statistically significant value.
    • The number of vectors is now restricted to be a multiple of the number of threads, so that the blocks are always all of the same size.
• README.md:
  • Added instructions and examples for the new features described above.
  • Rewrote the documentation for parallelize_loop() to make it clearer.

v3.1.0 (2022-07-13)

• BS_thread_pool.hpp:
  • Fixed an issue where wait_for_tasks() would sometimes get stuck if push_task() was executed immediately before wait_for_tasks().
  • Both the thread pool constructor and the reset() member function now determine the number of threads to use in the pool as follows. If the parameter is a positive number, then the pool will be created with this number of threads. If the parameter is non-positive, or a parameter was not supplied, then the pool will be created with the total number of hardware threads available, as obtained from std::thread::hardware_concurrency(). If the latter returns a non-positive number for some reason, then the pool will be created with just one thread. See #51 and #52.
  • Added the [[nodiscard]] attribute to classes and class members, in order to warn the user when accidentally discarding an important return value, such as a future or the return value of a function with no useful side-effects. For example, if you use submit() and don't save the future it returns, the compiler will now generate a warning. (If a future is not needed, then you should use push_task() instead.)
  • Removed the explicit specifier from all constructors, as it prevented the default constructor from being used with static class members. See #48.
• BS_thread_pool_test.cpp:
  • Improved count_unique_threads() using condition variables, to ensure that each thread in the pool runs at least one task regardless of how fast it takes to run the tasks.
  • When appropriate, check() now explicitly reports what the obtained result was and what it was expected to be.
  • check_task_monitoring() and check_pausing() now explicitly report the results of the monitoring at each step.
  • Changed all instances of std::vector<std::atomic<bool>> to std::unique_ptr<std::atomic<bool>[]>. See #44.
  • Converted a few more C-style casts to C++ cast expressions.
• README.md:
  • Added instructions for using this package with the Conan C/C++ package manager. Please refer to this package's page on ConanCenter to learn how to use Conan to include this package in your project with various build systems.
• If you found this project useful, please consider starring it on GitHub! This allows me to see how many people are using my code, and motivates me to keep working to improve it.

v3.0.0 (2022-05-30)

