A while back at the awesome maintainerati I chatted with a few great fellow maintainers about how to scale really big open source projects, and how github forces projects into a certain way of scaling. The linux kernel has an entirely different model, which maintainers hosting their projects on github don’t understand, and I think it’s worth explaining why and how it works, and how it’s different.

Another motivation to finally get around to typing this all up is the HN discussion on my “Maintainers Don’t Scale” talk, where the top comment boils down to “… why don’t these dinosaurs use modern dev tooling?”. A few top kernel maintainers vigorously defend mailing lists and patch submissions over something like github pull requests, but at least some folks from the graphics subsystem would love more modern tooling which would be much easier to script. The problem is that github doesn’t support the way the linux kernel scales out to a huge number of contributors, and therefore we can’t simply move, not even just a few subsystems. And this isn’t about just hosting the git data, that part obviously works, but how pull requests, issues and forks work on github.

Scaling, the Github Way

Git is awesome, because everyone can fork and create branches and hack on the code very easily. And eventually you have something good, and you create a pull request for the main repo and get it reviewed, tested and merged. And github is awesome, because it figured out an UI that makes this complex stuff all nice&easy to discover and learn about, and so makes it a lot simpler for new folks to contribute to a project.

But eventually a project becomes a massive success, and no amount of tagging, labelling, sorting, bot-herding and automating will be able to keep on top of all the pull requests and issues in a repository, and it’s time to split things up into more manageable pieces again. More important, with a certain size and age of a project different parts need different rules and processes: The shiny new experimental library has different stability and CI criteria than the main code, and maybe you have some dumpster pile of deprecated plugins that aren’t support, but you can’t yet delete them: You need to split up your humongous project into sub-projects, each with their own flavour of process and merge criteria and their own repo with their own pull request and issue tracking. Generally it takes a few tens to few hundreds of full time contributors until the pain is big enough that such a huge reorganization is necessary.

Almost all projects hosted on github do this by splitting up their monorepo source tree into lots of different projects, each with its distinct set of functionality. Usually that results in a bunch of things that are considered the core, plus piles of plugins and libraries and extensions. All tied together with some kind of plugin or package manager, which in some cases directly fetches stuff from github repos.

Since almost every big project works like this I don’t think it’s necessary to delve on the benefits. But I’d like to highlight some of the issues this is causing:

Your community fragments more than necessary. Most contributors just have the code and repos around that they directly contribute to, and ignore everything else. That’s great for them, but makes it much less likely that duplicated effort and parallel solutions between different plugins and libraries get noticed and the efforts shared. And people who want to steward the overall community need to deal with the hassle of tons of repos either managed through some script, or git submodules, or something worse, plus they get drowned in pull requests and issues by being subscribed to everything. Any kind of concern (maybe you have shared build tooling, or documentation, or whatever) that doesn’t neatly align with your repo splits but cuts across the project becomes painful for the maintainers responsible for that.

Even once you’ve noticed the need for it, refactoring and code sharing have more bureaucratic hurdles: First you have to release a new version of the core library, then go through all the plugins and update them, and then maybe you can remove the old code in the shared library. But since everything is massively spread around you can forget about that last step. Of course it’s not much work to do this, and many projects excel at making this fairly easy. But it is more effort than a simple pull request to the one single monorepo. Very simple refactorings (like just sharing a single new function) will happen less often, and over a long time that compounds and accumulates a lot of debt. Except when you go the node.js way with repos for single functions, but then you essentially replace git with npm as your source control system, and that seems somewhat silly too.

