如何检查内存限制

Question

I am trying to check if the memory limits in my OS works. So I am using this is simple c program:

#include <iostream>
#include <cstdlib>

int main()
{
    const size_t GB = 1024 * 1024 * 1024;
    const size_t mem = 1 * GB;
    std::cout << "allocating " << mem << " bytes" << std::endl;
    void* p = malloc(mem);
    if (p) { std::cout << "memory allocated" << std::endl; }
    else { std::cout << "cannot allocate memory" << std::endl; }
    char a;
    std::cin >> a;
    free(p);
}

I am compiling with -O0 but when I look to the performance monitor I see that my a.out is using only 128Kb. Why I don't see 1 GB?

Answer 1

You need to commit the memory. As long as you only reserve it you have allocated "virtual memory". You commit with reading or writing the memory. With your program just add

void* p = malloc(mem);
if (p) { 
    std::cout << "memory allocated" << std::endl;
    memset(p, 0, mem);
} else {
    std::cout << "cannot allocate memory" << std::endl; }

In the Windows operating system you can use the function VirtualQuery to see what chunks of memory are reserved and what is committed.

Answer 2

The OS will only "use" the memory that you are actually touching, so it will not show on performance monitor or such like when unless you actually "use" the memory. This is because some applications allocate large amounts of memory "just in case", and it would take a lot of extra time to ACTUALLY populate that memory into the process, when in fact it isn't being used.