CHANGELOG.md

A C++17 Thread Pool for High-Performance Scientific Computing

Version history

• 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.