Question

I am writing a C program which has to use a 2D array to store previously processed data for later using.

The size of this 2D array 33×33; matrix[33][33].

I define it as a global parameter, so it will be initialized for only one time. Dose this definition cost a lot of time when program is running? Because I found my program turn to be slower than previous version without using this matrix to store data.

Additional:

I initialize this matrix as a global parameter like this:

int map[33][33];

1. In one of function A, I need to store all of 33×33 data into this matrix.
2. In another function B, I will fetch 3×3 small matrix from map[33][33] for my next step of processing.

Above 2 steps will be repeated for about 8000 times. So, will it affect program running efficiency?

Or, I have another guess that the program truns to be slower because of there are couple of if-else branch statements were lately added into the program.

Answer 1

How ere you doing it before? The only problem I can think of is that extracting a 3×3 sub matrix from a 33×33 integer matrix is going to cause you cacheing issues every time you extract the sub matrix.

On most modern machines the cacheline is 64 bytes in size. Thats enough for 8 elements of the matrix. So for each extra line of the 3×3 sub matrix you will be performing a new cacheline fetch. If the matrix gets hammered very regularly then the matrix will probably sit mostly in the level 2 cache (or maybe even the level 1 if its big enough) but if you are doing lots of other data calculations in between each sub-matrix fetch then you will be getting 3expensive cacheline fetches each time you grab the sub matrix.

However even then its unlikely you’d see a HUGE difference in performance. As stated elsewhere we need to see before and after code to be able to hazard a guess at why performance has got worse …