Starting Clash is really slow on Windows with modern GHC's
Summary
At least GHC 9.4.8, and GHC 9.6.3 up to and including 9.12.2 are very, very slow on Windows when starting up Clash. We mainly observe this on Clash but discovered that the same problem occurs with doctest-parallel, which is a much smaller code base so easier to run and debug.
This is observed on GitHub Windows runners (windows-2022, Microsoft Windows Server 2022 10.0.20348 Datacenter) and on our own desktops, at least one of which runs Windows 11 Pro 24H2.
If I compile Clash with Stack LTS 23.19 on my old Core i5-4690k and Windows 11 Pro 24H2, and start clashi (which is a modification of ghci), it will output some text, and then spend 53 seconds before showing the prompt. Of course, this is a rather old machine.
Steps to reproduce
git clone https://github.com/martijnbastiaan/doctest-parallel.git
cd doctest-parallel
git checkout d3df7aa
cabal build doctests
cabal run doctestsOn GHC 9.4.8 and 9.6.3 through 9.8.4, cabal run doctests takes 14 seconds (except that 9.8.3 is uninstallable).
On newer versions, it's even slower:
| GHC | Mean runtime (s) |
|---|---|
| 9.12.2 | 18 |
| 9.12.1 | 17 |
| 9.10.1 | 15 |
The test was run on GitHub Actions at martijnbastiaan/benchmark-ghc:experiments and the result is attached as a CSV file. Additionally, the results are in this details block:
Execution speed
| OS | GHC | Mean runtime (s) |
|---|---|---|
| windows-latest | 9.12.2 | 18.1 |
| windows-latest | 9.12.1 | 17.3 |
| windows-latest | 9.10.1 | 14.9 |
| windows-latest | 9.8.4 | 13.6 |
| windows-latest | 9.8.2 | 13.7 |
| windows-latest | 9.8.1 | 14.0 |
| windows-latest | 9.6.7 | 14.0 |
| windows-latest | 9.6.6 | 14.4 |
| windows-latest | 9.6.5 | 13.7 |
| windows-latest | 9.6.4 | 14.0 |
| windows-latest | 9.6.3 | 14.0 |
| windows-latest | 9.6.2 | 4.7 |
| windows-latest | 9.6.1 | 4.7 |
| windows-latest | 9.4.8 | 13.8 |
| windows-latest | 9.4.7 | 4.2 |
| windows-latest | 9.4.6 | 4.7 |
| windows-latest | 9.4.5 | 4.3 |
| windows-latest | 9.4.4 | 4.3 |
| windows-latest | 9.4.3 | 4.5 |
| windows-latest | 9.4.2 | 4.3 |
| windows-latest | 9.4.1 | 4.3 |
| windows-latest | 9.2.8 | 3.7 |
| windows-latest | 9.2.7 | 3.8 |
| windows-latest | 9.2.6 | 3.6 |
| windows-latest | 9.2.5 | 3.5 |
| windows-latest | 9.2.4 | 3.6 |
| windows-latest | 9.2.3 | 3.7 |
| windows-latest | 9.2.2 | 3.6 |
| windows-latest | 9.2.1 | 3.5 |
| windows-latest | 8.10.7 | 3.6 |
| windows-latest | 8.10.6 | 3.7 |
| windows-latest | 8.10.5 | 3.6 |
| windows-latest | 8.10.4 | 3.8 |
| windows-latest | 8.10.3 | 3.5 |
| windows-latest | 8.10.1 | 3.7 |
| ubuntu-24.04 | 9.12.2 | 1.0 |
| ubuntu-24.04 | 9.12.1 | 1.0 |
| ubuntu-24.04 | 9.10.1 | 0.9 |
| ubuntu-24.04 | 9.8.4 | 0.9 |
| ubuntu-24.04 | 9.8.2 | 0.9 |
| ubuntu-24.04 | 9.8.1 | 0.8 |
| ubuntu-24.04 | 9.6.7 | 0.8 |
| ubuntu-24.04 | 9.6.6 | 0.9 |
| ubuntu-24.04 | 9.6.5 | 0.9 |
| ubuntu-24.04 | 9.6.4 | 1.0 |
| ubuntu-24.04 | 9.6.3 | 0.9 |
| ubuntu-24.04 | 9.6.2 | 0.9 |
| ubuntu-24.04 | 9.6.1 | 0.9 |
| ubuntu-24.04 | 9.4.8 | 0.9 |
| ubuntu-24.04 | 9.4.7 | 0.9 |
| ubuntu-24.04 | 9.4.6 | 0.9 |
| ubuntu-24.04 | 9.4.5 | 0.9 |
| ubuntu-24.04 | 9.4.4 | 0.9 |
| ubuntu-24.04 | 9.4.3 | 0.9 |
| ubuntu-24.04 | 9.4.2 | 0.9 |
| ubuntu-24.04 | 9.4.1 | 0.9 |
| ubuntu-24.04 | 9.2.8 | 0.8 |
| ubuntu-24.04 | 9.2.7 | 0.9 |
| ubuntu-24.04 | 9.2.6 | 0.9 |
| ubuntu-24.04 | 9.2.5 | 0.9 |
| ubuntu-24.04 | 9.2.4 | 0.9 |
| ubuntu-24.04 | 9.2.3 | 0.9 |
| ubuntu-24.04 | 9.2.2 | 0.9 |
| ubuntu-24.04 | 9.2.1 | 0.9 |
| ubuntu-24.04 | 8.10.7 | 0.9 |
| ubuntu-24.04 | 8.10.6 | 1.0 |
| ubuntu-24.04 | 8.10.5 | 1.0 |
| ubuntu-24.04 | 8.10.4 | 1.0 |
| ubuntu-24.04 | 8.10.3 | 1.0 |
| ubuntu-24.04 | 8.10.2 | 0.9 |
| ubuntu-24.04 | 8.10.1 | 1.0 |
Expected behavior
The run doctests should take about 4 seconds on a GitHub Actions Windows runner, which it does do for GHC 8.10.1 through 9.4.7 (except that 8.10.2 failed compilation and somehow we forgot to test on 9.0).
Environment
- Operating System: Windows Server 2022 / Windows 11 Pro 24H2
- System Architecture: x86_64