sccache-dist.rst - mozsearch

.. _sccache_dist:

==================================

Distributed sccache (sccache-dist)

==================================

`sccache <https://github.com/mozilla/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::

    [dist.auth]

    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``::

    [[dist.toolchains]]

    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"

    [[dist.toolchains]]

    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"

    [[dist.toolchains]]

    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 <https://hg.mozilla.org/mozilla-central/file/tip/taskcluster/kinds/toolchain/dist-toolchains.yml>`_.

  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::

    [[dist.toolchains]]

    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"

    [[dist.toolchains]]

    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 <https://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

  <https://github.com/mozilla/sccache/pull/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

  <https://github.com/mozilla/sccache/pull/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 <https://sourceware.org/ml/libc-alpha/2019-11/msg00431.html>`_.

* 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.

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