Re: [HACKERS] fork/exec for backend

>
>
> On 24 Jan 1998, Goran Thyni wrote:
>
> > Fork on modern unices (linux and (a think) *BSD) cost
> > almost nothing (in time and memory) thanks to COW (copy-on-write).
> > Exec in expensive as it breaks COW.
>
> Not so. Modern Unixs will share executable address space between
> processes. So if you fork and exec 10 identical programs, they will share
> most address space.
>
> 1. Code is probably not shared between postmaster and postgres
> processes.

I think it is shared. postmaster is a symlink to postgres, so by the
time it gets to the kernel exec routines, both processes are mapped to
the same inode number.

>
> 2. Some inits may be done once (by postmaster) and not repeated
> by every child.

Maybe.

>
> 3. (and most important)
> With no exec COW is in action, meaning:
> data pages in shared until changed.

This would also prevent us from attaching to shared memory because it
would already be in the address space.

>
> COW is the key to how Linux can fork faster than most unices
> starts a new thread. :-)

>
> Again, this only applies to "modern" systems, but FreeBSD definitely has
> this behaviour.
>
> I don't know if *BSD has COW, but if should think so.

All modern Unixes have COW.

--
Bruce Momjian
maillist(at)candle(dot)pha(dot)pa(dot)us