The combinatorial explosion of theoretically supported version mixes becomes unsupportable. As a user that means you end up having to do the integration testing. As a project you’ll end up with blessed combinations, or at least de-facto blessed combinations because developers just close bug reports with “please upgrade everything first”. Again that means defacto you have a monorepo, except once more it’s not managed in git. Well, except if you use submodules, and I’m not sure that’s considered git …

Reorganizing how you split the overall projects into sub-projects is a pain, since it means you need to reorganize your git repositories and how they’re split up. In a monorepo shifting around maintainership just amounts to updating OWNER or MAINTAINERS files, and if your bots are all good the new maintainers get auto-tagged automatically. But if your way of scaling means splitting git repos into disjoint sets, then any reorg is as painful as the initial step from a monorepo to a group of split up repositories. That means your project will be stuck with a bad organizational structure for too long.

Interlude: Why Pull Requests Exist

The linux kernel is one of the few projects I’m aware of which isn’t split up like this. Before we look at how that works - the kernel is a huge project and simply can’t be run without some sub-project structure - I think it’s interesting to look at why git does pull requests: On github pull request is the one true way for contributors to get their changes merged. But in the kernel changes are submitted as patches sent to mailing lists, even long after git has been widely adopted.

But the very first version of git supported pull requests. The audience of these first, rather rough, releases was kernel maintainers, git was written to solve Linus Torvalds’ maintainer problems. Clearly it was needed and useful, but not to handle changes from individual contributors: Even today, and much more back then, pull requests are used to forward the changes of an entire subsystem, or synchronize code refactoring or similar cross-cutting change across different sub-projects. As an example, the 4.12 network pull request from Dave S. Miller, committed by Linus: It contains 2k+ commits from 600 contributors and a bunch of merges for pull requests from subordinate maintainers. But almost all the patches themselves are committed by maintainers after picking up the patches from mailing lists, not by the authors themselves. This kernel process peculiarity that authors generally don’t commit into shared repositories is also why git tracks the committer and author separately.

Github’s innovation and improvement was then to use pull requests for everything, down to individual contributions. But that wasn’t what they were originally created for.

Scaling, the Linux Kernel Way

At first glance the kernel looks like a monorepo, with everything smashed into one place in Linus’ main repo. But that’s very far from it:

Almost no one who’s using linux is running the main repo from Linus Torvalds. If they run something upstream-ish it’s probably one of the stable kernels. But much more likely is that they run a kernel from their distro, which usually has additional patches and backports, and isn’t even hosted on kernel.org, so would be a completely different organization. Or they have a kernel from their hardware vendor (for SoC and pretty much anything Android), which often have considerable deltas compared to anything hosted in one of the “main” repositories.

No one (except Linus himself) is developing stuff on top of Linus’ repository. Every subsystem, and often even big drivers, have their own git repositories, with their own mailing lists to track submissions and discuss issues completely separate from everyone else.

Cross-subsystem work is done on top of the linux-next integration tree, which contains a few hundred git branches from about as many different git repositories.

All this madness is managed through the MAINTAINERS file and the get_maintainers.pl script, which for any given snippet of code can tell you who’s the maintainer, who should review this, where the right git repo is, which mailing lists to use and how and where to report bugs. And it’s not just strictly based on file locations, it also catches code patterns to make sure that cross-subsystem topics like device-tree handling, or the kobject hierarchy are handled by the right experts.

At first this just looks like a complicated way to fill everyone’s disk space with lots of stuff they don’t care about, but there’s a pile of compounding minor benefits that add up:

It’s dead easy to reorganize how you split things into sub-project, just update the MAINTAINERS file and you’re done. It’s a bit more work than it really needs to be, since you might need to create a new repo, new mailing lists and a new bugzilla. That’s just an UI problem that github solved with this neat little fork button.

It’s really really easy to reassign discussions on pull requests and issues between sub-projects, you simply adjust the Cc: list on your reply. Similarly, doing cross-subsystem work is much easier to coordinate, since the same pull request can be submitted to multiple sub-projects, and there’s just one overall discussions (since the Msg-Ids: tags used for mailing list threading are the same for everyone), despite that the mails are archived in a pile of different mailing list archives, go through different mailing lists and land in a few thousand different inboxes. Making it easier to discuss topics and code across sub-projects avoids fragmentation and makes it much easier to spot where code sharing and refactoring would be beneficial.

