Fuchsia-specific Ninja improvements

The Fuchsia build system uses a custom Ninja binary that provides several improvements to the developer experience. This page describes them.


The motivation for customizing Ninja for Fuchsia is detailed in RFC-0153.

In a nutshell, there are a number of features that would benefit Fuchsia developers significantly that are hard to get in the upstream version.

All Fuchsia-specific changes are performed on the local fuchsia-rfc-0153 branch of our local Ninja git mirror, and are rebased periodically to make them easy to send as Github pull requests to the upstream project, as described in the Strategy section of the RFC.

Feature: Status of running commands

Set NINJA_STATUS_MAX_COMMANDS in your environment to a strictly positive integer to let Ninja print, when run in a smart terminal, a table of the longest running commands, and their elapsed times, just under the status line. For example, with export NINJA_STATUS_MAX_COMMANDS=4, the status could look like:

[0/28477](260) STAMP host_x64/obj/tools/configc/configc_sdk_meta_generated_file.stamp
  0.4s | STAMP obj/sdk/zircon_sysroot_meta_verify.stamp
  0.4s | CXX obj/BUILD_DIR/fidling/gen/sdk/fidl/fuchsia.me...chsia.media/cpp/fuchsia.media_cpp_common.common_types.cc.o
  0.4s | CXX obj/BUILD_DIR/fidling/gen/sdk/fidl/fuchsia.me...fuchsia.media/cpp/fuchsia.media_cpp.natural_messaging.cc.o
  0.4s | CXX obj/BUILD_DIR/fidling/gen/sdk/fidl/fuchsia.me...dia/cpp/fuchsia.media_cpp_natural_types.natural_types.cc.o

The following animated image shows how this looks in practice:

Ninja multi-line status example

Note that:

  • This feature is automatically disabled in dry-run or verbose invocations of Ninja (that is, using the -n or --verbose flags).

  • This feature is automatically disabled when Ninja is not running in an interactive / smart terminal.

  • This feature is suspended when running console commands as well (visible in the example above when running Bazel actions).

  • This feature makes it easy to visualize bottlenecks in the build, that is, long-lasting commands that prevent other commands to run in parallel.

The commands table updates 10 times per second by default, which is very useful to understand which long commands are hobbling the build. It is possible to change the refresh period by setting NINJA_STATUS_REFRESH_MILLIS to a decimal value in milliseconds (not that anything lower than 100 will be ignored since elapsed times are only printed up to a single decimal fractional digit).

Feature: Persistent mode for faster startup times

Speed up successive Ninja invocations by setting NINJA_PERSISTENT_MODE=1 in your environment. This feature makes Ninja launch a background server process to read the build manifest once, then keep the build graph in memory between successive builds.

Note that:

  • This feature should be completely transparent, and should not affect Ninja's behavior otherwise. It you spot an issue or difference, please let us know at fuchsia-build-team@google.com!

  • Any change in the input .ninja file is automatically detected. In this case the existing server will be shutdown, and a new one will be started automatically. No additional user interaction is required after changing a GN build file or performing a jiri update.

  • The server process will shutdown gracefully after idling for 5 minutes. Set NINJA_PERSISTENT_TIMEOUT_SECONDS=<count> in your environment to change this delay.

  • Use fx build -t server status to retrieve the status of the server for the current build directory.

  • Use fx build -t server stop to stop any running instance of the server explicitly.

  • Set NINJA_PERSISTENT_LOG_FILE=<path> to send logs related to the persistent mode to a given file path.

  • Each server process currently takes about 1 GiB of RAM for a core.x64 build configuration. Exact figures will depend on the size of the Ninja graph, which depends on your args.gn configuration.

  • Each Ninja build directory can be served by at most one server process. But it is possible to have several processes when using several build directories.

  • Ninja tools (e.g. ninja -C <dir> -t commands <target>) do not run on the server yet, so will still use slow startup. This will be fixed in the future to speed up queries.

Known bugs / caveats, that will be worked out:

  • Mixing persistent and non-persistent builds on the same directory can confuse the server at the moment, because independent changes to the Ninja build and deps logs are not properly detected. This will be fixed.

    Work-around: use -t server stop to stop the server before unsetting NINJA_PERSISTENT_MODE in your environment.

  • "Fast startup" takes a few seconds. For now, every incremental build still needs to call stat() for all the files in the build graph, which currently takes a few seconds. This will be fixed in the future by using inotify / kqueue based filesystem watching features of the host operating system, to start immediately when only a few files have been modified instead.

  • Does not work on Windows (yet). This is due to a technical Win32 limitation that prevents copying console handles to other processes. This is mostly an issue for the upstream Ninja team, since Fuchsia development does not happen on Windows.