• This is a major new release with many changes and improvements! Please note that code written using previous releases will need to be slightly modified to work with the new release. The changes needed to migrate to the new API are explicitly indicated below for your convenience.
• Breaking changes to the library header file:
  • The header file has been renamed to BS_thread_pool.hpp to avoid potential conflict with other thread pool libraries.
    • API migration: The library must now be included by invoking #include "BS_thread_pool.hpp".
  • All the definitions in the library, including the thread_pool class and the helper classes, are now located in the namespace BS. This namespace will also be used for my other C++ projects, and is intended to ensure consistency between my projects while avoiding potential name conflicts with other libraries.
    • API migration: The thread pool class should now be invoked as BS::thread_pool. Alternatively, it is possible to employ using BS::thread_pool or even using namespace BS and then invoke thread_pool directly. Same for the BS::synced_stream and BS::timer helper classes.
  • The macro THREAD_POOL_VERSION, which contains the version number and release date of the library, has been renamed to BS_THREAD_POOL_VERSION to avoid potential conflicts.
    • API migration: The version must now be read from the macro BS_THREAD_POOL_VERSION.
  • The public member sleep_duration has been removed. The thread pool now uses condition variables instead of sleep to facilitate waiting. This significantly improves performance (by 10%-50% in my testing), drastically decreases idle CPU utilization, and eliminates the need to set an optimal sleep time. This was a highly-requested change; see issue #1, issue #12, and pull request #23.
    • API migration: Remove any code that relates to the public member sleep_duration.
  • The template specializations for submit() have been merged. Now instead of two versions, one for functions with a return value and one for functions without a return value, there is just one version, which can accept any function. This makes the code more compact (and elegant). If a function with no return value is submitted, an std::future<void> is returned (the previous version returned an std::future<bool>)
    • API migration: To wait for a task with no return value, simply call wait() or get() on the corresponding std::future<void>.
  • parallelize_loop() now returns a future in the form of a new BS::multi_future helper class template. The member function wait() of this future allows waiting until all of the loop's blocks finish executing. In previous versions, calling parallelize_loop() both parallelized the loop and waited for the blocks to finish; now it is possible to do other stuff while the loop executes.
    • API migration: Since parallelize_loop() no longer automatically blocks, you should either store the result in a BS::multi_future object and call its wait() member function, or simply call parallelize_loop().wait() to reproduce the old behavior.
• Non-breaking changes to the library header file:
  • It is now possible to use parallelize_loop() with functions that have return values and get these values from all blocks at once through the get() member function of the BS::multi_future.
  • The template specializations for push_task() have been merged. Now instead of two versions, one for functions with arguments and one for functions without arguments, there is just one version, which can accept any function.
  • Constructors have been made explicit. See issue #28.
  • submit() now uses std::make_shared instead of new to create the shared pointer. This means only one memory allocation is performed instead of two, which should improve performance. In addition, all unique pointers are now created using std::make_unique.
  • A new helper class template, BS::multi_future, has been added. It's basically just a wrapper around std::vector<std::future<T>>. This class is used by the new implementation of parallelize_loop() to allow waiting for the entire loop, consisting of multiple tasks with their corresponding futures, to finish executing.
  • BS::multi_future can also be used independently to handle multiple futures at once. For example, you can now keep track of several groups of tasks by storing their futures inside separate BS::multi_future objects and use either wait() to wait for all tasks in a specific group to finish or get() to get an std::vector with the return values of every task in the group.
  • Integer types are now chosen in a smarter way to improve portability, allow for better compatibility with 32-bit systems, and prevent potential conversion errors.
  • Added a new type, BS::concurrency_t, equal to the return type of std::thread::hardware_concurrency(). This is probably pointless, since the C++ standard requires this to be unsigned int, but it seems to me to make the code slightly more portable, in case some non-conforming compiler chooses to use a different integer type.
  • C-style casts have been converted to C++ cast expressions for added clarity.
  • Miscellaneous minor optimizations and style improvements.
• Changes to the test program:
  • The program has been renamed to BS_thread_pool_test.cpp to avoid potential conflict with other thread pool libraries.
  • The program now returns EXIT_FAILURE if any of the tests failed, for automation purposes. See pull request #42.
  • Fixed incorrect check order in check_task_monitoring(). See pull request #43.
  • Added a new test for parallelize_loop() with a return value.
  • Improved some of the tests to make them more reliable. For example, count_unique_threads() now uses futures (stored in a BS::multi_future<void> object).
  • The program now uses std::vector instead of matrices, for both consistency checks and benchmarks, in order to simplify the code and considerably reduce its length.
  • The benchmarks have been simplified. There's now only one test: filling a specific number of vectors of fixed size with random values. This may be replaced with something more practical in a future released, but at least on the systems I've tested on, it does demonstrate a very significant multithreading speedup.
  • In addition to multithreaded tests with different numbers of tasks, the benchmark now also includes a single-threaded test. This allows for more accurate benchmarks compared to previous versions, as the (slight) parallelization overhead is now taken into account when calculating the maximum speedup.
  • The program decides how many vectors to use for benchmarking by testing how many are needed to reach a target duration in the single-threaded test. This ensures that the test takes approximately the same amount of time on different systems, and is thus more consistent and portable.
  • Miscellaneous minor optimizations and style improvements.
• Changes to README.md:
  • Many sections have been rewritten and/or polished.
  • Explanations and examples of all the new features have been added.
  • Added an acknowledgements section.
• Miscellaneous changes:
  • Added a CITATION.bib file (in BibTeX format) to the GitHub repository. You can use it to easily cite this package if you use it in any research papers.
  • Added a CITATION.cff file (in YAML format) to the GitHub repository. This should add an option to get a citation in different formats directly from GitHub repository by clicking on "cite this repository" on the sidebar to the right.
  • Added templates for GitHub issues and pull requests.

v2.0.0 (2021-08-14)

