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Re: [Lennart Poettering] Re: A few observations about systemd

On Sun, Jul 17, 2011 at 06:51:17PM +0200, Lennart Poettering wrote:
> In fact, a minimal systemd system will win in almost very aspect against
> a remotely similarly powerful sysvinit system: you will need much fewer
> processes to boot. That means much shorter boot times.

This is, as far as I'm aware, an unproven assertion.  While it's true that
there is a cost to the additional processes used in init scripts, I have not
seen any serious attempt at measuring how big this impact actually is -
certainly not in terms that would be relevant to Debian, which uses dash as
its /bin/sh and insserv by default (in squeeze and above).

I'm sure that systemd does much better than a traditional sysvinit boot with
/bin/bash and no dependency-based booting.  But then, so does Debian's
current boot system, and so does upstart; and neither of the latter two
involve grandiose claims of a "shell-free boot".  Trying to take the shell
completely out of the boot means a definite tradeoff here between boot speed
and configurability/maintainability, and in the absence of hard numbers, I
suspect this is a false optimization and not a trade-off that we actually
want to make in a general distribution.  Which then calls into question the
use of such claims as a justification for a switch to systemd at all...

> > For a low-level piece of infrastructure, systemd interacts with a lot of
> > high-level software; while this might be okay for a workstation running
> > Gnome, it makes me wonder whether it will be usable on servers.
> > A cursory look shows that systemd intrinsically depends on D-Bus (the
> > *desktop* bus) and knows about Plymouth, RedHat's implementation of
> > a splash screen.  More on that below.

> Oh, come on.

> systemd does not depend on Plymouth, it merely interacts with it if it
> is around it. Where interaction simply means writing a single message
> every now and then to ply to keep it updated how far the boot
> proceeded. It's more or a less a single line of text we send over every
> now and then in very terse code.

One of these days I'll get around to writing that blog entry to set the
record straight on why plymouth is an indispensible component of boot with
any modern boot system, because when everything is starting in parallel, you
need something to handle I/O multiplexing to the user on console.  So in a
real sense, it *should* be a dependency.  Even if you don't care about
splash, you still need multiplexing.

Upstart has the same dependency, though pid 1 doesn't talk directly to
plymouth in the upstart model; instead this is handled by an out-of-process
plymouth-upstart "bridge", as well as by the out-of-process mountall service
that talks to plymouth for handling of fscks, skipping the mounting of
missing filesystems, etc.

> > He practices misleading advertising[2], likes to claim that the
> > universal adoption of systemd by all distributions is a done thing[3],
> > and attempts to bully anyone who has the gall to think that the
> > discussion is still open[4].

> Juliusz practices misleading anti-advertising [1], likes to ignore the
> fact that all major distros either made systemd the default or include
> it in their distro with the exception of Ubuntu.

Well, it's nice to see that Lennart is at least acknowledging Ubuntu as a
major distribution these days, unlike in some of his earlier rhetoric. ;)

Though this is still a pretty misleading comment, since both of these
statements are also true:

  All major distros either made sysvinit the default or include it in their

  All major distros either made upstart the default or include it in their


Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek@ubuntu.com                                     vorlon@debian.org

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