在任务管理器上看到分配的 memory

Question

I want to allocate 1 GB of memory. Assuming int takes 4 bytes, to allocate 1 GB I need an array of 250000000 elements:

int main() { 
    string s;

    cout << "Program paused" << flush;
    getline(cin, s);
    int * p = new int[250000000]{};
    cout << "Program paused" << flush;
    getline(cin, s);
    delete [] p;
    cout << "Program paused" << flush;
    getline(cin, s);
}

Windows Task Manager show that my program uses 954 MB. Why is it less than 1 GB?

EDIT: The task manager also shows that OS reserved 979,100 MB of virtual memory for the process what is still less than 1 GB.

Answer 1

1 GB is 1024x1024x1024 bytes (by convention)

So 250000000 x 4 is less than 1GB.

So you are using (250000000 x 4) / (1024x1024) MB which is about 953 MB

Answer 2

250,000,000 bytes * 4 is not 1GB. Remember a Gigabyte ( more accurately, a Gibibyte ) is 1024 megabytes (mebibytes), and each mebibyte is 1024 kilobytes (kibibytes) which is 1024 bytes.

To allocate 1 GiB (assuming sizeof(int) == 4 ) you want:

const size_t len = ( 1024 * 1024 * 1024 ) / sizeof(int); // 268,435,456 
int* p = new int[ len ]{};