• From now on, version numbers will adhere to the Semantic Versioning specification in the format major.minor.patch.
• A file named thread_pool_test.cpp has been added to the package. It will perform automated tests of all aspects of the package, and benchmark some multithreaded matrix operations. Please run it on your system and submit a bug report if any of the tests fail. In addition, the code is thoroughly documented, and is meant to serve as an extensive example of how to properly use the package.
• The package is now available through vcpkg. Instructions for how to install it have been added to README.md. See this pull request.
• The package now defines a macro THREAD_POOL_VERSION, which returns the version number and release date of the thread pool library as a string.
• parallelize_loop() has undergone some major changes (and is now incompatible with v1.x):
  • The second argument is now the index after the last index, instead of the last index itself. This is more consistent with C++ conventions (e.g. standard library algorithms) where the range is always [first, last). For example, for an array with n indices, instead of parallelize_loop(0, n - 1, ...) you should now write parallelize_loop(0, n, ...).
  • The loop function is now only called once per block, instead of once per index, as was the case before. This should provide a performance boost due to significantly reducing the number of function calls, and it also allows you to conserve resources by using them only once per block instead of once per index (an example can be found in the random_matrix_generator class in thread_pool_test.cpp). It also means that loop now takes two arguments: the first index in the block and the index after the last index in the block. Thus, loop(start, end) should typically involve a loop of the form for (T i = start; i < end; i++).
  • The first and last indices can now be of two different integer types. Previously, parallelize_loop(0, i, ...) did not work if i was not an int, because 0 was interpreted as int, and the two arguments had to be of the same type. Therefore, one had to use casting, e.g. parallelize_loop((size_t)0, i), to make it work. Now this is no longer necessary; the common type is inferred automatically using std::common_type_t.

v1.9 (2021-07-29)

• Fixed a bug in reset() which caused it to create the wrong number of threads.

v1.8 (2021-07-28)

• The version history has become too long to be included in README.md, so I moved it to a separate file, CHANGELOG.md.
• A button to open this repository directly in Visual Studio Code has been added to the badges in README.md.
• An internal variable named promise has been renamed to task_promise to avoid any potential errors in case the user invokes using namespace std.
• submit() now catches exceptions thrown by the submitted task and forwards them to the future. See this issue.
• Eliminated compiler warnings that appeared when using the -Weffc++ flag in GCC. See this pull request.

v1.7 (2021-06-02)

• Fixed a bug in parallelize_loop() which prevented it from actually running loops in parallel, see this issue.

v1.6 (2021-05-26)

• Since MSVC does not interpret and as && by default, the previous release did not compile with MSVC unless the /permissive- or /Za compiler flags were used. This has been fixed in this version, and the code now successfully compiles with GCC, Clang, and MSVC. See this pull request.

v1.5 (2021-05-07)

• This library now has a DOI for citation purposes. Information on how to cite it in publications has been added to the source code and to README.md.
• Added GitHub badges to README.md.

v1.4 (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.

v1.3 (2021-05-03)

• Fixed this issue: Removed std::move from the return statement in push_task(). This previously generated a -Wpessimizing-move warning in Clang. The assembly code generated by the compiler seems to be the same before and after this change, presumably because the compiler eliminates the std::move automatically, but this change gets rid of the Clang warning.
• Fixed this issue: Removed a debugging message printed to std::cout, which was left in the code by mistake.
• Fixed this issue: parallelize_loop() no longer sends references for the variables start and stop when calling push_task(), which may lead to undefined behavior.
• A companion paper is now published at arXiv:2105.00613, including additional information such as performance tests on systems with up to 80 hardware threads. The README.md has been updated, and it is now roughly identical in content to the paper.

v1.2 (2021-04-29)

• The worker function, which controls the execution of tasks by each thread, now sleeps by default instead of yielding. Previously, when the worker could not find any tasks in the queue, it called std::this_thread::yield() and then tried again. However, this caused the workers to have high CPU usage when idle, as reported by some users. Now, when the worker function cannot find a task to run, it instead sleeps for a duration given by the public member variable sleep_duration (in microseconds) before checking the queue again. The default value is 1000 microseconds, which I found to be optimal in terms of both CPU usage and performance, but your own optimal value may be different.
• If the constructor is called with an argument of zero for the number of threads, then the default value, std::thread::hardware_concurrency(), is used instead.
• Added a simple helper class, timer, which can be used to measure execution time for benchmarking purposes.
• Improved and expanded the documentation.

v1.1 (2021-04-24)

• Cosmetic changes only. Fixed a typo in the Doxygen comments and added a link to the GitHub repository.

v1.0 (2021-01-15)

• Initial release.