Thread pool - Thread pool using std::* primitives from C++17, with optional priority queue/greenthreading for POSIX.



Thread pool using std::* primitives from C++11. Also includes a class for a priority thread pool.

Requires concepts and C++17, including constexpr-if. Currently only GCC 7.0+ is sufficient. Use -std=c++17 -fconcepts to compile. The priority thread pool is only supported on POSIX/-like systems. But it's still easy to use the normal pool on non-POSIX; just don't compile priority_thread_pool.cpp or include the header.

If GCC 6.2 is prefered, revision 65879c9 is compatible. For just C++11, use 8bdfb9b. 5ea01d0 was the latest to support <= C++14.

The priority pool has the same API as described below, accept it has an int parameter first for the priority of the task. E.g. pool.async(5, func, arg1, arg2) for priority 5.

An example that computes the primality of 2 to 10,000 using 8 threads:

#include "thread_pool.hpp"

#include <iostream>
#include <list>
#include <utility>

using namespace std;

// Return the integer argument and a boolean representing its primality.
// We need to return the integer because the loop that retrieves results doesn't know which integer corresponds to which future.
pair<int, bool> is_prime(int n){
  for(int i = 2;i < n;i++)
    if(n % i == 0)
      return make_pair(i, false);
  return make_pair(n, true);

int main(){
  thread_pool pool(8);                   // Contruct a thread pool with 8 threads.
  list<future<pair<int, bool>>> results;
  for(int i = 2;i < 10000;i++){
  	// Add a task to the queue.
    results.push_back(pool.async(is_prime, i));
  for(auto i = results.begin();i != results.end();i++){
    pair<int, bool> result = i->get();  // Get the pair<int, bool> from the future<...>
    cout << result.first << ": " << (result.second ? "is prime" : "is composite") << endl;
  return 0;

thread_pool::async is a templated method that accepts any std::function and arguments to pass to the function. It returns a std::future<Ret> where Ret is the return type of the aforementioned std::function.

To submit a task: future<Ret> fut(pool.async(func, args...));.

To wait on fut to complete: Ret result = fut.get();

  • Test as number of threads decreased : infinite loop?

    Test as number of threads decreased : infinite loop?

    I simplified the original test code to be the following


    `#include "thread_pool.hpp" #include <vector> #include <iostream> // std::cout, std::endl using namespace std;


    • Intel Xeon ES-1620 v3 3.5GHz
    • git clone
    • git checkout -b cv origin/cv
    • $ gcc --version
    • gcc.exe (Rev3, Built by MSYS2 project) 6.3.0
    • $ g++ --std=c++11 test.cpp thread_pool.cpp
    • $ time ./a.exe
    • ...
    • real 5m23.883s for num_threads = 1 (aborted manually)
    • real 3m55.198s for num_threads = 2 (aborted manually)
    • why do above configurations never finish but the ones below do finish?
    • Further, if add "-O3" to compiler option above, the run in 26s and 13s,why?
    • real 0m12.690s for num_threads = 3
    • real 0m9.199s for num_threads = 4
    • real 0m7.971s for num_threads = 5
    • real 0m7.654s for num_threads = 6
    • real 0m7.304s for num_threads = 7
    • real 0m7.740s for num_threads = 8 */

    bool func_bool_int(int n) { if(n < 2) return false; for(int i = 2;i < n;i++) if(n % i == 0) return false; return true; }

    int main() {

    const int num_threads = 4; thread_pool p(num_threads);

    int n = 100000;
    int max = 5 * n;
    vector<future<bool>> f;
    for( int i = n; i < max; i++ )
    	f.emplace_back(p.async(function<bool(int)>(func_bool_int), i));

    std::cout << "..." << std::endl; size_t accum = 0; for(auto &i : f) { accum += i.get(); } std::cout << "accum = " << accum << std::endl;

    return 0;

    } `

    When I ran for num_threads = 8, 7, 6, 5, 4, 3 things went okay. But when I tried num_threads of 2 or 1 the runtime seemed to be in an infinite loop ie I was seeing about 7s to 12s runtimes above num_threads>2 but once I tried num_threads=2 or 1 I waited 3 minutes to 5 minutes (orders of magnitude longer than extrapolated runtime) before manually killing the process. Is this behavior to be expected? Note this was on a gcc 6.3 with "git checkout -b cv origin/cv" branch. Adding an "-O3" optimazation seemed to fix the issue; expected?

    opened by pocdn 4
  • High CPU usage

    High CPU usage

    I integrated thread_pool in one of our projects and replaced the current threading mechanism. Instead of creating new threads I wanted to reuse the threads. It was fairly easy to migrate to this lib, but I am experiencing high CPU load, when the thread pool is not used.

    opened by r0l1 2
  • How to pass a multiple arguments to my function?

    How to pass a multiple arguments to my function?

    I tried something like

    int myFunc(int a, int b, int c)
    ... myPool.async(std::function<int(int, int, int)>(myFunc), a, b, c)...

    and get a ton of errors. But if I using function with 1 argument everything works well. What i'm doing wrong?

    opened by lkjhgfda 2
  • Waiting for threads to finish

    Waiting for threads to finish


    How do I wait for threads to finish?

    I've got this:

        fut.emplace_back(tp.async(std::function<int(std::string)>(breakPacket), completePacket));

    I push into it about 3000 items for the threads ti process, but then I don't know how to wait for the threads to finish...



    opened by davidcsi 2
  • Error: underined reference to thread_pool::...

    Error: underined reference to thread_pool::...

    (Edited by Tyler to show error that OP had before he deleted the comment): ... main.cpp:(.text+0x31b): undefined reference to 'thread_pool::~thread_pool()' ...

    opened by davidcsi 2
  • C++20 build failure

    C++20 build failure

    I get this when using C++20:

    thread_pool.hpp:13:9: warning: the 'bool' keyword is not allowed in a C++20 concept definition 13 | concept bool Callable = requires(Fn f, Args... args) {

    opened by frli8848 1
  • Compilation warning Wsign-compare

    Compilation warning Wsign-compare

    Very silly, but annoying; I get a:

    thread_pool.hpp:107:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Wsign-compare]

    warning, which is easily silenced by:

    - for(int i = 0;i < num_threads;i++){
    + for(unsigned int i = 0;i < num_threads;i++){
    opened by dpellegr 0
Tyler Hardin
Tyler Hardin
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