The State of the Firefox Build System (2013 Q3 Review)

October 15, 2013 at 01:00 PM | categories: Mozilla, build system

As we look ahead to Q4 planning for the Firefox build system, I wanted to take the time to reflect on what was accomplished in Q3 and to simultaneously look forward to Q4 and beyond.

2013 Q3 Build System Improvements

There were notable improvements in the build system during the last quarter.

The issues our customers care most about is speed. Here is a list of accomplishments in that area:

  • MOZ_PSEUDO_DERECURSE work to change how make directory traversal works. This enabled the binaries make target, which can do no-op libxul-only builds in just a few seconds. Of all the changes that landed this quarter, this is the most impactful to local build times. This change also enables C++ compilation to scale out to as many cores as you have. Previously, the build system was starved in many parts of the tree when compiling C++. Mike Hommey is responsible for this work. I reviewed most of it.

  • WebIDL and IPDL bindings are now compiled in unified mode, reducing compile times and linker memory usage. Nathan Froyd wrote the code. I reviewed the patches.

  • XPIDL files are generated much more efficiently. This removed a few minutes of CPU core time from builds. I wrote these patches and Mike Hommey reviewed.

  • Increased reliance on install manifests to process file installs. They have drastically reduced the number of processes required to build by performing all actions inside Python processes as system calls and removing the clownshoes of having to delete parts of the object directory at the beginning of builds. When many mochitests were converted to manifests, no-op build times dropped by ~15% on my machine. Many people are responsible for this work. Mike Hommey wrote the original install code for packaging a few months ago. I built in manifest file support, support for symlinks, and made the code a bit more robust and faster. Mike Hommey reviewed these patches.

  • Many bugs and issues around dependency files on Windows have been discovered and fixed. These were a common source of clobbers. Mike Hommey found most of these, many during his work to make MOZ_PSEUDO_DERECURSE work.

  • The effort to reduce C++ include hell is resulting in significantly shorter incremental builds. While this effort is largely outside the build config module, it is worth mentioning. Ehsan Akhgari is leading this effort. He's been assisted by too many people to mention.

  • The build system now has different build modes favoring faster building vs release build options depending on the environment. Mike Hommey wrote most (all?) of the patches.

A number of other non-speed related improvements have been made:

  • The build system now monitors resource usage during builds and can graph the results. I wrote the code. Ted Mielczarek, Mike Hommey, and Mike Shal had reviews.

  • Support for test manifests has been integrated with the build system. This enabled some build speed wins and is paving the road for better testing UX, such as the automagical mach test command, which will run the appropriate test suite automatically. Multiple people were involved in the work to integrate test manifests with the build system. I wrote the patches. But Ted Mielczarek got primary review. Joel Maher, Jeff Hammel, and Ms2ger provided excellent assistance during the design and implementation phase. The work around mochitest manifests likely wouldn't have happened this quarter if all of us weren't attending an A*Team work week in August.

  • There are now in-tree build system docs. They are published automatically. Efforts have been made to purge MDN of cruft. I am responsible for writing the code and most of the docs. Benjamin Smedberg and Mike Shal performed code reviews.

  • Improvements have been made to object directory detection in mach. This was commonly a barrier to some users using mach. I am responsible for the code. Nearly every peer has reviewed patches.

  • We now require Python 2.7.3 to build, making our future Python 3 compatibility story much easier while eliminating a large class of Python 2.7.2 and below bugs that we constantly found ourselves working around.

  • mach bootstrap has grown many new features and should be more robust than ever. There are numerous contributors here, including many community members that have found and fixed bugs and have added support for additional distributions.

  • The boilerplate from has disappeared. Mike Hommey is to thank.

  • dumbmake integrated with mach. Resulted in friendlier build interface for a nice UX win. Code by Nick Alexander. I reviewed.

  • Many variables have been ported from to We started Q3 with support for 47 variables and now support 73. We started with 1226 and 1517 and currently have 941 and 1568 Many people contributed to this work. Worth mentioning are Joey Armstrong, Mike Shal, Joshua Cranmer, and Ms2ger.

  • Many build actions are moving to Python packages. This enabled pymake inlining (faster builds) and is paving the road towards no .pyc files in the source directory. (pyc files commonly are the source of clobber headaches and make it difficult to efficiently perform builds on read-only filesystems.) I wrote most of the patches and Mike Shal and Mike Hommey reviewed.

  • is now more strict about what it accepts. We check for missing files at config parse time rather than build time, causing errors to surface faster. Many people are responsible for this work. Mike Shal deserves kudos for work around C/C++ file validation.

  • mach has been added to the B2G repo. Jonathan Griffin and Andrew Halberstadt drove this.

Current status of the build bystem

Q3 was a very significant quarter for the build system. For the first time in years, we made fundamental changes to how the build system goes about building. The work to free our build config from the shackles of make files had enabled us to consume that data and do new and novel things with it. This has enabled improvements in build robustness and - most importantly - speed.

This is most evident with the MOZ_PSEUDO_DERECURSE work, which effectively replaces how make traverses directories. The work there has allowed Gecko developers focused on libxul to go from e.g. 50s no-op build times to less than 5s. Combined with optimized building of XPIDL, IPDL, and WebIDL files, processing of file installs via manifests, and C++ header dependency reduction, and a host of other changes, and we are finally turning a corner on build times! Much of this work wouldn't have been possible without files providing a whole world view of our build config.

The quarter wasn't all roses. Unfortunately, we also broke things. A lot. The total number of required clobbers this quarter grew slightly from 38 in Q2 to 43 in Q3. Many of these clobbers were regressions from supposed improvements to the build system. Too many of these regressions were Windows/pymake only and surely would have been found prior to landing if more build peers were actively building on Windows. There are various reasons we aren't. We should strive to fix them so more build development occurs on Windows and Windows users aren't unfairly punished.

