dijkstra-omp.c

迪杰克斯特拉最短路径算法的OpenMP实现。体现了OpenMP并行编程的结构

// Dijkstra.c

// OpenMP example program:  Dijkstra shortest-path finder in a
// bidirectional graph; finds the shortest path from vertex 0 to all
// others

// 编 译:  icl /Qopenmp Dijkstra-omp.c(Intel 编译)
// 
// usage:  dijkstra nv print  (例如: Dijkstra-omp.exe 22000 0)

// where nv is the size of the graph, and print is 1 if graph and min
// distances are to be printed out, 0 otherwise

#include <omp.h>
#include <time.h>
// global variables, shared by all threads by default

int nv,  // number of vertices
    *notdone, // vertices not checked yet
    nth,  // number of threads
    chunk,  // number of vertices handled by each thread
    md,  // current min over all threads
    mv,  // vertex which achieves that min
    largeint = -1;  // max possible unsigned int

unsigned *ohd,  // 1-hop distances between vertices; "ohd[i][j]" is
         // ohd[i*nv+j]
         *mind;  // min distances found so far

void init(int ac, char **av)
{  int i,j,tmp;
   nv = atoi(av[1]);
   ohd = malloc(nv*nv*sizeof(int));
   mind = malloc(nv*sizeof(int));
   notdone = malloc(nv*sizeof(int)); 
   // random graph
   for (i = 0; i < nv; i++)  
      for (j = i; j < nv; j++)  {
         if (j == i) ohd[i*nv+i] = 0;
         else  {
            ohd[nv*i+j] = rand() % 20;
            ohd[nv*j+i] = ohd[nv*i+j];
         }
      }
   for (i = 1; i < nv; i++)  {
      notdone[i] = 1;
      mind[i] = ohd[i];
   }
}

// finds closest to 0 among notdone, among s through e
void findmymin(int s, int e, unsigned *d, int *v)
{  int i;
   *d = largeint; 
   for (i = s; i <= e; i++)
      if (notdone[i] && mind[i] < *d)  {
         *d = ohd[i];
         *v = i;
      }
}

// for each i in [s,e], ask whether a shorter path to i exists, through
// mv
void updatemind(int s, int e)
{  int i;
   for (i = s; i <= e; i++)
      if (mind[mv] + ohd[mv*nv+i] < mind[i])  
         mind[i] = mind[mv] + ohd[mv*nv+i];
}

void dowork()
{  
   #pragma omp parallel  
   {  int startv,endv,  // start, end vertices for my thread
          step,  // whole procedure goes nv steps
          mymv,  // vertex which attains the min value in my chunk
          me = omp_get_thread_num();  
          unsigned mymd;  // min value found by this thread
      #pragma omp single   
      {  nth = omp_get_num_threads();  
         if (nv % nth != 0) {
            printf("nv must be divisible by nth\n");
            exit(1);
         }
         chunk = nv/nth;  
         printf("there are %d threads\n",nth);  
      }
      startv = me * chunk; 
      endv = startv + chunk - 1;
      for (step = 0; step < nv; step++)  {
         // find closest vertex to 0 among notdone; each thread finds
         // closest in its group, then we find overall closest
         #pragma omp single 
         {  md = largeint; mv = 0;  }
         findmymin(startv,endv,&mymd,&mymv);
         // update overall min if mine is smaller
         #pragma omp critical  
         {  if (mymd < md)  
               {  md = mymd; mv = mymv;  }
         }
         #pragma omp barrier 
         // mark new vertex as done 
         #pragma omp single 
         {  notdone[mv] = 0;  }
         // now update my section of mind
         updatemind(startv,endv);
         #pragma omp barrier 
         }
   }
}

int main(int argc, char **argv)
{  int i,j,print;
	clock_t start, stop;
   init(argc,argv);
   start = clock();
   // parallel
   dowork();  
   // back to single thread
   stop = clock();
   print = atoi(argv[2]);
   if (print)  {
      printf("graph weights:\n");
      for (i = 0; i < nv; i++)  {
         for (j = 0; j < nv; j++)  
            printf("%u  ",ohd[nv*i+j]);
         printf("\n");
      }
      printf("minimum distances:\n");
      for (i = 1; i < nv; i++)
         printf("%u\n",mind[i]);
	  
   }
   printf("use time: %f seconds\n",((double)(stop - start)/1000.0));
   return 0;
}