Re: [RFD] Combined fork-exec syscall.

[dean gaudet <dean-list-linux-kernel@arctic.org>]

On Sun, 28 Apr 2003, Måns Rullgård wrote:

> Mark Grosberg <mark@nolab.conman.org> writes:
>
> > > If you do this, _please_ make it compat with NT.
> >
> > Actually, I thought about this. My first thought is this could benefit
> > WINE running on Linux. Then (not like I'm a Wine expert by any means) I
> > figured it might be an issue as far as having to do some preliminary
> > wineserver setup work (if anybody on this list knows better than me, speak
> > up!)
> >
> > But yeah, basically, something similar to NT's CreateProcess(). For the
> > cases where the one-step process creation is sufficient.
>
> Is that the call that takes dozens of parameters?  Copying :-) that
> is, IMHO, straight against the UNIX philosophy.

unfortunately you want those dozen parameters, they all have a purpose...
which is what makes such a call suspect in the first place.

vfork() solves the mm copying problem, which eliminates half the reason
for a combined fork-exec syscall.

the only time fork-exec is inefficient, given the existence of vfork, is
when you need to fork a process which has a lot of fd.  and by "a lot" i
mean thousands.

in that case even F_CLOEXEC isn't a good answer -- because it's a pain in
the ass to set because it requires an extra system call for the most
important case -- sockets.  otherwise you have to iterate over the entire
fd array to close things... which isn't so hot for multiprocessor setups.

but even this has a potential work-around using procfs -- use clone() to
get the vfork semantics without also copying the fd array.  then open
/proc/$ppid/fd/N for any file descriptors you want opened in the forked
process.

given both vfork and procfs i'm not sure there's any other performance
benefit a combined fork+exec syscall offers...  and if procfs isn't fast
enough for this then that's a better place to focus effort :)

-dean