Investigate speed regressions

💥 Regression Report

Speed regression in Jest@23.x

Last working version

Worked up to version:

Jest@22.x

Stopped working in version:

A big chunk of the regression was introduced with this PR https://github.com/facebook/jest/pull/5932

To Reproduce

Steps to reproduce the behavior:

I’ve been using a small repo, https://github.com/rogeliog/jest-benchmark(I can give you access if you need to) to run simple benchmarks across Jest versions.

Expected behavior

Test should run faster

Here are is progress that I’ve made on the speed regression investigation.

1. A big chunk of the speed regression was introduced here https://github.com/facebook/jest/pull/5932
2. I’ve been testing with https://github.com/rogeliog/jest-benchmark and the differences that I get between jest@22 and jest@23 are the following

   ------------------- Jest@22 -------------------------
   0m2.338s
   0m2.317s
   ------------------- Jest@23 -------------------------
   0m6.068s
   0m6.048s
   

Initial findings

1. In jest@22 https://github.com/facebook/jest/blob/v22.4.4/packages/jest-config/src/normalize.js#L128 returns null which causes https://github.com/facebook/jest/blob/v22.4.4/packages/jest-config/src/normalize.js#L130-L132 not to get executed.

2. In jest@23 https://github.com/facebook/jest/blob/master/packages/jest-config/src/normalize.js#L143 returns a path and https://github.com/facebook/jest/blob/master/packages/jest-config/src/normalize.js#L144-L146 gets executed.

3. If I comment out https://github.com/facebook/jest/blob/master/packages/jest-config/src/normalize.js#L144-L146 then I get the following stats.

   ------------------- Jest@22 -------------------------
   0m2.537s
   0m2.331s
   ------------------- Jest@dev -------------------------
   0m3.658s
   0m3.633s
   

4. I’m not to familiar the reasoning behind it, but it seems that since the original there were some comments about them returning different values https://github.com/facebook/jest/pull/5932#discussion_r179545920.

cc: @cpojer, @arcanis

[jkillian]

For what it’s worth, turns out I misdiagnosed the cause of the problem. It was actually an upgrade from ts-jest 23.1.4 -> 23.10.5 which was causing the large slowdown and memory usage increase (babel and jest proper were red herrings). We found switching our jest config from:

    globals: {
      'ts-jest': {
        diagnostics: false,
      },
    },

to

    globals: {
      'ts-jest': {
        diagnostics: false,
        isolatedModules: true,
      },
    },

resolved the performance issues for us! Just posting the info here to absolve jest of the blame I put on it 😁 and to help anyone else if they have similar issues with ts-jest

[nasreddineskandrani]

please check my comment here: problem: https://github.com/facebook/jest/issues/6783#issuecomment-445560998 investigation: https://github.com/facebook/jest/issues/6783#issuecomment-445660665 cause: https://github.com/facebook/jest/issues/6783#issuecomment-446123323 solution: https://github.com/facebook/jest/pull/7518

I am sure 100% that the TestScheduler is creating the regression for the case i reported in the issue. I don’t know yet if solving this will help with the run on all files => if you think i should isolate this in a specific issue let me know.