Distributed sccache (sccache-dist)

sccache is a ccache-like tool written in Rust by Mozilla and many contributors.

sccache-dist, its distributed variant, elevates this functionality by enabling the distribution and caching of Rust compilations across multiple machines. Please consider using sccache-dist when you have several machines compiling Firefox on the same network.

The steps for setting up your machine as an sccache-dist server are detailed below.

In addition to improved security properties, distributed sccache offers distribution and caching of rust compilation, so it should be an improvement above and beyond what we see with icecc. Build servers run on Linux and distributing builds is currently supported from Linux, macOS, and Windows.

Steps for distributing a build as an sccache-dist client

Start by following the instructions at https://github.com/mozilla/sccache/blob/master/docs/DistributedQuickstart.md#configure-a-client to configure your sccache distributed client. NOTE If you’re distributing from Linux a toolchain will be packaged automatically and provided to the build server. If you’re distributing from Windows or macOS, start by using the cross-toolchains provided by ./mach bootstrap rather than attempting to use icecc-create-env. sccache 0.2.12 or above is recommended, and the auth section of your config must read:

type = "mozilla"
  • If you’re compiling from a macOS client, there are a handful of additional considerations outlined here: https://github.com/mozilla/sccache/blob/master/docs/DistributedQuickstart.md#considerations-when-distributing-from-macos.

    Run ./mach bootstrap to download prebuilt toolchains to ~/.mozbuild/clang-dist-toolchain.tar.xz and ~/.mozbuild/rustc-dist-toolchain.tar.xz. This is an example of the paths that should be added to your client config to specify toolchains to build on macOS, located at ~/Library/Application Support/Mozilla.sccache/config:

    type = "path_override"
    compiler_executable = "/path/to/home/.rustup/toolchains/stable-x86_64-apple-darwin/bin/rustc"
    archive = "/path/to/home/.mozbuild/rustc-dist-toolchain.tar.xz"
    archive_compiler_executable = "/builds/worker/toolchains/rustc/bin/rustc"
    type = "path_override"
    compiler_executable = "/path/to/home/.mozbuild/clang/bin/clang"
    archive = "/path/to/home/.mozbuild/clang-dist-toolchain.tar.xz"
    archive_compiler_executable = "/builds/worker/toolchains/clang/bin/clang"
    type = "path_override"
    compiler_executable = "/path/to/home/.mozbuild/clang/bin/clang++"
    archive = "/path/to/home/.mozbuild/clang-dist-toolchain.tar.xz"
    archive_compiler_executable = "/builds/worker/toolchains/clang/bin/clang"

    Note that the version of rustc found in rustc-dist-toolchain.tar.xz must match the version of rustc used locally. The distributed archive will contain the version of rustc used by automation builds, which may lag behind stable for a few days after Rust releases, which is specified by the task definition in this file. For instance, to specify 1.37.0 rather than the current stable, run rustup toolchain add 1.37.0 and point to /path/to/home/.rustup/toolchains/1.37.0-x86_64-apple-darwin/bin/rustc in your client config.

    The build system currently requires an explicit target to be passed with HOST_CFLAGS and HOST_CXXFLAGS e.g.:

    export HOST_CFLAGS="--target=x86_64-apple-darwin16.0.0"
    export HOST_CXXFLAGS="--target=x86_64-apple-darwin16.0.0"
  • Compiling from a Windows client is supported but hasn’t seen as much testing as other platforms. The following example mozconfig can be used as a guide:

    ac_add_options CCACHE="C:/Users/<USER>/.mozbuild/sccache/sccache.exe"
    export CC="C:/Users/<USER>/.mozbuild/clang/bin/clang-cl.exe --driver-mode=cl"
    export CXX="C:/Users/<USER>/.mozbuild/clang/bin/clang-cl.exe --driver-mode=cl"
    export HOST_CC="C:/Users/<USER>/.mozbuild/clang/bin/clang-cl.exe --driver-mode=cl"
    export HOST_CXX="C:/Users/<USER>/.mozbuild/clang/bin/clang-cl.exe --driver-mode=cl"

