calloc v/s malloc and time efficiency

Question

I've read with interest the post C difference between malloc and calloc. I'm using malloc in my code and would like to know what difference I'll have using calloc instead.

My present (pseudo)code with malloc:

Scenario 1

int main()
{  
   allocate large arrays with malloc

   INITIALIZE ALL ARRAY ELEMENTS TO ZERO

   for loop //say 1000 times
    do something and write results to arrays
   end for loop

   FREE ARRAYS with free command

} //end main

If I use calloc instead of malloc, then I'll have:

Scenario2

int main()
{  

   for loop //say 1000 times
    ALLOCATION OF ARRAYS WITH CALLOC 

    do something and write results to arrays

    FREE ARRAYS with free command

   end for loop


} //end main

I have three questions:

1. Which of the scenarios is more efficient if the arrays are very large?

2. Which of the scenarios will be more time efficient if the arrays are very large?

3. In both scenarios,I'm just writing to arrays in the sense that for any given iteration in the for loop, I'm writing each array sequentially from the first element to the last element. The important question: If I'm using malloc as in scenario 1, then is it necessary that I initialize the elements to zero? Say with malloc I have array z = [garbage1, garbage2, garbage 3]. For each iteration, I'm writing elements sequentially i.e. in the first iteration I get z =[some_result, garbage2, garbage3], in the second iteration I get in the first iteration I get z =[some_result, another_result, garbage3] and so on, then do I need specifically to initialize my arrays after malloc?

Answer 1

For 1 and 2, both do the same thing: allocate and zero, then use the arrays.

For 3, if you don't need to zero the arrays first, then zeroing is unnecessary and not doing it is faster.

There is a possibility that calloc's zeroing is more efficient than the code you write, but this difference will be small compared to the rest of the work the program does. The real savings of calloc is not having to write that code yourself.

Answer 2

Assuming the total amount of memory being initialized in your two examples is the same, allocating the memory with calloc() might be faster than allocating the memory with malloc() and then zeroing them out in a separate step, especially if in the malloc() case you zero the elements individually by iterating over them in a loop. A malloc() followed by a memset() will likely be about as fast as calloc().

If you do not care that the array elements are garbage before you actually store the computation results in them, there is no need to actually initialize your arrays after malloc().

Answer 3

The calloc and memset approaches should be about the same, and maybe slightly faster than zeroing it yourself.

Regardless, it's all relative to what you do inside your main loop, which could be orders of magnitude larger.

Answer 4

Your point stated in 3. seems to indicate a case or unnecessary initialization. That is pretty bad speed wise, not only the time spent doing it is wasted but a whole lot of cache eviction happened because of it.

Doing a memset() or bzero() (that are called by calloc() anyway) is a good way to invalidate huge portion of your cache. Don't do it unless you are sure you won't overwrite everything yet can read parts of the buffer that will not have been written (as if 0 is an acceptable default value). If you write over everything anyway by all mean don't initialize your memory unnecessarily.

Unnecessary memory writing will not only ruin your app performance but also the performance of all applications sharing the same CPU with it.