From: koenraad.devleeschauwer@lid.kviv.be
Date: Mon Jan 22 2001 - 06:55:01 EST
At this moment, uClinux implements vfork() instead of fork.
In vfork(), child and parent share the same data and stack.
This implies the child should
- be careful not to modify data in the parent process.
- not return from the function where vfork() was called.
Some software would be easier to port to uClinux if uClinux had a real fork() instead.
I've been experimenting with a wrapper around vfork() which does the following:
- Before forking:
- make a backup copy of the parents' data and bss segments.
- make a backup copy of the parents' malloced data.
- make a backup copy of the parents' stack.
- After forking:
- In the child process:
- Silently ignore any request by the child process to free data
malloced by the parent. The child process is allowed to free data
malloced by itself only.
- In the parent process, after the child has exec()ed or _exit()ed:
- restore backup copy of the data and bss segments.
- restore backup copy of the malloced data.
- restore backup copy of the stack.
This results in very fork()-like behavior. The main differences between the
above and a real fork are:
- The parent still is blocked until the child execs or _exits.
- The parent does not process any signals until the child has exec-ed or exited.
The cost of the above seems to be reasonable. As an example,
a fork in bash results in about 100k additional memory being allocated before
the parent forks (and freed after the child has exec-ed).
bash-2.04# wc /etc/rc
nfork: forking, 20758 bytes in 421 mallocs, stack 358 bytes, heap 87245 bytes.
12 42 327 /etc/rc
bash-2.04# exit
The downside is that I've implemented all of this in userland, and in retrospect
this seems to have been a poor choice. Especially saving and restoring the
stack is very tricky in userland.
Has anyone else experimented along similar lines? Implemented something
like this in the kernel? What did it look like?
koen
This message resent by the uclinux-dev@uclinux.org list server http://www.uClinux.org/
This archive was generated by hypermail 2.1.4 : Thu Sep 19 2002 - 13:19:27 EDT