[vbezhenar]

Let's say you have 1GB RAM. You're running program that occupies 600 MB. Now this program wants to launch second small program that occupies 1 MB.

You're doing fork + exec.

If you're overcommiting, fork will not reserve another 600 MB, and exec immediately after fork will cause total system usage to be 601 MB.

If you're not overcommiting, that fork will fail, because total memory consumption will be 1200 MB which is more than 1GB. That somewhat restricts program design.

[agwa]

I think that on Unixes without overcommit, people allocate massive amounts of swap so that fork never fails.

[dwattttt]

> Let's say you have 1GB RAM. You're running program that occupies 600 MB. Now this program wants to launch second small program that occupies 1 MB.

> You're doing fork + exec.

This is the clear problem: you don't want another process that's a duplicate of the current one, that's just a detail of what you actually want: a 1mb process. Right now it's a badly leaky detail which you're forced to work around.

[cylemons]

> If you're not overcommiting, that fork will fail, because total memory consumption will be 1200 MB which is more than 1GB. That somewhat restricts program design.

Does this accounting apply to vfork as well?

[vbezhenar]

As I understand it, the whole purpose of vfork is to avoid that problem. So vfork does not copy memory in any way and you're not allowed to do anything but exec after vfork. But at this point question arises: why not call it `vfork_and_exec` and get rid of undefined behaviour. Or choose better name like `CreateProcessW` hehe

[cylemons]

In the manpage for vfork, Linux in particular

> As with fork(2), the child process created by vfork() inherits copies of various of the caller's process attributes (e.g., file descriptors, signal dispositions, and current working directory); the vfork() call differs only in the treatment of the virtual address space, as described above.

so it seems Linux does define the behavior of vfork, but if you rely on it, your code won't be portable to other POSIX systems

https://man7.org/linux/man-pages/man2/vfork.2.html

[cryptonector]

Correct: it does NOT apply to vfork().