randomread.c

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include "pcg/pcg_basic.h"

struct timespec diff_t(struct timespec, struct timespec);

int main(void) {
	pcg32_srandom(time(NULL) ^ (intptr_t)&printf, rand());
	struct timespec start, end;
	long long int diff;
	int a;
	int rand[200000];
	a = 200000;
	while(a--) {
		rand[a] = (int)pcg32_boundedrand(1000);
	}
	char* a1 = malloc(1000*1000);
	char** a2 = malloc(1000);
	a = 1000;
	while(a--) {
		a2[a] = malloc(1000);
	}
	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
	// 2D array of 1000 x 1000 elements.
	int c;
	a = 200000;
	while(a-->0) {
		c = a1[rand[a--]*1000+rand[a]];
		c+=2;
	}
	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
	diff = (diff_t(end, start)).tv_nsec;
	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
	// Jagged array of 1000 x 1000 elements.
	a = 200000;
	while(a-->0) {
		c = a2[rand[a--]][rand[a]];
		c+=2;
	}
	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);

	printf("Perf:\n%llins normal array\n%llins jagged array\n", diff, (long long int)(diff_t(end, start)).tv_nsec);
	return EXIT_SUCCESS;

}


struct timespec diff_t(struct timespec end, struct timespec start)
{
	struct timespec temp;
	if ((end.tv_nsec-start.tv_nsec)<0) {
		temp.tv_sec = end.tv_sec-start.tv_sec-1;
		temp.tv_nsec = 1000000000+end.tv_nsec-start.tv_nsec;
	} else {
		temp.tv_sec = end.tv_sec-start.tv_sec;
		temp.tv_nsec = end.tv_nsec-start.tv_nsec;
	}
	return temp;
}

randomreadresult

With compiler optimizations:
Perf:
908ns normal array
333ns jagged array
Perf:
419ns normal array
138ns jagged array
Perf:
454ns normal array
140ns jagged array

Without:
Perf:
517845ns normal array
582495ns jagged array
Perf:
1255826ns normal array
1478993ns jagged array
Perf:
979111ns normal array
1466433ns jagged array

results

With optimizations:
Perf:
580ns normal array
225ns jagged array
Perf:
652ns normal array
225ns jagged array
Perf:
1062ns normal array
716ns jagged array
Without:
Perf:
5775760ns normal array
2605375ns jagged array
Perf:
5751747ns normal array
6772963ns jagged array
Perf:
4785553ns normal array
2549643ns jagged array

source.c

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

struct timespec diff_t(struct timespec, struct timespec);

int main(void) {
        struct timespec start, end;
        long long int diff;
        int a, x, tmp1 = 0;
        char a1[1000*1000];
        char* a2[1000];
        a = 1000;
        while(a--) {
                a2[a] = malloc(1000);
        }
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
        // 2D array of 1000 x 1000 elements.
        for (a = 0; a < 1000; a++) {
                for (x = 0; x < 1000; x++) {
                        int c = a1[tmp1+x];
                }
                tmp1 += 1000;
        }

        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
        diff = (diff_t(end, start)).tv_nsec;

        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
        // Jagged array of 100 x 100 elements.
        for (a = 0; a < 1000; a++) {
                for (x = 0; x < 1000; x++) {
                        int c = a2[a][x];
                }
        }
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);

        printf("Perf:\n%llins normal array\n%llins jagged array\n", diff, (long long int)(diff_t(end, start)).tv_nsec);
        return EXIT_SUCCESS;

}


struct timespec diff_t(struct timespec end, struct timespec start)
{
        struct timespec temp;
        if ((end.tv_nsec-start.tv_nsec)<0) {
                temp.tv_sec = end.tv_sec-start.tv_sec-1;
                temp.tv_nsec = 1000000000+end.tv_nsec-start.tv_nsec;
        } else {
                temp.tv_sec = end.tv_sec-start.tv_sec;
                temp.tv_nsec = end.tv_nsec-start.tv_nsec;
        }
        return temp;
}