slow execution of built executables on a Mac

Ben Gamari ben at
Wed Nov 7 23:09:44 UTC 2018

Richard Eisenberg <rae at> writes:

> Sadly, none of the suggestions on this thread worked.
> But here is some more detail:
> - During stage-1, my machine's CPU is maxed out (or nearly so) in user mode.
> - After stage-1 (most obviously during rts_dist_HC), my machine spends roughly 80% of its CPU in *system* mode.
> - Testing on my other machine (which is slower, but much faster at building GHC), I never see high *system* percentages.
> - Both machines use APFS, which was one candidate for the slowdown.
> - The slow machine uses XCode 10.1; the fast one uses XCode 9.4.1
> - The slow machine uses clang 10.0.0; the fast one uses clang 9.1.0
> - `brew install gmp` on the slow machine tells me that gmp is already installed.
> - As a reminder: the slow machine is macOS 10.13.6; the fast one is macOS 10.13.5. I don't wish to try upgrading the fast one... lest that slow it down!
> Does anyone have any insight?
Were you able to collect a few stacks from the slow processes? It sounds
like the author of the original post was somehow able to do this. Under
Linux I would just fire up perf and grab a system-wide profile. Knowing
precisely what slow path you are hitting would help localize any
possible problem in GHC.


- Ben
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