systemd (Was Re: tmpfs for strategic directories)

Casey Dahlin cdahlin at
Wed May 26 14:16:48 UTC 2010

On Wed, May 26, 2010 at 01:26:48PM +0200, Lennart Poettering wrote:
> On Tue, 25.05.10 23:02, Casey Dahlin (cdahlin at wrote:
> > > Why do you say "cgroups are a dead end"? Sure, Scott claims that, but
> > > uh, it's not the only place where he is simply wrong and his claims
> > > baseless. In fact it works really well, and is one of the strong points
> > > in systemd. I simply see no alternative for it. The points Scott raised
> > > kinda showed that he never really played around with them.
> > > 
> > > Please read up on cgroups, before you just dismiss technology like that
> > > as "dead end".
> > > 
> > 
> > I did. When upstart was about to use them. 2 years ago. We chucked them by the
> > following LPC.
> Who is "we"?

Upstart has about 1.7 developers. The 1 is Scott, myself and Johann Kiviniemi
fight over the .7.

> > The problem we've found is that cgroups are too aggressive. They don't have a
> > notion of sessions and count too much as being part of your service, so you end
> > up with your screen session being counted as part of gdm.
> Well, how exactly you set up the groups is up to you, but the way we do
> it in systemd is stick every service in a seperate cgroup, plus every
> user in a seperate one, too. Some examples:
> And so on.

Someone else already pointed out the issues here.

> And the nice thing is that these cgroups are shown when you do "ps -eo
> cgroup,...". You can always figure out from "ps" to which service a
> process belongs, even if if it fork()ed a gazillions of times. And all
> the keeping track is done by the kernel, basically for free. No
> involvement from userspace.
> > This is why setsid was added to the netlink connector.
> Well, this is just flawed, on so many levels, that it
> enforcement for your services, since you simply have no cgroups. And no
> nice interfacing with the other libcgroup tools either.

There's no reason Upstart couldn't offer cgroups for resource limits, its
just passed them up as a process supervision mechanism. And they are broken for
the screen case, though perhaps that could be fixed in the kernel.

> Meh, and this lists gets on like this.
> People should not use cn_proc. It's evil. And if you are using for
> anything except logging you are doomed. And even for logging it isnt
> really useful.
> > 1) Socket activation. Part of Upstart's roadmap. Would happen sooner if you
> > cared to submit the patch. We don't think its good enough by itself, hence the
> > rest of Upstart, but a socket activation subsystem that could reach as far as
> > systemd and even work standalone in settings where systemd can work is
> > perfectly within Upstart's scope. I'd be happy to firm up the design details
> > with you if you wanted to contribute patches.
> Well, for once, it would be nice to judge things due to actually
> existign features, not of big plans nobody is working on as you
> apparently admit outright.

You worked on it, and finished it. You just didn't do it within Upstart. The
manpower was there it was just misapplied.

> And then, the socket activtion is nice for various reasons, and
> lazy-loading is just one of them. The bigger advantage is that it does
> automatic dependency handling -- which of course is nothign that really
> fits into upstart's design, since that is based on "events" not
> dependencies -- events are just broken, as I might note. And adding
> dependency would turn around upstart's design, making it a completely
> different beast.

Event-driven in Upstart's case just talks about the internal operation of
Upstart. Its rather exposed in current iterations but its just a design
philosophy. You can implement "dependencies" within it, though in the past
Upstart has been hesitant with the term because it tends to imply a
dependency-solving algorithm, where as Upstart provides its dependencies
reactively, in response to change.

> I mean, you called socket activation "xinetd-style activation" in the
> earlier mail of yours -- that is just completely besides the point,
> because this all is not so much about doing on-demand starting of
> internet services. systemd-style activation is about parallelizing
> startup of (mostly) local services and making fully written dependencies
> obsolete. And that's what is so magic about it. And an init systemd
> should be designed around this at its core! And systemd is.

Yes. I get it.

> > 2) Bus activation. Missed opportunity here to actually become the launchpoint
> > for activated services. I won't criticize that too much though, as its
> > usefulness is largely dependent on kernelspace DBus, which I've been trying to
> > bludgeon Marcel Holtmann into turning over to the public for a year
> > now.
> Not sure what this has to do with kernel space D-Bus, except that that
> is practically dead.
> If people want to reinvestigate the kernel/dbus issue they should not
> focus on an AF_DBUS, but instead just use netlink and use BSD socket
> filters for minimizing wakeups, plus come up with something inspired by
> iptables/ebtables to filter netlink traffic, for the permission
> problems. But that's a completely different story.
> > 3) Cutting down on the forking by replacing some of the shell scripts... cool
> >    3a) With C code... really?
> Yes, really. MacOS could do it, and so can we. Its not that hard. And as
> I my add here I already hacked up a big part of it now for the servcies
> we start by default.

Good for Apple? Maybe now they can design a better kernel? This is linux. We
configure our systems here.

> > 4) Process environment control. No complaints, and also nothing Upstart doesn't
> > want to do.
> Well, have you seen the functionality we provide thre, and the limited
> stuff upstart has there?
> supervisor too? The fact that we support different boot targets? The
> fact we have a UI? the sane copyright? The fact that we don't use bloody
> bzr? And so on and so forth, i could repeat the whole blog story
> here. Please just read that instead.

Again, could have been Upstart patches (except for the last couple).

> > > We did systemd because we thought that technically Upstart is
> > > fundamentally flawed and misses out on so many opportunities.
> > 
> > And we think the same of systemd.
> We?
> OK, I am listening. What's flawed about systemd? I wrote very detailed
> explanation why Upstart is just wrong in its core design. Please be so
> kind and be more specific if you claim the same abotu systemd, because
> otherwise all you are doing is spreading FUD.

Apart from the issue of circular dependencies, which I know you've already had
explained to you, I'd like to see how you intend to manage, say Veritas
clustering products. Like it or not there are systems out there running
software that's unstable, not open source, written by people who don't like
you, will /never/ take your socket passing patch and would probably find clever
ways to break or do it wrong if they did (yes, because of a 10-line patch).

Upstart doesn't ask anything of the userspace it intends to manage, and that
userspace is a harsher place than you seem to realize.

> > And mail it to upstart-devel-list. I'm being a bit hyperbolic, but actually not
> > much. That would have been a valid move.
> Please read my blog story. Thank you. It actually answers you that very
> question:
> "Why didn't you just add this to Upstart, why did you invent something
> new?
>     Well, the point of the part about Upstart above was to show that the
>     core design of Upstart is flawed, in our opinion. Starting
>     completely from scratch suggests itself if the existing solution
>     appears flawed in its core. However, note that we took a lot of
>     inspiration from Upstart's code-base otherwise."
> If there is a valid reason for starting anew if something exists
> already, then it is that the basic design of the existing system is just
> flawed. And hence we started anew.
> > You should really come work with us. We're fun guys.
> Man, you are suggesting I didn't talk to the Upstart folks. That's a
> ridiculous claim, and quite insulting that you imply that. Kay and I
> have had long discussions with Scott, during the last three years or so,
> at various confernces. And as far as I can see Upstart is Scott and
> Scott is Upstart, and the bzr repo commits underline that. And I am
> sorry if weren't part of those discussions.

See Scott's reply a few mails down.


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