Why aren't my Minitest tests being run in parallel?

Question

I just recently figured out that my Minitest test cases can run in parallel. All I had to do was

require "minitest/hell"

and so I did. Unfortunately, my tests run exactly like before. Everything passes and it takes exactly as much time as it usually does. I checked htop while running my test suite, and only one core is utilized.

I set a breakpoint in a random test to check if the tests were actually set to run in parallel:

(byebug) Minitest::Test.test_order
:parallel

So what's going on?

---

My first assumption was that Minitest counts the amount of CPU cores when deciding how many processes to spawn. I had multiple physical processors (in a virtual machine) but just 1 core per processor. I have changed my VPS to having two physical processors, each with 4 cores, and my tests still aren't being run in parallel.

$ lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                8
On-line CPU(s) list:   0-7
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             2
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 62
Stepping:              4
CPU MHz:               2600.000
BogoMIPS:              5200.00
Hypervisor vendor:     VMware
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              20480K
NUMA node0 CPU(s):     0-7

Answer 1

Minitest uses threads and not processes for parallel test execution.

Since MRI (the standard Ruby interpreter) has a Global Interpreter Lock only one thread can be executed at a time. Therefore your tests do not run in parallel when using MRI.

You can get you tests to run in parallel by using a Ruby interpreter that supports concurrent threads like JRuby or Rubinius.

Read this article for more details.

Answer 2

severin's answer about MRI not being able to execute in parallel because of the GIL is correct. (Disclaimer: I wrote the article he links to.) The language gets a little fuzzy in many writeups, but you can read this article for a very straightforward description.

If you're still interested in running tests in parallel and can't change Ruby interpreters, take a look at the parallel_tests gem as an alternative for doing that, albeit with some limitations.