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Add --semaphore#6851

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mpilgrem merged 8 commits intocommercialhaskell:masterfrom
horriblename:sema
Feb 21, 2026
Merged

Add --semaphore#6851
mpilgrem merged 8 commits intocommercialhaskell:masterfrom
horriblename:sema

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@horriblename horriblename commented Feb 20, 2026 •
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implements #6131

Please include the following checklist in your pull request:

  • Any changes that could be relevant to users have been recorded in ChangeLog.md.
  • The documentation has been updated, if necessary

Please also shortly describe how you tested your change. Bonus points for added tests!

(I'm not sure what kind of test I can add for this?)

a quick benchmark matches claims in https://well-typed.com/blog/2023/08/reducing-haskell-parallel-build-times/ regarding compiling lens (112s vs 140s, ~20% improvement).

$ time stack exec -- stack build --work-dir .temp-work lens
  298.37s user 43.14s system 242% cpu 2:20.69 total

# with --semaphore
$ time stack exec -- stack build --semaphore --work-dir .temp-work lens
  349.93s user 47.79s system 353% cpu 1:52.50 total

rm -r $(stack exec -- stack path --snapshot-pkg-db) was deleted before each run

Some notes about the implementation

  • this uses semaphore-compat, which is also used by cabal

@horriblename horriblename changed the title Sema Add --semaphore Feb 20, 2026
horriblename
horriblename commented Feb 20, 2026
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doc/commands/build_command.md Outdated
Comment on lines +1149 to +1150
Uses the semaphore feature in Cabal/GHC to build multiple modules from the same
package in parallel. (see GHC's documentation on `-jsem`)
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@horriblename horriblename Feb 20, 2026

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probably better if I can refer to cabal's docs for the --semaphore flag, but I couldn't find any (aside from the extremely short description in --help). There are some docs for cabal-install's --semaphore but that's not quite the same thing

@horriblename horriblename force-pushed the sema branch 2 times, most recently from d66961a to 26128a1 Compare February 20, 2026 00:15
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mpilgrem commented Feb 20, 2026 •
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@horriblename, thanks! Don't worry about the CI for Linux with --alpine (that is waiting for Stack 3.9.3 to appear in Docker images).

I am wondering about the user experience if they accidently specify --semaphore when building with GHC < 9.8.

EDIT: --semaphore appears to have been introduced with Cabal-3.12.0.0, which was a boot package of GHC 9.10.1.

I had not paid attention to --semaphore, as the Cabal project has not yet documented its existence. However, I see that stack runghc -- Setup.hs build --help offers up the (largely unhelpful):

Flags for build:
...
--semaphore=SEMAPHORE          semaphore

I have opened an issue at Cabal's repository:

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mpilgrem commented Feb 20, 2026 •
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@horriblename, other than handling the Stack user experience when --semaphore=SEMAPHORE is not supported by Cabal, this looks good to me.

I've pushed a commit to better conform the online and in-app documentation to other documentation.

EDIT: I have also pushed a commit to ignore the flag with a warning when it is unsupported.

EDIT2: I have also pushed a commit to update a relevant unit test.

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mpilgrem commented Feb 21, 2026

Tested on Windows 11 by building Stack itself: reduced from 83 s to 50 s (~40% less time).

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horriblename commented Feb 21, 2026 •
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EDIT: I have also pushed a commit to ignore the flag with a warning when it is unsupported.

I suppose there's nothing I need to do anymore? (thanks!)

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mpilgrem commented Feb 21, 2026

@horriblename, many thanks. I'll likely merge this once the current CI passes.

@mpilgrem mpilgrem merged commit bac8a3d into commercialhaskell:master Feb 21, 2026
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@horriblename horriblename deleted the sema branch February 21, 2026 19:56
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mpilgrem commented Feb 21, 2026

@horriblename, some disappointing news. I have learned that GHC's semaphore feature has major problems on Linux, because musl and non-musl semaphores are incompatible:

In the light of that, I am going to add something to Stack's documentation and label the feature as experimental.

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