virtual memory vs. RSS in Linux

I wrote a small program to understand the difference between the VSZ and RSS. If you malloc, it only means that you can use the memory address. No real memory will be used until that page is accssed.


First I find out my page size using:
%getconf PAGE_SIZE
4096

(1)
size_t length=0x10000000; //256M ~const size_t pagesize=4096;
char* x=(char*)malloc(length);

after start the program:
%ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
user     25125  0.0  0.0 273552   912 pts/1    S+   10:54   0:00 a.out

Here the VSZ just mean the maxium address space you can use, and the real memory RSS is still 912K although the VSZ is 270M
(2) and then execute
for (int i=0; i<1000; i++)
      x[i]='a';

%ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
user     25395  0.0  0.0 273552   916 pts/1    S+   10:56   0:00 a.out

Only one page is written, so the RSS just changed for an extra one page space (916K now).

(3) and then execute to write/read one character in 1000 pages
   for (int i=0; i<1000; i++)
      x[i*pagesize]='a';


%ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
user     25285  0.0  0.2 273552  4912 pts/1    S+   10:55   0:00 a.out

Now every page written/read will be come the RSS (1000*4K+912K=4912K).