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Jorge Manrubia ecc5afed30 Parallelize tests only when overhead is justified 
Parallelizing tests has a cost in terms of database setup and fixture
loading. This change makes Rails disable parallelization when the number
of tests is below a configurable threshold.

When running tests in parallel each process gets its own database
instance. On each execution, each process will update each database
schema (if needed) and load all the fixtures. This can be very expensive
for non trivial datasets.

As an example, for HEY, when running a single file with 18 tests,
running tests in parallel in my box adds an overhead of 13 seconds
versus not parallelizing them. Of course parallelizing is totally worthy
when there are many tests to run, but not when running just a few tests.

The threshold is configurable via
config.active_support.test_parallelization_minimum_number_of_tests,
which is 30 50 by default.

This also adds some tracing to know how tests are being executed:

When in parallel:

```
Running 2829 tests in parallel in 8 processes
```

When not in parallel:

```
Running 15 tests in a single process (parallelization threshold is 30)
```
2021-07-14 13:36:28 +02:00
..
bin
lib Parallelize tests only when overhead is justified 2021-07-14 13:36:28 +02:00
test Fix CI failure caused by error_highlight gem 2021-07-02 17:44:10 +09:00
.gitignore
activesupport.gemspec Rails 7 requires Ruby 2.7 and prefer Ruby 3+ 2021-02-04 16:34:53 +00:00
CHANGELOG.md Parallelize tests only when overhead is justified 2021-07-14 13:36:28 +02:00
MIT-LICENSE
Rakefile
README.rdoc

README.rdoc 

= Active Support -- Utility classes and Ruby extensions from Rails

Active Support is a collection of utility classes and standard library
extensions that were found useful for the Rails framework. These additions
reside in this package so they can be loaded as needed in Ruby projects
outside of Rails.

You can read more about the extensions in the {Active Support Core Extensions}[https://edgeguides.rubyonrails.org/active_support_core_extensions.html] guide.

== Download and installation

The latest version of Active Support can be installed with RubyGems:

  $ gem install activesupport

Source code can be downloaded as part of the Rails project on GitHub:

* https://github.com/rails/rails/tree/main/activesupport


== License

Active Support is released under the MIT license:

* https://opensource.org/licenses/MIT


== Support

API documentation is at:

* https://api.rubyonrails.org

Bug reports for the Ruby on Rails project can be filed here:

* https://github.com/rails/rails/issues

Feature requests should be discussed on the rails-core mailing list here:

* https://discuss.rubyonrails.org/c/rubyonrails-core