How to optimize N-queen with openmp - C - c

Question

I am learning OpenMP and found the following code to solve N-Queens problem. I want to make faster solution with parallel code, but I don't know how to do it. Everytime I tried something, it is only go to worse and slower solution.
This is code:
#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
#define BOARD_SIZE 15
int board[20],solutions;
int main() {
double start_time, end_time, result_time;
void queen(int row,int n);
start_time = omp_get_wtime();
queen(1, BOARD_SIZE);
end_time = omp_get_wtime();
result_time = end_time - start_time;
printf("Time = %.3f seconds\n", result_time);
printf("Number of solutions: %d\n", solutions);
return 0;
}
/*funtion to check conflicts
If no conflict for desired postion returns 1 otherwise returns 0*/
int place(int row, int column) {
for(int i=1;i<=row-1;++i) {
//checking column and diagonal conflicts
if(board[i]==column)
return 0;
else
if(abs(board[i]-column)==abs(i-row))
return 0;
}
return 1; //no conflicts
}
//function to check for proper positioning of queen
void queen(int row,int n) {
for(int column=1;column<=n;++column) {
if(place(row,column)) {
board[row]=column; //no conflicts so place queen
if(row==n) //dead end
solutions++;
else //try queen with next position
queen(row+1,n);
}
}
}
What can I do for optimize this code?

Answer 1

To parallelize your code with OpenMP I did the following:
I have added #pragma omp parallel and #pragma omp single before the first call of function queen.
To avoid data race I added #pragma omp atomic before solutions++; (Note that I have also tried using reduction, but in that case tasks have to be synchronized, which makes it a slower option).
To make parallel recursive calls I had to create a (private) copy of the board (and to pass its pointer to function queen). To create tasks I used #pragma omp task if(row<4). Note that if(row<4) is used to limit the number of tasks created.
The measured speedup is about 6x on 8 cores.
#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
#define BOARD_SIZE 15
int solutions;
int main() {
double start_time, end_time, result_time;
int board[BOARD_SIZE+1];
void queen(int row,int n, int*);
start_time = omp_get_wtime();
#pragma omp parallel
#pragma omp single
queen(1, BOARD_SIZE, board);
end_time = omp_get_wtime();
result_time = end_time - start_time;
printf("Time = %.3f seconds\n", result_time);
printf("Number of solutions: %d\n", solutions);
return 0;
}
/*funtion to check conflicts
If no conflict for desired postion returns 1 otherwise returns 0*/
int place(int row, int column, int* board) {
for(int i=1;i<=row-1;++i) {
//checking column and diagonal conflicts
if(board[i]==column)
return 0;
else
if(abs(board[i]-column)==abs(i-row))
return 0;
}
return 1; //no conflicts
}
//function to check for proper positioning of queen
void queen(int row,int n, int* board) {
for(int column=1;column<=n;++column) {
if(place(row,column, board)) {
if(row==n) //dead end
{
#pragma omp atomic
solutions++;
}
else
//try queen with next position
{
int local_board[BOARD_SIZE+1];
for(int i=1;i<row;i++) local_board[i]=board[i]; //copy board to local_board
local_board[row]=column; //no conflicts so place queen
#pragma omp task if(row<4)
queen(row+1,n, local_board);
}
}
}
}
Please indicate if you need more explanation.

Answer 2

I see no omp directives in your code.
As Jerome suggested, the way to do this is with tasks.
I coded this a while ago. Download my textbook and see the chapter with OMP examples (or html version. The solution uses taskloop and omp cancel taskloop.
Unfortunately, you get no speedup: the sequential code finds the first solution very fast, and going parallel doesn't make it faster.
Oh, sorry, this uses C++. I'm sure the translation to C is simple enough. But C++ is a better language these days.