Free up memory immediately by swap trick (C++) [closed]

I have a very large vector as follows.

vector<vector<int>> a(100000, vector<int>(100000);
// do some work using a here
......

After the work is done, I use

vector<vector<int>>().swap(a);

in order to free the memory used by "a"immediately. I add

sleep(100)

after the swap and use "top" command to see whether memory usage is reduced immediately. But I observe that nothing has changed. What happens here?