That upstart Upstart
pgoetz at mail.utexas.edu
Mon Jun 29 21:11:53 UTC 2009
(Apologies in advance for the length of this message -- if you don't
care about order and manner in which system daemons are launched, please
invoke the Delete key operator immediately.)
Everyone knows the canonical (no pun intended) unix interview question,
Q: how many processes does the kernel start on boot?
A: Only one -- init
Apparently this isn't necessarily true any more, or soon won't be? I
usually don't dig into random subsystems until something fails. In this
case, I recently found out that my coworker had "fixed" an autofs
startup timing problem in our 8.04 rollout by simply inserting a few
sleep loops into the autofs init script. This hack was no longer
working in 9.04, which we're testing now (not to mention dramatically
increased boot times). Since autofs is a core system service, I
suggested that he contact Canonical support and let them know that there
is a problem; i.e. as installed by default, autofs fails to start. The
support technician who got back to us made a number of comments about
how processes are launched which were simply wrong. At this point,
Houston: we have a problem. The Ubuntu tech staff isn't even clear on
how daemons are launched at startup.
So, this brings us to upstart, the init replacement. After a couple of
days of looking through both the on-line documentation I could find
(http://upstart.ubuntu.com/getting-started.html) and the actual
installed files I find that I still have more than a number of
questions about how this is supposed to work currently and in the
future, as alluded to in this snippet from something posted to one of
the ubuntu devel lists:
> Date: Fri, 26 Jun 2009 14:55:16 -0500
> From: Robbie Williamson <robbie at ubuntu.com>
> Subject: Debian/Ubuntu Boot Performance Sprint Summary
<snip/>Scott James Remnant stated that he expects to have the
entire Ubuntu boot sequence transitioned over to upstart
by 10.04, with some work already planned for the 9.10 cycle.<snip/>
1. the previously referenced site says that jobs files are placed in
/etc/init/jobs.d; actually, they seem to be in /etc/event.d -- what's up
lizard:~~$ which init
lizard:~~$ dpkg -S /sbin/init
So, I'm confused: is init still being used, or what? /etc/inittab no
longer exists in 9.04/9.10, and the jobs in /etc/event.d follow the
event driven model outlined in the documentation, however there is still
an init executable. Is this init an upstarted init? There don't appear
to be any other candidate executables in the upstart package.
3. There's much discussion about improving the startup system by moving
to an event driven model. The current implementation in 9.04/9.10alpha
simply mimics System-V init. OK, this is transitional, but how are we
going to get from A to B? Package implementers are not getting
initialization scripts right now (e.g. autofs), what's the plan for
rolling out a new event-driven system and how is it going to work?
4. Related: The concept of run levels is fairly useful, and the core
concept in System-V init. Are run levels going to go way under full
upstart implementation? If so, how will the system distinguish between,
say, a maintenance single-user boot and what is currently rc2? And if
run levels are not going away, how is process of startinɡ system daemons
going to be event driven in any meaningful way?
5. Even after reading through the "Getting Started" page twice, the
implementation under 9.04 is still confusing. For example, to quote
Getting Started: "You list the events you want to start your job with
start on, and the events that stop your job with stop on." OK, fair
enough. However, here is /etc/event.d/rcS:
start on startup
stop on runlevel
runlevel --set S >/dev/null || true
export PREVLEVEL RUNLEVEL
Doesn't this mean that rcS is stopped before /etc/init.d/rcS is ever
exec'd, since the runlevel is set before the script is run? In
particular, when exactly does the runlevel event stop the script?
Clearly after the rcS script is exec'd (otherwise none of the goodies in
/etc/rc2.d would ever get started), but this needs to be documented much
more carefully, since not every case will be this clear cut. Again,
package implementors can't get the timing down right now under System-V
init. This is going to turn into complete pandemonium if the
termination of events is ill-defined.
6. "If you're using the example jobs, you will also have runlevel X
events, where X is one of 0–6 or S. Jobs will be run alongside the init
scripts for that runlevel."
Presumably this means in an event driven model, user-installed jobs are
triggered in parallel with runlevel events? Otherwise: Um, what?
7. Finally, is there better documentation available, say something in
between http://upstart.ubuntu.com/getting-started.html and slogging
through the actual source code?
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