How To Add Your Build Configuration To LLVM Buildbot Infrastructure

Introduction

This document contains information about adding a build configuration and buildbot-worker to private worker builder to LLVM Buildbot Infrastructure.

Buildmasters

There are two buildmasters running.

  • The main buildmaster at https://lab.llvm.org/buildbot. All builders attached to this machine will notify commit authors every time they break the build.
  • The staging buildmaster at https://lab.llvm.org/staging. All builders attached to this machine will be completely silent by default when the build is broken.

In order to remain connected to the main buildmaster (and thus notify developers of failures), a builbot must:

  • Be building a supported configuration. Builders for experimental backends should generally be attached to staging buildmaster.
  • Be able to keep up with new commits to the main branch, or at a minimum recover to tip of tree within a couple of days of falling behind.

Additionally, we encourage all bot owners to point their bots towards the staging master during maintenance windows, instability troubleshooting, and such.

Roles & Expectations

Each buildbot has an owner who is the responsible party for addressing problems which arise with said buildbot. We generally expect the bot owner to be reasonably responsive.

For some bots, the ownership responsibility is split between a “resource owner” who provides the underlying machine resource, and a “configuration owner” who maintains the build configuration. Generally, operational responsibility lies with the “config owner”. We do expect “resource owners” - who are generally the contact listed in a workers attributes - to proxy requests to the relevant “config owner” in a timely manner.

Most issues with a buildbot should be addressed directly with a bot owner via email. Please CC Galina Kistanova.

Steps To Add Builder To LLVM Buildbot

Volunteers can provide their build machines to work as build workers to public LLVM Buildbot.

Here are the steps you can follow to do so:

  1. Check the existing build configurations to make sure the one you are interested in is not covered yet or gets built on your computer much faster than on the existing one. We prefer faster builds so developers will get feedback sooner after changes get committed.

  2. The computer you will be registering with the LLVM buildbot infrastructure should have all dependencies installed and you can actually build your configuration successfully. Please check what degree of parallelism (-j param) would give the fastest build. You can build multiple configurations on one computer.

  3. Install buildbot-worker (currently we are using buildbot version 2.8.5). Depending on the platform, buildbot-worker could be available to download and install with your package manager, or you can download it directly from http://trac.buildbot.net and install it manually.

  4. Create a designated user account, your buildbot-worker will be running under, and set appropriate permissions.

  5. Choose the buildbot-worker root directory (all builds will be placed under it), buildbot-worker access name and password the build master will be using to authenticate your buildbot-worker.

  6. Create a buildbot-worker in context of that buildbot-worker account. Point it to the lab.llvm.org port 9990 (see Buildbot documentation, Creating a worker for more details) by running the following command:

    $ buildbot-worker create-worker <buildbot-worker-root-directory> \
                 lab.llvm.org:9990 \
                 <buildbot-worker-access-name> \
                 <buildbot-worker-access-password>
    

    To point a worker to silent master please use lab.llvm.org:9994 instead of lab.llvm.org:9990.

  7. Fill the buildbot-worker description and admin name/e-mail. Here is an example of the buildbot-worker description:

    Windows 7 x64
    Core i7 (2.66GHz), 16GB of RAM
    
    g++.exe (TDM-1 mingw32) 4.4.0
    GNU Binutils 2.19.1
    cmake version 2.8.4
    Microsoft(R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
    
  8. Make sure you can actually start the buildbot-worker successfully. Then set up your buildbot-worker to start automatically at the start up time. See the buildbot documentation for help. You may want to restart your computer to see if it works.

  9. Send a patch which adds your build worker and your builder to zorg. Use the typical LLVM workflow.

    • workers are added to buildbot/osuosl/master/config/workers.py
    • builders are added to buildbot/osuosl/master/config/builders.py

    Please make sure your builder name and its builddir are unique through the file.

    All new builders should default to using the “‘collapseRequests’: False” configuration. This causes the builder to build each commit individually and not merge build requests. To maximize quality of feedback to developers, we strongly prefer builders to be configured not to collapse requests. This flag should be removed only after all reasonable efforts have been exhausted to improve build times such that the builder can keep up with commit flow.

    It is possible to allow email addresses to unconditionally receive notifications on build failure; for this you’ll need to add an InformativeMailNotifier to buildbot/osuosl/master/config/status.py. This is particularly useful for the staging buildmaster which is silent otherwise.

