First SwiftTesting test always slow

Development Swift Testing
1

I have a Swift package with a test suite that contains some tests implemented with Swift Testing. Locally, they run quickly, but when I run them on Semaphore CI, the first Swift Testing test to execute incurs a performance penalty.

I'm running the tests with xcodebuild on iOS Simulator:

xcodebuild test \
  -scheme "Scheme" \
  -workspace Workspace.xcworkspace \
  -destination "platform=iOS Simulator,name=iPhone 16,OS=18.2"

The scheme is configured to use a test plan that has parallelization disabled.

Here's an excerpt from the output showing what I'm seeing:

Test Suite 'All tests' started at 2025-04-03 07:47:37.328.
◇ Test run started.
↳ Testing Library Version: 102 (arm64-apple-ios13.0-simulator)
◇ Iteration 1 started.
◇ Suite <redacted> started.
◇ Test foo() started.
✔ Test foo() passed after 23.063 seconds.

When foo() is not the first test it runs in under 100 ms.

The reason that I have parallelization disabled is that I was initially seeing all of the tests in this suite incur a performance hit. But now it's clear that there must be some startup cost.

Things I'm wondering:

  1. What is this startup penalty?
  2. Why don't I encounter it locally?
  3. Why is it attributed to the first test? (this seems like a bug)

My wild guesses around 1 so far have been…

  • maybe some simulator clone is booting. I've tried to rule that out by disabling parallelization, but maybe there's still something there.
  • maybe swift testing is getting loaded lazily and there's some kind of dynamic linking cost

Thoughts on 2…

  • maybe there's some one-time penalty when using swift testing that I've already incurred locally but that has not yet been incurred in the CI image

Guidance welcome!

x-posted:

1 Like
2

I'll take a look at the feedback you provided to Apple. While I do so, if you haven't already, it would be immensely helpful if you could reproduce the issue and capture a sysdiagnose (or at least sudo spindump) immediately after the test starts but before it finishes. Once it's finished saving, please attach it to the feedback report you created. It will help Apple's engineers see where the time is being spent. It may be the case that this is simply a quirk of how the iOS Simulator works.

3

@grynspan thanks for the reply. I captured a spindump and attached it to FB17102970

4

I took a look at the spindump you attached (thanks!) From what I can see, Swift Testing is not directly involved in the hang. As an Apple employee, I can't go into further detail here (sorry!)

Thanks for filing the bug report!

1 Like