Slow start-up of large programs

Question

The following program

#include <stdio.h>

int main() {
    static char a[1 << 28] = {1};
    printf("%d\n", a[0]);
    return 0;
}

Compiles on Windows 7 x64 to this executable

19/06/2015  21:57       268,519,936 a.exe

Which takes 5.6 seconds to run, even when run several times in succession to make sure it's cached in memory.

I would have expected Windows to either read in the entire executable at the start (in which case the machine is capable of stream reading at many hundreds of megabytes per second) or page in only the parts that are needed (in which case only a few kilobytes should have been read); either way, the program should run in a fraction of a second. If you tweak the array size, runtime is proportional to the size of the executable, so Windows behaves as though it's reading the entire executable but by some very slow method.

What's the reason for this behaviour, and is there anything that can be done about it?

Answer 1

It turns out the delay is the Microsoft antivirus program scanning the executable each time it's run. Disabling protection on that file cuts the time to 47 milliseconds.