minitest-parallel_fork adds fork-based parallelization to Minitest. Each test/spec suite is run in one of the forks, allowing this to work correctly when using before_all/after_all/around_all hooks provided by minitest-hooks. Using separate processes via fork can significantly improve spec performance when using MRI, and can work in cases where Minitest's default thread-based parallelism do not work, such as when tests/specs modify the constant namespace.


gem install minitest-parallel_fork

Source Code

Source code is available on GitHub at


You can enable fork-based parallelism just by requiring minitest/parallel_fork. One easy to do so without modifying the spec code itself is to use the RUBYOPT environment variable. So if you execute your specs using:

rake spec

You can switch to fork-based parallelism using:

RUBYOPT=-rminitest/parallel_fork rake spec

To control the number of forks, you can set the NCPU environment variable:

NCPU=8 RUBYOPT=-rminitest/parallel_fork rake spec

If you don't set the NCPU environment variable, minitest-parallel_fork will use 4 forks by default.


In some cases, especially when using external databases, you'll need to do some before fork or after fork setup. minitest/parallel_fork supports before_parallel_fork and after_parallel_fork hooks.

before_parallel_fork is called before any child processes are forked:

Minitest.before_parallel_fork do

after_parallel_fork is called after each child process is forked, with the number of the child process, starting at 0:

Minitest.after_parallel_fork do |i|
  DB.opts[:database] += (i+1).to_s

The above examples show a fairly easy way to use minitest-parallel_fork with an external database when using Sequel. Before forking, all existing database connections are disconnected, and after forking, the database name is changed in each child to reference a child-specific database, so that the child processes do not share a database and are thus independent.

There is also a hook for debugging. on_parallel_fork_marshal_failure is called if there is an error unmarshalling data sent from the child process to the parent process. This can happen if one of the child processes exits unexpected during the test, before it reports results.

Minitest.on_parallel_fork_marshal_failure do
  # Gather relevant logs for more debugging


To use this with Rails/ActiveRecord, you probably want to use hooks similar to:

Minitest.before_parallel_fork do

Minitest.after_parallel_fork do |i|
  db_config = Rails.application.config.database_configuration[Rails.env].clone
  db_config['database'] += (i+1).to_s


The speedup you get greatly depends on your specs. Here's some examples using Sequel's specs:

                   2 forks         4 forks
spec_core:      1.25x - 1.36x        1.5x   
spec_model:     1.29x - 1.62x   1.72x - 2.02x
spec_plugin:    1.57x - 1.76x   2.29x - 2.37x    
spec_sqlite:    1.75x - 1.86x   2.26x - 2.65x
spec_postgres:  1.32x - 1.40x     Untested




Jeremy Evans <[email protected]>