Cross-subsystem work doesn’t need any kind of release dance. You simply change the code, which is all in your one single repository. Note that this is strictly more powerful than what a split repo setup allows you: For really invasive refactorings you can still space out the work over multiple releases, e.g. when there’s so many users that you can just change them all at once without causing too big coordination pains. A huge benefit of making refactoring and code sharing easier is that you don’t have to carry around so much legacy gunk. That’s explained at length in the kernel’s no stable api nonsense document.

It doesn’t prevent you from creating your own experimental additions, which is one of the key benefits of the multi-repo setup. Add your code in your own fork and leave it at that - no one ever forces you to push the code back, or push it into the one single repo or even to the main organization, because there simply is no central repositories. This works really well, maybe too well, as evidenced by the millions of code lines which are out-of-tree in the various Android hardware vendor repositories.

In short, I think this is a strictly more powerful model, since you can always fall back to doing things exactly like you would with multiple disjoint repositories. Heck there’s even kernel drivers which are in their own repository, disjoint from the main kernel tree, like the proprietary Nvidia driver. Well it’s just a bit of source code glue around a blob, but since it can’t contain anything from the kernel for legal reasons it is the perfect example.

This looks like a monorepo horror show!

Yes and no.

At first glance the linux kernel looks like a monorepo because it contains everything. And lots of people learned that monorepos are really painful, because past a certain size they just stop scaling.

But looking closer, it’s very, very far away from a single git repository. Just looking at the upstream subsystem and driver repositories gives you a few hundred. If you look at the entire ecosystem, including hardware vendors, distributions, other linux-based OS and individual products, you easily have a few thousand major repositories, and many, many more in total. Not counting any git repo that’s just for private use by individual contributors.

The crucial distinction is that linux has one single file hierarchy as the shared namespace across everything, but lots and lots of different repos for all the different pieces and concerns. It’s a monotree with multiple repositories, not a monorepo.

Examples, please!

Before I go into explaining why github cannot currently support this workflow, at least if you want to retain the benefits of the github UI and integration, we need some examples of how this works in practice. The short summary is that it’s all done with git pull requests between maintainers.

The simple case is percolating changes up the maintainer hierarchy, until it eventually lands in a tree somewhere that is shipped. This is easy, because the pull request only ever goes from one repository to the next, and so could be done already using the current github UI.

Much more fun are cross-subsystem changes, because then the pull request flow stops being an acyclic graph and morphs into a mesh. The first step is to get the changes reviewed and tested by all the involved subsystems and their maintainers. In the github flow this would be a pull request submitted to multiple repositories simultaneously, with the one single discussion stream shared among them all. Since this is the kernel, this step is done through patch submission with a pile of different mailing lists and maintainers as recipients.

The way it’s reviewed is usually not the way it’s merged, instead one of the subsystems is selected as the leading one and takes the pull requests, as long as all other maintainers agree to that merge path. Usually it’s the subsystem most affected by a set of changes, but sometimes also the one that already has some other work in-flight which conflicts with the pull request. Sometimes also an entirely new repository and maintainer crew is created, this often happens for functionality which spans the entire tree and isn’t neatly contained to a few files and directories in one place. A recent example is the DMA mapping tree, which tries to consolidate work that thus far has been spread across drivers, platform maintainers and architecture support groups.

But sometimes there’s multiple subsystems which would both conflict with a set of changes, and which would all need to resolve some non-trivial merge conflict. In that case the patches aren’t just directly applied (a rebasing pull request on github), but instead the pull request with just the necessary patches, based on a commit common to all subsystems, is merged into all subsystem trees. The common baseline is important to avoid polluting a subsystem tree with unrelated changes. Since the pull is for a specific topic only, these branches are commonly called topic branches.

One example I was involved with added code for audio-over-HDMI support, which spanned both the graphics and sound driver subsystems. The same commits from the same pull request where both merged into the Intel graphics driver and also merged into the sound subsystem.

