Shared memory (System V-style) - High usage of phys memory and page outs

Question

Hi!
I get a much higher usage of physical memory when I use shared memory than I would expect. Please, I would really need someone to confirm my conclusions, so that I can revise my ignorance in this subject.
In my experiments I create a shared memory segment of 200 MB and I have 7 processes attaching to it. I have a ws of 1 GB.
I expect to see what I see when I attach to the shared memory segment in terms of virtual size, i.e. SIZE in prstat. After attaching (mapping it) to the process all 7 processes are about ~203 MB each and this makes sense. RSS, in prstat, is about 3 MB for each process and this is ok to me too.
It is what I see when each of the 7 processes start to write a simple string like 'Hello!' in parallel to each page in their shared memory segment that I get surprised. I run out of memory on my ws after a while and the system starts to wildly page out physical memory to disk so that the next 'Hello!' can be written to the next page in the shared memory segment. It seems that each page written to in the shared memory chunk is mapped into each process private address space. This means that the shared memory is not physically shared, just virtually shared. Is this correct?
Can memory be physically shared, so that my 7 processes only use 200 MB? ISM? DISM?
I create a shared memory segment in a C-program with the following calls:
shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT)
data = shmat(shmid, (void *)0, 0);Thanks in advance
/Sune

Answer 1

Your problem seemed reasonable. What were you doing wrong?
Darren