The other class of avoidable clobbers mostly revolves around the theme that the build system is complicated, particularly when it comes to integration with release automation. Build automation has its build logic currently coded in Buildbot config files. This means it's all but impossible for build peers to test and reproduce that build environment and flow without time-intensive, stop-energy abundant excessive try pushes or loading out build slaves. The RelEng effort to extract this code from buildbot to mozharness can't come soon enough. See my overview on how automation works for more.

This quarter, the sheriffs have been filing bugs whenever a clobber is needed. This has surfaced clobber issues to build peers better and I have no doubt their constant pestering caused clobber issues to be resolved sooner. It's a terrific incentive for us to fix the build system.

I have mixed feelings on the personnel/contribution front in Q3. Kyle Huey no longer participates in active build system development or patch review. Ted Mielczarek is also starting to drift away from active coding and review. Although, he does constantly provide knowledge and historical context, so not all is lost. It is disappointing to see fantastic people and contributors no longer actively participating on the coding front. But, I understand the reasons behind it. Mozilla doesn't have a build team with a common manager and decree (a mistake if you ask me). Ted and Kyle are both insanely smart and talented and they work for teams that have other important goals. They've put in their time (and suffering). So I see why they've moved on.

On the plus side, Mike Hommey has been spending a lot more time on build work. He was involved in many of the improvements listed above. Due to review load and Mike's technical brilliance, I don't think many of our accomplishments would have happened without him. If there is one Mozillian who should be commended for build system work in Q3, it should be Mike Hommey.

Q3 also saw the addition of new build peers. Mike Shal is now a full build config module peer. Nick Alexander is now a peer of a submodule covering just the Fennec build system. Aside from his regular patch work, Mike Shal has been developing his review skills and responsibilities. Without him, we would likely be drowning in review requests and bug investigations due to the departures of Kyle and Ted. Nick is already doing what I'd hope he'd do when put in charge of the Fennec build system: looking at a proper build backend for Java (not make) and Eclipse project generation. (I still can't believe many of our Fennec developers code Java in vanilla text editors, not powerful IDEs. If there is one language that would miss IDEs the most, I'd think it would be Java. Anyway.)

There was a steady stream of contributions from people not in the build config module. Joshua Cranmer has been keeping up with conversions for comm-central. Nathan Froyd and Boris Zbarsky have helped with all kinds of IDL work. Trevor Saunders has helped keep things clean. Ms2ger has been eager to provide assistance through code and reviews. Various community contributors have helped with conversion patches and improvements to mach and the bootstrapper. Thank you to everyone who contributed last quarter!

Looking to the future

At the beginning of the quarter, I didn't think it would be possible to attain no-op build speeds with make as quickly as make binaries now does. But, Mike Hommey worked some magic and this is now possible. This was a game changer. The code he wrote can be applied to other build actions. And, our other solutions involving files to autogenerated make files seems to be working pretty well too. This raises some interesting questions with regards to priortization.

Long term, we know we want to move away from make. It is old and clumsy. It's easy to do things wrong. It doesn't scale to handle a single DAG as large as our build system. The latter is particularly important if we are to ever have a build system that doesn't require clobbers periodically.

Up to this point we've prioritized work on conversion, with the rationale being that it would more soon enable a clean break from make and thus we'd arrive at drastically faster builds sooner. The assumption in that argument was that drastically faster builds weren't attainable with make. Between the directory traversal overhaul and the release of GNU make 4.0 last week (which actually seems to work on Windows, making the pymake slowness a non-issue), the importance of breaking away from make now seems much less pressing.

While we would like to actively move off make, developments in the past few weeks seem to say that we can reassess priorities. I believe that we can drive down no-op builds with make to a time that satisfies many - let's say under 10s to be conservative. Using clever tricks optimizing for common developer workflows, we can probably get that under 5s everywhere, including Windows (people only caring about libxul can get 2.5s on mozilla-central today). This isn't the 250ms we could get with Tup. But it's much better than 45s. If we got there, I don't think many people would be complaining.

So, the big question for goals setting this quarter will be whether we want to focus on a new build backend (likely Tup) or whether we should continue with an emphasis on make. Now, a lot of the work involved applies to both make and any other build backend. But, I have little doubt it would be less overall work to support one build backend (make) than two. On the other hand, we know we want to support multiple build backends eventually. Why wait? In the balance are numerous other projects that have varying impact for developers and release automation. While important in their own right, it is difficult to balance them against build speed. While we could strive towards instantaneous builds, at some point we'll hit good enough and the diminishing returns that accompany them. There is already a small vocal faction advocating for Ninja support, even though it would only decrease no-op libxul build times from ~2.5s to 250ms. While a factor of 10x improvement, I think this is dangerously close to diminishing returns territory and our time investment would be better spent elsehwere. (Of course, once we can support building libxul with Ninja, we could easily get it for Tup. And, I believe Tup wins that tie.). Anyway, I'm sure it will be an interesting discussion!

Whatever the future holds, it was a good quarter for the build system and the future is looking brighter than ever. We have transitioned from a maintain-and-react mode (which I understand has largely been the norm since the dawn of Firefox) to a proactive and future-looking approach that will satisfy the needs of Firefox and its developers for the next ten years. All of this progress is even more impressive when you consider that we still react to an aweful lot of fire drills and unwanted maintenance!

The Firefox build system is improving. I'm as anxioux as you are to see various milestones in terms of build speed and other features. But it's hard work. Wish us luck. Please help out where you can.