An entirely different example that this isn’t insane is the only other relevant general purpose large scale OS project in the world also decided to have a monotree, with a commit flow modelled similar to what’s going on in linux. I’m talking about the folks with such a huge tree that they had to write an entire new GVFS virtual filesystem provider to support it …

Dear Github

Unfortunately github doesn’t support this workflow, at least not natively in the github UI. It can of course be done with just plain git tooling, but then you’re back to patches on mailing lists and pull requests over email, applied manually. In my opinion that’s the single one reason why the kernel community cannot benefit from moving to github. There’s also the minor issue of a few top maintainers being extremely outspoken against github in general, but that’s a not really a technical issue. And it’s not just the linux kernel, it’s all huge projects on github in general which struggle with scaling, because github doesn’t really give them the option to scale to multiple repositories, while sticking to with a monotree.

In short, I have one simple feature request to github:

Please support pull requests and issue tracking spanning different repos of a monotree.

Simple idea, huge implications.

Repositories and Organizations

First, it needs to be possible to have multiple forks of the same repo in one organization. Just look at git.kernel.org, most of these repositories are not personal. And even if you might have different organizations for e.g. different subsystems, requiring an organization for each repo is silly amounts of overkill and just makes access and user managed unnecessarily painful. In graphics for example we’d have 1 repo each for the userspace test suite, the shared userspace library, and a common set of tools and scripts used by maintainers and developers, which would work in github. But then we’d have the overall subsystem repo, plus a repository for core subsystem work and additional repositories for each big drivers. Those would all be forks, which github doesn’t do. And each of these repos has a bunch of branches, at least one for feature work, and another one for bugfixes for the current release cycle.

Combining all branches into one repository wouldn’t do, since the point of splitting repos is that pull requests and issues are separated, too.

Related, it needs to be possible to establish the fork relationship after the fact. For new projects who’ve always been on github this isn’t a big deal. But linux will be able to move at most a subsystem at a time, and there’s already tons of linux repositories on github which aren’t proper github forks of each another.

Pull Requests

Pull request need to be attached to multiple repos at the same time, while keeping one unified discussion stream. You can already reassign a pull request to a different branch of repo, but not at multiple repositories at the same time. Reassigning pull requests is really important, since new contributors will just create pull requests against what they think is the main repo. Bots can then shuffle those around to all the repos listed in e.g. a MAINTAINERS file for a given set of files and changes a pull request contains. When I chatted with githubbers I originally suggested they’d implement this directly. But I think as long as it’s all scriptable that’s better left to individual projects, since there’s no real standard.

There’s a pretty funky UI challenge here since the patch list might be different depending upon the branch the pull request is against. But that’s not always a user error, one repo might simple have merged a few patches already.

Also, the pull request status needs to be different for each repo. One maintainer might close it without merging, since they agreed that the other subsystem will pull it in, while the other maintainer will merge and close the pull. Another tree might even close the pull request as invalid, since it doesn’t apply to that older version or vendor fork. Even more fun, a pull request might get merged multiple times, in each subsystem with a different merge commit.

Issues

Like pull requests, issues can be relevant for multiple repos, and might need to be moved around. An example would be a bug that’s first reported against a distribution’s kernel repository. After triage it’s clear it’s a driver bug still present in the latest development branch and hence also relevant for that repo, plus the main upstream branch and maybe a few more.

Status should again be separate, since once push to one repo the bugfix isn’t instantly available in all of them. It might even need additional work to get backported to older kernels or distributions, and some might even decide that’s not worth it and close it as WONTFIX, even thought the it’s marked as successfully resolved in the relevant subsystem repository.

Summary: Monotree, not Monorepo

The Linux Kernel is not going to move to github. But moving the Linux way of scaling with a monotree, but mutliple repos, to github as a concept will be really beneficial for all the huge projects already there: It’ll give them a new, and in my opinion, more powerful way to handle their unique challenges.