Grand central dispatch is giving me a slower execution time - c

Question

So, I am about 99% certain that I have implemented something wrong, but heres the deal.
I have been playing around with Grand Central Dispatch, and put together an experiment calculating MD5 hashes. I am running a macbook air with an i5, so have 4 cores available. This would led me to believe that using Grand Central Dispatch to calculate the hashes would be approx 4 times faster. But, for some reason, it appears to be slower.
Code below
Using GCD
#include <stdio.h>
#include <time.h>
#import <CommonCrypto/CommonDigest.h>
#import <dispatch/dispatch.h>
int main (int argc, const char * argv[])
{
int i,j,k,l,a;
int num_chars = 4, total;
clock_t start, end;
double elap;
printf("Calculating hashes for %d chars\n", num_chars);
total = num_chars ^ 64;
printf("Calculating %d hashes\n", total);
dispatch_queue_t queue = dispatch_get_global_queue(0,0);
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_group_t group = dispatch_group_create();
start = clock();
printf("Starting calculation queue\n");
for(i=0;i<62;i++) {
for(j=0;j<62;j++) {
for(k=0;k<62;k++) {
for(l=0;l<62;l++) {
dispatch_group_async(group, queue, ^{
char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
char buffer[10];
char out[100];
unsigned char hash[16];
sprintf(buffer, "%c%c%c%c", letters[i], letters[j], letters[k], letters[l]);
CC_MD5(buffer, strlen(buffer), hash);
});
}
}
}
}
printf("Finished calculation queue\n");
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
end = clock();
elap = ((double) end - start) / CLOCKS_PER_SEC;
printf("Time taken %2f\n", elap);
return 0;
}
Compile and run...
gcc -o a.out main.c
./a.out
Calculating hashes for 4 chars
Calculating 68 hashes
Starting calculation queue
Finished calculation queue
Time taken 35.193133
Looking at Activity Monitor, I can see all 4 cores max out while the script is running.
Now, comment out the dispatching....
#include <stdio.h>
#include <time.h>
#import <CommonCrypto/CommonDigest.h>
#import <dispatch/dispatch.h>
int main (int argc, const char * argv[])
{
int i,j,k,l,a;
int num_chars = 4, total;
clock_t start, end;
double elap;
printf("Calculating hashes for %d chars\n", num_chars);
total = num_chars ^ 64;
printf("Calculating %d hashes\n", total);
dispatch_queue_t queue = dispatch_get_global_queue(0,0);
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_group_t group = dispatch_group_create();
start = clock();
printf("Starting calculation queue\n");
for(i=0;i<62;i++) {
for(j=0;j<62;j++) {
for(k=0;k<62;k++) {
for(l=0;l<62;l++) {
//dispatch_group_async(group, queue, ^{
char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
char buffer[10];
char out[100];
unsigned char hash[16];
sprintf(buffer, "%c%c%c%c", letters[i], letters[j], letters[k], letters[l]);
CC_MD5(buffer, strlen(buffer), hash);
//});
}
}
}
}
printf("Finished calculation queue\n");
//dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
end = clock();
elap = ((double) end - start) / CLOCKS_PER_SEC;
printf("Time taken %2f\n", elap);
return 0;
}
Compile and run
gcc -o b.out main.c
./b.out
Calculating hashes for 4 chars
Calculating 68 hashes
Starting calculation queue
Finished calculation queue
Time taken 7.511273
Looking at Activity Monitor, it only shows 1 core active while the script runs.

Answer 1

There's probably too little work being done in the dispatch that it doesn't make the overhead involved with dispatching worthwhile. I would try and increase the amount of work done in each dispatch. I wouldn't have a clue whether this would help, but try:
Moving the dispatch up a few loops, perhaps wrap the k or j loop inside the dispatched block instead, to get it to do more work.
Remove the call sprintf and strlen. In fact, the block could be simplified to:
static const char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
unsigned char hash[16];
char buffer[4] = { letters[i], letters[j], letters[k], letters[l] };
CC_MD5(buffer, sizeof buffer, hash);