    The client config should be located at ~/AppData/Roaming/Mozilla/sccache/config/config, and as on macOS custom toolchains should be obtained with ./mach bootstrap and specified in the client config, for example:

    type = "path_override"
    compiler_executable = "C:/Users/<USER>/.mozbuild/clang/bin/clang-cl.exe"
    archive = "C:/Users/<USER>/.mozbuild/clang-dist-toolchain.tar.xz"
    archive_compiler_executable = "/builds/worker/toolchains/clang/bin/clang"
    type = "path_override"
    compiler_executable = "C:/Users/<USER>/.rustup/toolchains/stable-x86_64-pc-windows-msvc/bin/rustc.exe"
    archive = "C:/Users/<USER>/.mozbuild/rustc-dist-toolchain.tar.xz"
    archive_compiler_executable = "/builds/worker/toolchains/rustc/bin/rustc"
  • Add the following to your mozconfig:

    ac_add_options CCACHE=/path/to/home/.mozbuild/sccache/sccache

    If you’re compiling from a macOS client, you might need some additional configuration:

    # Set the target flag to Darwin
    export CFLAGS="--target=x86_64-apple-darwin16.0.0"
    export CXXFLAGS="--target=x86_64-apple-darwin16.0.0"
    export HOST_CFLAGS="--target=x86_64-apple-darwin16.0.0"
    export HOST_CXXFLAGS="--target=x86_64-apple-darwin16.0.0"
    # Specify the macOS SDK to use
    ac_add_options --with-macos-sdk=/path/to/MacOSX-SDKs/MacOSX13.3.sdk

    You can get the right macOS SDK by downloading an old version of XCode from developer.apple.com and unpacking the SDK from it.

  • When attempting to get your client running, the output of sccache -s should be consulted to confirm compilations are being distributed. To receive helpful logging from the local daemon in case they aren’t, run SCCACHE_NO_DAEMON=1 SCCACHE_START_SERVER=1 SCCACHE_LOG=sccache=trace path/to/sccache in a terminal window separate from your build prior to building. NOTE use RUST_LOG instead of SCCACHE_LOG if your build of sccache does not include pull request 822. (sccache binaries from mach bootstrap do include this PR.)

  • Run ./mach build -j<value> with an appropriately large <value>. sccache --dist-status should provide the number of cores available to you (or a message if you’re not connected). In the future this will be integrated with the build system to automatically select an appropriate value.

This should be enough to distribute your build and replace your use of icecc. Bear in mind there may be a few speedbumps, and please ensure your version of sccache is current before investigating further. Please see the common questions section below and ask for help if anything is preventing you from using it over email (dev-builds), on slack in #sccache, or in #build on irc.

Steps for setting up a server

Build servers must run linux and use bubblewrap 0.3.0+ for sandboxing of compile processes. This requires a kernel 4.6 or greater, so Ubuntu 18+, RHEL 8, or similar.

  • Run ./mach bootstrap or ./mach artifact toolchain --from-build linux64-sccache to acquire a recent version of sccache-dist. Please use a sccache-dist binary acquired in this fashion to ensure compatibility with statically linked dependencies.

  • The instructions at https://github.com/mozilla/sccache/blob/master/docs/DistributedQuickstart.md#configure-a-build-server should contain everything else required to configure and run the server.

    NOTE Port 10500 will be used by convention for builders. Please use port 10500 in the public_addr section of your builder config.

    Extra logging may be helpful when setting up a server. To enable logging, run your server with sudo env SCCACHE_LOG=sccache=trace ~/.mozbuild/sccache/sccache-dist server --config ~/.config/sccache/server.conf (or similar). NOTE sudo must come before setting environment variables for this to work. NOTE use RUST_LOG instead of SCCACHE_LOG if your build of sccache does not include pull request 822. (sccache binaries from mach bootstrap do include this PR.)

Common questions/considerations

  • My build is still slow: scache-dist can only do so much with parts of the build that aren’t able to be parallelized. To start debugging a slow build, ensure the “Successful distributed compilations” line in the output of sccache -s dominates other counts. For a full build, at least a 2-3x improvement should be observed.

  • My build output is incomprehensible due to a flood of warnings: clang will treat some warnings differently when it’s fed preprocessed code in a separate invocation (preprocessing occurs locally with sccache-dist). Adding rewrite_includes_only = true to the dist section of your client config will improve this; however, setting this will cause build failures with a commonly deployed version of glibc. This option will default to true once the fix is more widely available. Details of this fix can be found in this patch.

  • My build fails with a message about incompatible versions of rustc between dependent crates: if you’re using a custom toolchain check that the version of rustc in your rustc-dist-toolchain.tar.xz is the same as the version you’re running locally.