Jumbo / Unity builds

To improve compilation times it is possible to use “unity builds”, called Jumbo builds, in Chromium. The idea is to merge many translation units (“source files”) and compile them together. Since a large portion of Chromium's code is in shared header files that dramatically reduces the total amount of work needed.

Build instructions

If jumbo isn't already enabled, you enable it in gn by setting use_jumbo_build = true then compile as normal.


Jumbo is currently implemented as a combined gn template and a python script. Eventually it may become a native gn feature. By (indirectly) using the template internal_jumbo_target, each target will split into one action to “merge” the files and one action to compile the merged files and any files left outside the merge.

Template file: //build/config/jumbo.gni Merge script: //build/config/merge_for_jumbo.py


The “merge” is currently done by creating wrapper files that #include the source files.

Jumbo Pros and Cons


  • Everything compiles significantly faster. When fully enabled everywhere this can save hours for a full build (binaries and tests) on a moderate computer. Linking is faster because there is less redundant data (debug information, inline functions) to merge.
  • Certain code bugs can be statically detected by the compiler when it sees more/all the relevant source code.


  • By merging many files, symbols that have internal linkage in different cc files can collide and cause compilation errors.
  • The smallest possible compilation unit grows which can add 10-20 seconds to some single file recompilations (though link times often shrink).

Mixed blessing

  • Slightly different compiler warnings will be active.


By default at most 200 files are merged at a time. The more files are merged, the less total CPU time is needed, but parallelism is reduced. This can be changed by setting jumbo_file_merge_limit to something else than 200.


The term jumbo is used to avoid the confusion resulting from talking about unity builds since unity is also the name of a graphical environment, a 3D engine, a webaudio filter and part of the QUIC congestion control code. Jumbo has been used as name for a unity build system in another browser engine.

Want to make your favourite piece of code jumbo?

  1. Add import("//build/config/jumbo.gni") to BUILD.gn.
  2. Change your target, for instance static_library, to jumbo_static_library. So far source_set, component, static_library and split_static_library are supported.
  3. Recompile and test.


Change from:

source_set("foothing") {
  sources = [


import("//build/config/jumbo.gni")  # ADDED LINE
jumbo_source_set("foothing") {      # CHANGED LINE
  sources = [

If you see some compilation errors about colliding symbols, resolve those by renaming symbols or removing duplicate code. If it's impractical to change the code, add a jumbo_excluded_sources variable to your target in BUILD.gn:

jumbo_excluded_sources = [ "problematic_file.cc" ]

More information and pictures

There are more information and pictures in a Google Document

Mailing List

Public discussions happen on the generic blink-dev and chromium-dev mailing lists.


Bugs / feature requests

Related bugs use the label jumbo in the bug database. See the open bugs.