Thread pool on a queue in C++

Question

I've been trying to solve a problem concurrently, which fits the thread pool pattern very nicely. Here I will try to provide a minimal representative example:

Say we have a pseudo-program like this:

Q : collection<int>
while (!Q.empty()) {
    for each q in Q {
        // perform some computation
    }
    // assign a new value to Q
    Q = something_completely_new();
}

I'm trying to implement that in a parallel way, with n-1 workers and one main thread. The workers will perform the computation in the inner loop by grabbing elements from Q .

I tried to solve this using two conditional variables, work , on which the master threads notifies the workers that Q has been assigned to, and another, work_done , where the workers notify master that the entire computation might be done.

Here's my C++ code:

#include <iostream>
#include <mutex>
#include <condition_variable>
#include <queue>
#include <thread>

using namespace std;

std::queue<int> Q;
std::mutex mut;
std::condition_variable work;
std::condition_variable work_done;

void run_thread() {
    for (;;) {
        std::unique_lock<std::mutex> lock(mut);
        work.wait(lock, [&] { return Q.size() > 0; });

        // there is work to be done - pretend we're working on something
        int x = Q.front(); Q.pop();
        std::cout << "Working on " << x << std::endl;

        work_done.notify_one();
    }
}

int main() {
    // your code goes here
    std::vector<std::thread *> workers(3);

    for (size_t i = 0; i < 3; i++) {
        workers[i] = new std::thread{
            [&] { run_thread(); }
        };
    }

    for (int i = 4; i > 0; --i) {
        std::unique_lock<std::mutex> lock(mut);
        Q = std::queue<int>();
        for (int k = 0; k < i; k++) {
            Q.push(k);
        }
        work.notify_all();
        work_done.wait(lock, [&] { return Q.size() == 0; });
    }

    for (size_t i = 0; i < 3; i++) {
        delete workers[i];
    }

    return 0;
}

Unfortunately, after compiling it on OS X with g++ -std=c++11 -Wall -o main main.cpp I get the following output:

Working on 0
Working on 1
Working on 2
Working on 3
Working on 0
Working on 1
Working on 2
Working on 0
Working on 1
Working on 0
libc++abi.dylib: terminating
Abort trap: 6

After a while of googling it looks like a segmentation fault. It probably has to do with me misusing conditional variables. I would appreciate some insight, both architectural (on how to approach this type of problem) and specific, as in what I'm doing wrong here exactly.

I appreciate the help

Answer 1

Your application was killed by std::terminate .

Body of your thread function is infinite-loop, so when these lines are executed

for (size_t i = 0; i < 3; i++) {
    delete workers[i];
}

you want to delete threads which are still running (each thread is in joinable state). When you call destructor of thread which is in joinable state the following thing happens (from http://www.cplusplus.com/reference/thread/thread/~thread/ )

If the thread is joinable when destroyed, terminate() is called.

so if you want terminate not to be called, you should call detach() method after creating threads.

  for (size_t i = 0; i < 3; i++) {
    workers[i] = new std::thread{
        [&] { run_thread(); }
    };
    workers[i]->detach();  // <---
}

Answer 2

Just because the queue is empty doesn't mean the work is done.

finished = true;
work.notify_all();
for (size_t i = 0; i < 3; i++) {
    workers[i].join(); // wait for threads to finish
    delete workers[i];
}

and we need some way to terminate the threads

for (;!finshed;) {
    std::unique_lock<std::mutex> lock(mut);
    work.wait(lock, [&] { return Q.size() > 0 || finished; });
    if (finished)
      return;