Re: [HACKERS] fork/exec for backend

On 24 Jan 1998, Goran Thyni wrote:

> 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.

A backend is execed for every connection. All the backends will share
code space.

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

Not relevant. I'm only concerned with the children.

> 3. (and most important)
> With no exec COW is in action, meaning:
> data pages in shared until changed.
>
> COW is the key to how Linux can fork faster than most unices
> starts a new thread. :-)

COW is old news. Perhaps you can find some old SCO systems that don't
do COW :)

> 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.

I'm not speaking just about COW, but about being able share code between
separately execed processes.

> best regards,
> --
> ---------------------------------------------
> Gran Thyni, sysadm, JMS Bildbasen, Kiruna

Tom