for loop optimization c ++ - c++

Question

this is my first time posting in this site and I hope I get some help/hint. I have an assignment where I need to optimize the performance to the inner for loop but I have no idea how to do that. the code was given in the assignment. I need to count the time(which I was able to do) and improve the performance.
Here is the code:
//header files
#define N_TIMES 200 //This is originally 200000 but changed it to test the program faster
#define ARRAY_SIZE 9973
int main (void) {
int *array = (int*)calloc(ARRAY_SIZE, sizeof(int));
int sum = 0;
int checksum = 0;
int i;
int j;
int x;
// Initialize the array with random values 0 to 13.
srand(time(NULL));
for (j=0; j < ARRAY_SIZE; j++) {
x = rand() / (int)(((unsigned)RAND_MAX + 1) / 14);
array[j] = x;
checksum += x;
}
//printf("Checksum is %d.\n",checksum);
for (i = 0; i < N_TIMES; i++) {
// Do not alter anything above this line.
// Need to optimize this for loop----------------------------------------
for (j=0; j < ARRAY_SIZE; j++) {
sum += array[j];
printf("Sum is now: %d\n",sum);
}
// Do not alter anything below this line.
// ---------------------------------------------------------------
// Check each iteration.
//
if (sum != checksum) {
printf("Checksum error!\n");
}
sum = 0;
}
return 0;
}
The code takes about 695 seconds to run. Any help on how to optimize it please?
thanks a lot.

Answer 1

The bottleneck in that loop is obviously the IO done by printf; since you are probably writing the output on a console, the output is line buffered, which means that the stdio buffer is flushed at each iteration, which slows down things a lot.
If you have to do all that prints, you can greatly enhance the performance by forcing the stream to do block buffering: before the for add a
setvbuf(stdout, NULL, _IOFBF, 0);
In alternative, if this approach is not considered valid, you can do your own buffering by allocating a big buffer on your own and do your own buffering: write in your buffer using sprintf, periodically emptying it in the output stream with a fwrite.
Also, you can use the poor man's approach to buffering - just use a buffer big enough to write all that stuff (you can calculate how big it must be quite easily) and write in it without worrying about when it's full, when to empty it, ... - just empty it at the end of the loop. edit: see #paxdiablo's answer for an example of this
Applying just the first optimization, what I get with time is
real 0m6.580s
user 0m0.236s
sys 0m2.400s
vs the original
real 0m8.451s
user 0m0.700s
sys 0m3.156s
So, we got down of ~3 seconds in real time, half a second in user time and ~0.7 seconds in system time. But what we can see here is the huge difference between user+sys and real, which means that the time is not spent in doing something inside the process, but waiting.
Thus, the real bottleneck here is not in our process, but in the process of the virtual terminal emulator: sending huge quantities of text to the console is going to be slow no matter what optimizations we do in our program; in other words, your task is not CPU-bound, but IO-bound, so CPU-targeted optimizations won't be of much benefit, since at the end you have to wait anyway for your IO device to do his slow stuff.
The real way to speed up such a program would be much simpler: avoid the slow IO device (the console) and just write the data to file (which, by the way, is block-buffered by default).
matteo#teokubuntu:~/cpp/test$ time ./a.out > test
real 0m0.369s
user 0m0.240s
sys 0m0.068s

Answer 2

Since there's absolutely no variation in that loop based on i (the outer loop), you don't need to calculate it each time.
In addition, the printing of the data should be outside the inner loop so as not to impose I/O costs on the calculation.
With those two things in mind, one possibility is:
static int sumCalculated = 0;
if (!sumCalculated) {
for (j=0; j < ARRAY_SIZE; j++) {
sum += array[j];
}
sumCalculated = 1;
}
printf("Sum is now: %d\n",sum);
although that has different output to the original which may be an issue (one line at the end rather than one line per addition).
If you do need to print the accumulating sum within the loop, I'd simply buffer that as well (since it doesn't vary each time through the i loop.
The string Sum is now: 999999999999\n (12 digits, it may vary depending on your int size) takes up 25 bytes (excluding terminating NUL). Multiply that by 9973 and you need a buffer of about 250K (including a terminating NUL). So something like this:
static char buff[250000];
static int sumCalculated = 0;
if (!sumCalculated) {
int offset = 0;
for (j=0; j < ARRAY_SIZE; j++) {
sum += array[j];
offset += sprintf (buff[offset], "Sum is now: %d\n",sum);
}
sumCalculated = 1;
}
printf ("%s", buff);
Now that sort of defeats the whole intent of the outer loop as a benchmark tool but loop-invariant removal is a valid approach to optimisation.

Answer 3

Move the printf outside the for loop.
// Do not alter anything above this line.
//Need to optimize this for loop----------------------------------------
for (j=0; j < ARRAY_SIZE; j++) {
sum += array[j];
}
printf("Sum is now: %d\n",sum);
// Do not alter anything below this line.
// ---------------------------------------------------------------

Answer 4

Getting the I/O out of the loop is a big help.
Depending on the compiler and machine, you might get a tiny increase in speed by using pointers rather than indexing (though on modern hardware, it generally doesn't make a difference).
Loop unrolling might help to increase the ratio of useful work to loop overhead.
You could use vector instructions (e.g., SIMD) to do a bunch of calculation in parallel.
Are you allowed to pack the array? Can you use an array of a smaller type than int (given that all the values are very small)? Making the array physically shorter improves locality.
Loop unrolling might look something like this:
for (int j = 0; j < ARRAY_SIZE; j += 2) {
sum += array[j] + array[j+1];
}
You'd have to figure out what to do if the array isn't an exact multiple of the unrolling size (which is probably why the assignment uses a prime number).
You would have to experiment to see how much unrolling would be the right amount.