GPU621/NoName

NoName

Our project: C++11 Threads Library Comparison to OpenMP

Group Members

1. Saad Toor [1] Research etc.
2. Danylo Medinski [2] Research etc.

Progress

Oct 17th:

1. Picked topic
2. Picked presentation date.
3. Gathering information

Oct 20th:

1. Created Wiki page

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OpenMp vs C++ 11 Threads

What are C++ 11 Threads

With the introduction of C++ 11, there were major changes and additions made to the C++ Standard libraries. One of the most significant changes was the inclusion of multi-threading libraries. Before C++ 11 in order to implement multi-threading, external libraries or language extensions such as OpenMp was required. The C++ 11 thread support library includes these 4 files to enable multi-threading

• <thread> - class and namespace for working with threads
• <mutex> - provides support for mutual exclusion
• <contition_variable> - a synchronization primitive that can be used to block a thread, or multiple threads at the same time, until another thread both modifies a shared variable (the condition), and notifies the condition_variable.
• <future> - Describes components that a C++ program can use to retrieve in one thread the result (value or exception) from a function that has run in the same thread or another thread.

Programming Models

SPMD

An example of the SPMD programming model in STD Threads using an atomic barrier

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <chrono>
#include <vector>
#include <thread>
#include <atomic>
using namespace std::chrono;

std::atomic<double> pi;

void reportTime(const char* msg, steady_clock::duration span) {
     auto ms = duration_cast<milliseconds>(span);
     std::cout << msg << " - took - " <<
     ms.count() << " milliseconds" << std::endl;
}
void run(int ID, double stepSize, int nthrds, int n)
{
     double x;
     double sum = 0.0;
     for (int i = ID; i < n; i = i + nthrds){
      	   x = (i + 0.5)*stepSize;
          sum += 4.0 / (1.0 + x*x);
     }
     sum = sum * stepSize;
     pi = pi + sum;
}

int main(int argc, char** argv) {
    if (argc != 3) {
         std::cerr << argv[0] << ": invalid number of arguments\n";
         return 1;
    }

    int n = atoi(argv[1]);
    int numThreads = atoi(argv[2]);

    steady_clock::time_point ts, te;

    // calculate pi by integrating the area under 1/(1 + x^2) in n steps 
    ts = steady_clock::now();

    std::vector<std::thread> threads(numThreads);

    double stepSize = 1.0 / (double)n;

    for (int ID = 0; ID < numThreads; ID++) {
         int nthrds = std::thread::hardware_concurrency();
         if (ID == 0) numThreads = nthrds;
         threads[ID] = std::thread(run, ID, stepSize, 8, n);
    }

    te = steady_clock::now();

    for (int i = 0; i < numThreads; i++){
         threads[i].join();
    }
	
    std::cout << "n = " << n << std::fixed << std::setprecision(15) << "\n pi(exact) = " << 3.141592653589793 << "\n pi(calcd) = " << pi << std::endl;

    reportTime("Integration", te - ts);

    // terminate
    char c;
    std::cout << "Press Enter key to exit ... ";
    std::cin.get(c);
}