Reusing thread in loop c++

Question

I need to parallelize some tasks in a C++ program and am completely new to parallel programming. I've made some progress through internet searches so far, but am a bit stuck now. I'd like to reuse some threads in a loop, but clearly don't know how to do what I'm trying for.

I am acquiring data from two ADC cards on the computer (acquired in parallel), then I need to perform some operations on the collected data (processed in parallel) while collecting the next batch of data. Here is some pseudocode to illustrate

//Acquire some data, wait for all the data to be acquired before proceeding
std::thread acq1(AcquireData, boardHandle1, memoryAddress1a);
std::thread acq2(AcquireData, boardHandle2, memoryAddress2a);
acq1.join();
acq2.join();

while(user doesn't interrupt)
{

//Process first batch of data while acquiring new data
std::thread proc1(ProcessData,memoryAddress1a);
std::thread proc2(ProcessData,memoryAddress2a);
acq1(AcquireData, boardHandle1, memoryAddress1b);
acq2(AcquireData, boardHandle2, memoryAddress2b);
acq1.join();
acq2.join();
proc1.join();
proc2.join();
/*Proceed in this manner, alternating which memory address 
is written to and being processed until the user interrupts the program.*/
}

That's the main gist of it. The next run of the loop would write to the "a" memory addresses while processing the "b" data and continue to alternate (I can get the code to do that, just took it out to prevent cluttering up the problem).

Anyway, the problem (as I'm sure some people can already tell) is that the second time I try to use acq1 and acq2, the compiler (VS2012) says "IntelliSense: call of an object of a class type without appropriate operator() or conversion functions to pointer-to-function type". Likewise, if I put std::thread in front of acq1 and acq2 again, it says " error C2374: 'acq1' : redefinition; multiple initialization".

So the question is, can I reassign threads to a new task when they have completed their previous task? I always wait for the previous use of the thread to end before calling it again, but I don't know how to reassign the thread, and since it's in a loop, I can't make a new thread each time (or if I could, that seems wasteful and unnecessary, but I could be mistaken).

Thanks in advance

Answer 1

The easiest way is to use a waitable queue of std::function objects. Like this:

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


class ThreadPool
{
    public:

    ThreadPool (int threads) : shutdown_ (false)
    {
        // Create the specified number of threads
        threads_.reserve (threads);
        for (int i = 0; i < threads; ++i)
            threads_.emplace_back (std::bind (&ThreadPool::threadEntry, this, i));
    }

    ~ThreadPool ()
    {
        {
            // Unblock any threads and tell them to stop
            std::unique_lock <std::mutex> l (lock_);

            shutdown_ = true;
            condVar_.notify_all();
        }

        // Wait for all threads to stop
        std::cerr << "Joining threads" << std::endl;
        for (auto& thread : threads_)
            thread.join();
    }

    void doJob (std::function <void (void)> func)
    {
        // Place a job on the queu and unblock a thread
        std::unique_lock <std::mutex> l (lock_);

        jobs_.emplace (std::move (func));
        condVar_.notify_one();
    }

    protected:

    void threadEntry (int i)
    {
        std::function <void (void)> job;

        while (1)
        {
            {
                std::unique_lock <std::mutex> l (lock_);

                while (! shutdown_ && jobs_.empty())
                    condVar_.wait (l);

                if (jobs_.empty ())
                {
                    // No jobs to do and we are shutting down
                    std::cerr << "Thread " << i << " terminates" << std::endl;
                    return;
                 }

                std::cerr << "Thread " << i << " does a job" << std::endl;
                job = std::move (jobs_.front ());
                jobs_.pop();
            }

            // Do the job without holding any locks
            job ();
        }

    }

    std::mutex lock_;
    std::condition_variable condVar_;
    bool shutdown_;
    std::queue <std::function <void (void)>> jobs_;
    std::vector <std::thread> threads_;
};

void silly (int n)
{
    // A silly job for demonstration purposes
    std::cerr << "Sleeping for " << n << " seconds" << std::endl;
    std::this_thread::sleep_for (std::chrono::seconds (n));
}

int main()
{
    // Create two threads
    ThreadPool p (2);

    // Assign them 4 jobs
    p.doJob (std::bind (silly, 1));
    p.doJob (std::bind (silly, 2));
    p.doJob (std::bind (silly, 3));
    p.doJob (std::bind (silly, 4));
}

Answer 2

The std::thread class is designed to execute exactly one task (the one you give it in the constructor) and then end. If you want to do more work, you'll need a new thread. As of C++11, that's all we have. Thread pools didn't make it into the standard. (I'm uncertain what C++14 has to say about them.)

Fortunately, you can easily implement the required logic yourself. Here is the large-scale picture:

• Start n worker threads that all do the following:
  • Repeat while there is more work to do:
    • Grab the next task t (possibly waiting until one becomes ready).
    • Process t.
• Keep inserting new tasks in the processing queue.
• Tell the worker threads that there is nothing more to do.
• Wait for the worker threads to finish.

The most difficult part here (which is still fairly easy) is properly designing the work queue. Usually, a synchronized linked list (from the STL) will do for this. Synchronized means that any thread that wishes to manipulate the queue must only do so after it has acquired a std::mutex so to avoid race conditions. If a worker thread finds the list empty, it has to wait until there is some work again. You can use a std::condition_variable for this. Each time a new task is inserted into the queue, the inserting thread notifies a thread that waits on the condition variable and will therefore stop blocking and eventually start processing the new task.

The second not-so-trivial part is how to signal to the worker threads that there is no more work to do. Clearly, you can set some global flag but if a worker is blocked waiting at the queue, it won't realize any time soon. One solution could be to notify_all() threads and have them check the flag each time they are notified. Another option is to insert some distinct “toxic” item into the queue. If a worker encounters this item, it quits itself.

Representing a queue of tasks is straight-forward using your self-defined task objects or simply lambdas.

All of the above are C++11 features. If you are stuck with an earlier version, you'll need to resort to third-party libraries that provide multi-threading for your particular platform.

While none of this is rocket science, it is still easy to get wrong the first time. And unfortunately, concurrency-related bugs are among the most difficult to debug. Starting by spending a few hours reading through the relevant sections of a good book or working through a tutorial can quickly pay off.