优化文件写入 C

Question

how can I improve speed while working with a file? Here's my code:

#include <stdio.h>
#include <string.h>
#include <sys/time.h>


void swap(char *x, char *y)
{
  char temp;
  temp = *x;
  *x = *y;
  *y = temp;
}


void permute(char *a, int l, int r)
{
  int i;
  if (l == r) {
    printf("%s\n", a);
    FILE *fp;
    fp = fopen("test.txt", "a");
    fprintf(fp,"%s\n", a );

    fclose(fp);
  }

  else
  {
    for (i = l; i <= r; i++)
    {
      swap((a+l), (a+i));
      permute(a, l+1, r);
      swap((a+l), (a+i)); //backtrack
    }
  }
}


int main()
{
  remove("test.txt");
  struct timeval  tv1, tv2;
  gettimeofday(&tv1, NULL);
  char str[] = "ABCD";
  int n = strlen(str);
  permute(str, 0, n-1);

  gettimeofday(&tv2, NULL);
  printf ("Total time = %f seconds\n",
  (double) (tv2.tv_usec - tv1.tv_usec) / 1000000 +
  (double) (tv2.tv_sec - tv1.tv_sec));


  return 0;
}

If I run it saving the permutations to a file I get Total time = 0.161452 seconds

Without saving permutations to a file I get Total time = 0.00000 seconds

Thanks!

Answer 1

Basic optimizations:

• Leave the file open for as long as you write to it.
• There is no need to remove the file at start up, you can as well "w" write to it and it will have the same effect.
• Replace recursion with a loop.

These will make a vast difference. Opening a file takes lots of time, as does writing to it or removing it. Everything file-related are the main bottlenecks in this program.

As for the recursion, you have written a version that isn't likely tail-call optimized, so it will bloat up the stack needlessly.

Advanced optimizations:

• Separate algorithm from printing to screen and/or file. Store all results in a RAM buffer. When done, print to screen or file.
• Outsource printing to file to a dedicated thread, which can chew away on the file write in the background while the main thread does other things. That way, the file write isn't a slow, blocking call.
• Investigate the algorithm itself. Could you do things in different order? Minimize the number of branches?

Micro-optimizations (will only have a small effect):

• Replace strlen with sizeof .
• Replace printf / fprintf with puts / fputs .
• restrict qualify the pointers in swap (probably not needed since it should get inlined).

Answer 2

Try open/close file only once, in the main() function, like below:

void permute(char *a, int l, int r, FILE *fp)
{
    int i;
    if (l == r) 
    {
        printf("%s\n", a);
        fprintf(fp, "%s\n", a);
    }
    else
    {
        for (i = l; i <= r; i++)
        {
            swap((a+l), (a+i));
            permute(a, l+1, r, fp);
            swap((a+l), (a+i)); //backtrack
        }
    }
}

int main()
{
    remove("test.txt");
    struct timeval  tv1, tv2;
    gettimeofday(&tv1, NULL);
    char str[] = "ABCD";
    int n = strlen(str);

    // open file
    FILE *fp;
    fp = fopen("test.txt", "a");

    permute(str, 0, n-1, fp);

    // close file
    fclose(fp);

    gettimeofday(&tv2, NULL);
    printf ("Total time = %f seconds\n",
        (double) (tv2.tv_usec - tv1.tv_usec) / 1000000 +
        (double) (tv2.tv_sec - tv1.tv_sec));

    return 0;
}