How does a sorting network beat generic sorting algorithms?

Question

In reference to fastest sort of fixed length 6 int array, I do not fully understand how this sorting network beats an algorithm like insertion sort.

Form that question, here is a comparison of the number of CPU cycles taken to complete the sort :

Linux 32 bits, gcc 4.4.1, Intel Core 2 Quad Q8300, -O2

• Insertion Sort (Daniel Stutzbach) : 1425
• Sorting Networks (Daniel Stutzbach) : 1080

The code used is as follows :

Insertion Sort (Daniel Stutzbach)

static inline void sort6_insertion_sort_v2(int *d){
    int i, j;
    for (i = 1; i < 6; i++) {
            int tmp = d[i];
            for (j = i; j >= 1 && tmp < d[j-1]; j--)
                    d[j] = d[j-1];
            d[j] = tmp;
    }
}

Sorting Networks (Daniel Stutzbach)

static inline void sort6_sorting_network_v1(int * d){
#define SWAP(x,y) if (d[y] < d[x]) { int tmp = d[x]; d[x] = d[y]; d[y] = tmp; }
    SWAP(1, 2);
    SWAP(0, 2);
    SWAP(0, 1);
    SWAP(4, 5);
    SWAP(3, 5);
    SWAP(3, 4);
    SWAP(0, 3);
    SWAP(1, 4);
    SWAP(2, 5);
    SWAP(2, 4);
    SWAP(1, 3);
    SWAP(2, 3);
#undef SWAP
}

I understand that sorting networks are really good for sorting in parallel, because some of the steps are independent of the other steps. But here we are not using the parallelization.

I expect it to be faster, as it has the advantage of knowing the exact number of elements beforehand. Where and why exactly does insertion sort make unnecessary comparisons?

EDIT1:

This is the input set these codes are compared against:

int d[6][6] = {\
    {1, 2, 3, 4, 5, 6},\
    {6, 5, 4, 3, 2, 1},\
    {100, 2, 300, 4, 500, 6},\
    {100, 2, 3, 4, 500, 6},\
    {1, 200, 3, 4, 5, 600},\
    {1, 1, 2, 1, 2, 1}\
};\

Answer 1

But here we are not using the parallelization.

Modern CPUs can figure out when instructions are independent and will execute them in parallel. Hence, even though there's only one thread, the sorting network's parallelism can be exploited.

Where exactly does insertion sort make unnecessary comparisons?

The easiest way to see the extra comparisons is to do an example by hand.

Insertion sort:
6 5 4 3 2 1
5 6 4 3 2 1
5 4 6 3 2 1
4 5 6 3 2 1
4 5 3 6 2 1
4 3 5 6 2 1
3 4 5 6 2 1
3 4 5 2 6 1
3 4 2 5 6 1
3 2 4 5 6 1
2 3 4 5 6 1
2 3 4 5 1 6
2 3 4 1 5 6
2 3 1 4 5 6
2 1 3 4 5 6
1 2 3 4 5 6

Sorting network:
6 5 4 3 2 1
6 4 5 3 2 1
5 4 6 3 2 1
4 5 6 3 2 1 # These three can execute in parallel with the first three
4 5 6 3 1 2 #
4 5 6 2 1 3 #
4 5 6 1 2 3
1 5 6 4 2 3
1 2 6 4 5 3
1 2 3 4 5 6
1 2 3 4 5 6