Colin Watson
   


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Mon, 24 Oct 2011

Quality in Ubuntu 12.04 LTS

As is natural for an LTS cycle, lots of people are thinking and talking about work focused on quality rather than features. With Canonical extending LTS support to five years on the desktop for 12.04, much of this is quite rightly focused on the desktop. I'm really not a desktop hacker in any way, shape, or form, though. I spent my first few years in Ubuntu working mainly on the installer - I still do, although I do some other things now too - and I used to say only half-jokingly that my job was done once X started. Of course there are plenty of bugs I can fix, but I wanted to see if I could do something with a bit more structure, so I got to thinking about projects we could work on at the foundations level that would make a big difference.

Image build pipeline

One difficulty we have is that quite a few of our bugs - especially installer bugs, although this goes for some other things too - are only really caught when people are doing coordinated image testing just before a milestone release. Now, it takes a while to do all the builds and then it takes a while to test them. The excellent work of the QA team has meant that testing is much quicker now than it used to be, and a certain amount of smoke-testing is automated (particularly for server images). On the other hand, the build phase has only got longer as we've added more flavours and architectures, particularly as some parts of the process are still serialised per architecture or subarchitecture so ARM builds in particular take a very long time indeed. Exact timings are a bit difficult to get for various reasons, but I think the minimum time between a developer uploading a fix and us having a full set of candidate images on all architectures including that fix is currently somewhere north of eight hours, and that's with people cutting corners and pulling strings which is a suboptimal thing to have to do around release time. This obviously makes us reluctant to respin for anything short of showstopper bugs. If we could get things down to something closer to two hours, respins would be a much less horrible proposition and so we might be able to fix a few bugs that are serious but not showstoppers, not to mention that the release team would feel less burned out.

We discussed this problem at the release sprint, and came up with a laundry list of improvements; I've scheduled this for discussion at UDS in case we can think of any more. Please come along if you're interested!

One thing in particular that I'm working on is refactoring Germinate, a tool which dates right back to our first meeting before Ubuntu was even called Ubuntu and whose job is to expand dependencies starting from our lists of "seed" packages; we use this, among other things, to generate Task fields in the archive and to decide which packages to copy into our images. This was acceptably quick in 2004, but now that we run it forty times (eight flavours multiplied by five architectures) at the end of every publisher run it's actually become rather a serious performance problem: cron.germinate takes about ten minutes, which is over a third of the typical publisher runtime. It parses Packages files eight times as often as it needs to, Sources files forty times as often as it needs to, and recalculates the dependency tree of the base system five times as often as it needs to. I am confident that we can significantly reduce the runtime here, and I think there's some hope that we might be able to move the publisher back to a 30-minute cycle, which would increase the velocity of Ubuntu development in general.

Maintaining the development release

Our release cycle always starts with syncing and merging packages from Debian unstable (or testing in the case of LTS cycles). The vast majority of packages in Ubuntu arrive this way, and generally speaking if we didn't do this we would fall behind in ways that would be difficult to recover from later. However, this does mean that we get a "big bang" of changes at the start of the cycle, and it takes a while for the archive to be usable again. Furthermore, even once we've taken care of this, we have a long-established rhythm where the first part of the cycle is mainly about feature development and the second part of the cycle is mainly about stabilisation. As a result, we've got used to the archive being fairly broken for the first few months, and we even tell people that they shouldn't expect things to work reliably until somewhere approaching beta.

This makes some kind of sense from the inside. But how are you supposed to do feature development that relies on other things in the development release?

In the first few years of Ubuntu, this question didn't matter very much. Nearly all the people doing serious feature development were themselves serious Ubuntu developers; they were capable of fixing problems in the development release as they went along, and while it got in their way a little bit it wasn't all that big a deal. Now, though, we have people focusing on things like Unity development, and we shouldn't assume that just because somebody is (say) an OpenGL expert or a window management expert that they should be able to recover from arbitrary failures in development release upgrades. One of the best things we could do to help the 12.04 desktop be more stable is to have the entire system be less unstable as we go along, so that developers further up the stack don't have to be distracted by things wobbling underneath them. Plus, it's just good software engineering to keep the basics working as you go along: it should always build, it should always install, it should always upgrade. Ubuntu is too big to do something like having everyone stop any time the build breaks, the way you might do in a smaller project, but we shouldn't let things slide for months either.

I've been talking to Rick Spencer and the other Ubuntu engineering leads at Canonical about this. Canonical has a system of "rotations", where you can go off to another team for a while if you're in need of a change or want to branch out a bit; so I proposed that we allow our engineers to spend a month or two at a time on what I'm calling the +1 Maintenance Team, whose job is simply to keep the development release buildable, installable, and upgradeable at all times. Rick has been very receptive to this, and we're going to be running this as a trial throughout the 12.04 cycle, with probably about three people at a time. As well as being professional archive gardeners, these people will also work on developing infrastructure to help us keep better track of what we need to do. For instance, we could deploy better tools from Debian QA to help us track uninstallable packages, or we could enhance some of our many existing reports to have bug links and/or comment facilities, or we could spruce up the weather report; there are lots of things we could do to make our own lives easier.

By 12.04, I would like, in no particular order:

  • Precise to have been more or less continuously usable from Alpha 1 onward for people with reasonable general technical ability
  • Canonical engineering teams outside Ubuntu (DX, Ubuntu One, Launchpad, etc.) to be comfortable with running the development release on at least one system from Alpha 2 onward
  • Installability problems in daily image builds to be dealt with within one working day, or preferably before they even make it to daily builds
  • The archive to be close to consistent as we start milestone preparation, rather than the release team having to scramble to make it so
  • A very significant reduction in our long-term backlog of automatically-detected problems

Of course, this overlaps to a certain extent with the kinds of things that the MOTU team have been doing for years, not to mention with what all developers should be doing to keep their own houses in reasonable order, and I'd like us to work together on this; we're trying to provide some extra hands here to make Ubuntu better for everyone, not take over! I would love this to be an opportunity to re-energise MOTU and bring some new people on board.

I've registered a couple of blueprints (priorities, infrastructure) for discussion at UDS. These are deliberately open-ended skeleton sessions, and I'll try to make sure they're scheduled fairly early in the week, so that we have time for break-out sessions later on. If you're interested, please come along and give your feedback!

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