parallel quicksort in c

Question

After a lot of searching for an implementation of parallel quicksort in c, I'm about to dive in and code it myself. (I need to sort an array of about 1 million text strings.) It seems that all the implementations I have found divide the work inside the qsort function itself, which creates a huge amount of overhead in partitioning the relatively small amount of work per thread.

Would it not be much faster to divide the 1 million strings by the number of threads (in my case, 24 threads), and have them each work on a section, and then do a mergesort? Granted, this has the theoretical disadvantage that it is not an in-place sort, but with gobs of memory available it is not a problem. The machine this runs on has 12 (very fast) physical/24 logical cores and 192 GB (yes, gigabytes) of memory. Currently, even on this machine, the sort takes almost 8 minutes!

Answer 1

Would it not be much faster to divide the 1 million strings by the number of threads (in my case, 24 threads), and have them each work on a section, and then do a mergesort?

Its a good idea.

But you can make some observation by writing toy programs for quick-sort and merge-sort and take advantages of their algorithmic-/run-time-behavior.

For example. quick-sort sorts while dividing process (pivot element will be put in its final place at the end of that iteration) and merge-sort sorts while merging (sorting is done after the whole working-set is broken down (divided) into very granular-units where it can be directly compared with other granular-units (== or strcmp()).

Mixing up algorithms based on the nature of the working set is a good idea.

With respect to the parallel sorting, here is my parallel merge-sort for you to get started.

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>

#define NOTHREADS 2

/*

gcc -ggdb -lpthread parallel-mergesort.c 


NOTE: 
The mergesort boils downs to this.. 
Given two sorted array's how do we merge this?

We need a new array to hold the result of merging
otherwise it is not possible to do it using array, 
so we may need a linked list

*/

int a[] = {10, 8, 5, 2, 3, 6, 7, 1, 4, 9};

typedef struct node {
int i;
int j;
} NODE;

void merge(int i, int j)
{
        int mid = (i+j)/2;
        int ai = i;
        int bi = mid+1;

        int newa[j-i+1], newai = 0;

        while(ai <= mid && bi <= j) {
                if (a[ai] > a[bi])
                        newa[newai++] = a[bi++];
                else                    
                        newa[newai++] = a[ai++];
        }

        while(ai <= mid) {
                newa[newai++] = a[ai++];
        }

        while(bi <= j) {
                newa[newai++] = a[bi++];
        }

        for (ai = 0; ai < (j-i+1) ; ai++)
                a[i+ai] = newa[ai];

}

void * mergesort(void *a)
{
                NODE *p = (NODE *)a;
                NODE n1, n2;
                int mid = (p->i+p->j)/2;
                pthread_t tid1, tid2;
                int ret;

                n1.i = p->i;
                n1.j = mid;

                n2.i = mid+1;
                n2.j = p->j;

                if (p->i >= p->j) return;

                ret = pthread_create(&tid1, NULL, mergesort, &n1);
                if (ret) {
                        printf("%d %s - unable to create thread - ret - %d\n", __LINE__, __FUNCTION__, ret);    
                        exit(1);
                }


                ret = pthread_create(&tid2, NULL, mergesort, &n2);
                if (ret) {
                        printf("%d %s - unable to create thread - ret - %d\n", __LINE__, __FUNCTION__, ret);    
                        exit(1);
                }

                pthread_join(tid1, NULL);
                pthread_join(tid2, NULL);

                merge(p->i, p->j);
                pthread_exit(NULL);
}


int main()
{
                int i;
                NODE m;
                m.i = 0;
                m.j = 9;
                pthread_t tid;

                int ret; 

                ret=pthread_create(&tid, NULL, mergesort, &m);
                if (ret) {
                        printf("%d %s - unable to create thread - ret - %d\n", __LINE__, __FUNCTION__, ret);    
                        exit(1);
                }

                pthread_join(tid, NULL);

                for (i = 0; i < 10; i++)
                                printf ("%d ", a[i]);

                printf ("\n");

                // pthread_exit(NULL);
                return 0;
}

Good luck!