it would lead to elseone 1G of memory occupied.

??

Could anyone tell me either how to make a program occupy a lot of memory

Easy:

my $mem = chr(0); $mem x= 1024**3; ## 1 GiB.
[download]

or answer a question:
How much resources would be occupied totally if I run following code from a program that occupies, say, 1G of memory itself? ]

Try it :)

Hmm, no, I've made a mistake. It uses an extra 1 GiB as free -m says. Anyways, thanks for the code :) Now I know that I cannot execute fork()-s from heavy programs.

It uses an extra 1 GiB as free -m says.

I don't use *nix, but I think that the tool you are using is giving you bad info.

AFAIK: That memory will not be cloned unless the forked child accesses it.

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Thanks a lot! This is what I've needed. I tried to launch your code, then fork, and it didn't take much more memory. I just thought that fork() call would duplicate the memory related to the parent process, but it didn't as it seems...

POSIX fork() itself will not (significantly) increase overall memory usage. If the parent uses 1GiB of memory and fork()s, the child will use the same 1GiB worth of pages in the operating system. If you go ahead and change any part of that memory in the child or parent, the operating system will make a copy of the affected page(s) of virtual memory.

This is known as copy-on-write (CoW) memory.

In your example code (which differs in an important way from your question about fork()), you exec another system command via backticks, which actually invokes a new shell and starts a fresh process from that shell rather than directly from your program. The implied exec() here essentially creates a new process image. In this case the OS has nothing to copy, and hence will not be able to take advantage of CoW and will be forced to allocate fresh pages for all of the runtime memory you request, because your program asks it to. In other words, if the memory has already been allocated and initialized pre-fork(), it will remain so post-fork(), in both the child and parent process.

None of this is at all new or Perl-specific, by the way, so reading up on the underlying fork(2) and exec(3) manual pages on any POSIX system would probably provide some useful background.