C++多线程加速for循环

• 引用传参需要使用std::ref
• 普通循环

#include <iostream>
#include <vector>
#include <sys/time.h>
#include <math.h>
using namespace std;

int main(int argc, char **argv) {
  struct timeval start, end;
  vector<vector<double> > a(10000, vector<double>(10000));
  gettimeofday(&start, NULL);
  for(int i = 0; i < 10000; ++i) {
    for(int j = 0; j < 10000; ++j) {
      a[i][j] = cos(1.05);
    }
  }
  gettimeofday(&end, NULL);
  float cost_time = (end.tv_usec-start.tv_usec)/1000000.0 + end.tv_sec-start.tv_sec;
  cout << cost_time << endl;
  cout << a[666][6] << endl;

  return 0;
}
// 输出 0.311707
//     0.497571

• 多线程

#include <iostream>
#include <vector>
#include <sys/time.h>
#include <math.h>
#include <thread>
using namespace std;

void parallel(vector<vector<double> > &a, int index){
  for(int i = index; i < index+2000; ++i) {
    for(int j = 0; j < 10000; ++j) {
      a[i][j] = cos(1.05);
    }
  }
}

int main(int argc, char **argv) {
  struct timeval start, end;
  vector<vector<double> > a(10000, vector<double>(10000));
  gettimeofday(&start, NULL);
  thread t1(parallel, std::ref(a), 0);
  thread t2(parallel, std::ref(a), 2000);
  thread t3(parallel, std::ref(a), 4000);
  thread t4(parallel, std::ref(a), 6000);
  thread t5(parallel, std::ref(a), 8000);
  t1.join();
  t2.join();
  t3.join();
  t4.join();
  t5.join();
  gettimeofday(&end, NULL);
  float cost_time = (end.tv_usec-start.tv_usec)/1000000.0 + end.tv_sec-start.tv_sec;
  cout << cost_time << endl;
  cout << a[666][6] << endl;

  return 0;
}
// 输出 0.067235
//     0.497571

• 均分成5个线程，速度提高差不多5倍