Extend the life of threads with synchronization (C++11)

Question

I have a program with a function which takes a pointer as arg, and a main. The main is creating n threads, each of them running the function on different me

Answer 1

The following is a simple compiling and working code performing some random stuffs. It implements aleguna's concept of barrier. The task length of each thread is different so it is really necessary to have a strong synchronization mechanism. I will try to do a pool on the same tasks and benchmark the result, and then maybe with futures as pointed out by Andy Prowl.

#include 
#include 
#include 
#include 
#include 
#include 
#include 

const unsigned int n_threads=4; //varying this will not (almost) change the total amount of work
const unsigned int task_length=30000/n_threads;
const float task_length_variation=task_length/n_threads;
unsigned int rep=1000; //repetitions of tasks

class t_chronometer{
 private: 
  std::chrono::steady_clock::time_point _t;

 public:
  t_chronometer(): _t(std::chrono::steady_clock::now()) {;}
  void reset() {_t = std::chrono::steady_clock::now();}
  double get_now() {return std::chrono::duration_cast>(std::chrono::steady_clock::now() - _t).count();}
  double get_now_ms() {return 
      std::chrono::duration_cast>(std::chrono::steady_clock::now() - _t).count();}
};

class t_barrier {
 private:
   std::mutex m_mutex;
   std::condition_variable m_cond;
   unsigned int m_threshold;
   unsigned int m_count;
   unsigned int m_generation;

 public:
   t_barrier(unsigned int count):
    m_threshold(count),
    m_count(count),
    m_generation(0) {
   }

   bool wait() {
      std::unique_lock lock(m_mutex);
      unsigned int gen = m_generation;

      if (--m_count == 0)
      {
          m_generation++;
          m_count = m_threshold;
          m_cond.notify_all();
          return true;
      }

      while (gen == m_generation)
          m_cond.wait(lock);
      return false;
   }
};


using namespace std;

void do_something(complex * c, unsigned int max) {
  complex a(1.,0.);
  complex b(1.,0.);
  for (unsigned int i = 0; i * c, unsigned int max, t_barrier* start_barrier, t_barrier* end_barrier) {
  while (!done) {
    start_barrier->wait ();
    do_something(c,max);
    end_barrier->wait ();
  }
  cout << "task finished" << endl;
}

int main() {
  t_chronometer t;

  std::default_random_engine gen;
  std::normal_distribution dis(.0,1000.0);

  complex cpx[n_threads];
  for (unsigned int i=0; i < n_threads; i++) {
    cpx[i] = complex(dis(gen), dis(gen));
  }

  t_barrier start_barrier (n_threads + 1); // child threads + main thread
  t_barrier end_barrier (n_threads + 1); // child threads + main thread

  std::thread mythread[n_threads];
  unsigned long int sum=0;
  for (unsigned int i=0; i < n_threads; i++) {
    unsigned int max = task_length +  i * task_length_variation;
    cout << i+1 << "th task length: " << max << endl;
    mythread[i] = std::thread(task, &cpx[i], max, &start_barrier, &end_barrier);
    sum+=max;
  }
  cout << "total task length " << sum << endl;

  complex c(0,0);
  for (unsigned long int j=1; j < rep+1; j++) {
    start_barrier.wait (); //give to the threads the missing call to start
    if (j==rep) done=true;
    end_barrier.wait (); //wait for the call from each tread
    if (j%100==0) cout << "cycle: " << j << endl;
    for (unsigned int i=0; i