  10. Send the buildbot-worker access name and the access password directly to Galina Kistanova, and wait till she will let you know that your changes are applied and buildmaster is reconfigured.

  11. Check the status of your buildbot-worker on the Waterfall Display to make sure it is connected, and the Workers Display to see if administrator contact and worker information are correct.

  12. Wait for the first build to succeed and enjoy.

Best Practices for Configuring a Fast Builder

As mentioned above, we generally have a strong preference for builders which can build every commit as they come in. This section includes best practices and some recommendations as to how to achieve that end.

The goal
In 2020, the monorepo had just under 35 thousand commits. This works out to an average of 4 commits per hour. Already, we can see that a builder must cycle in less than 15 minutes to have a hope of being useful. However, those commits are not uniformly distributed. They tend to cluster strongly during US working hours. Looking at a couple of recent (Nov 2021) working days, we routinely see ~10 commits per hour during peek times, with occasional spikes as high as ~15 commits per hour. Thus, as a rule of thumb, we should plan for our builder to complete ~10-15 builds an hour.
Resource Appropriately
At 10-15 builds per hour, we need to complete a new build on average every 4 to 6 minutes. For anything except the fastest of hardware/build configs, this is going to be well beyond the ability of a single machine. In buildbot terms, we likely going to need multiple workers to build requests in parallel under a single builder configuration. For some rough back of the envelope numbers, if your build config takes e.g. 30 minutes, you will need something on the order of 5-8 workers. If your build config takes ~2 hours, you’ll need something on the order of 20-30 workers. The rest of this section focuses on how to reduce cycle times.
Restrict what you build and test

Think hard about why you’re setting up a bot, and restrict your build configuration as much as you can. Basic functionality is probably already covered by other bots, and you don’t need to duplicate that testing. You only need to be building and testing the unique parts of the configuration. (e.g. For a multi-stage clang builder, you probably don’t need to be enabling every target or building all the various utilities.)

It can sometimes be worthwhile splitting a single builder into two or more, if you have multiple distinct purposes for the same builder. As an example, if you want to both a) confirm that all of LLVM builds with your host compiler, and b) want to do a multi-stage clang build on your target, you may be better off with two separate bots. Splitting increases resource consumption, but makes it easy for each bot to keep up with commit flow. Additionally, splitting bots may assist in triage by narrowing attention to relevant parts of the failing configuration.

In general, we recommend Release build types with Assertions enabled. This generally provides a good balance between build times and bug detection for most buildbots. There may be room for including some debug info (e.g. with -gmlt), but in general the balance between debug info quality and build times is a delicate one.

Use Ninja & LLD
Ninja really does help build times over Make, particularly for highly parallel builds. LLD helps to reduce both link times and memory usage during linking significantly. With a build machine with sufficient parallism, link times tend to dominate critical path of the build, and are thus worth optimizing.
Use CCache and NOT incremental builds

Using ccache materially improves average build times. Incremental builds can be slightly faster, but introduce the risk of build corruption due to e.g. state changes, etc… At this point, the recommendation is not to use incremental builds and instead use ccache as the latter captures the majority of the benefit with less risk of false positives.

One of the non-obvious benefits of using ccache is that it makes the builder less sensitive to which projects are being monitored vs built. If a change triggers a build request, but doesn’t change the build output (e.g. doc changes, python utility changes, etc..), the build will entirely hit in cache and the build request will complete in just the testing time.

With multiple workers, it is tempting to try to configure a shared cache between the workers. Experience to date indicates this is difficult to well, and that having local per-worker caches gets most of the benefit anyways. We don’t currently recommend shared caches.

CCache does depend on the builder hardware having sufficient IO to access the cache with reasonable access times - i.e. a fast disk, or enough memory for a RAM cache, etc.. For builders without, incremental may be your best option, but is likely to require higher ongoing involvement from the sponsor.

Enable batch builds
As a last resort, you can configure your builder to batch build requests. This makes the build failure notifications markedly less actionable, and should only be done once all other reasonable measures have been taken.
Leave it on the staging buildmaster
While most of this section has been biased towards builders intended for the main buildmaster, it is worth highlighting that builders can run indefinitely on the staging buildmaster. Such a builder may still be useful for the sponsoring organization, without concern of negatively impacting the broader community. The sponsoring organization simply has to take on the responsibility of all bisection and triage.