slow execution of built executables on a Mac

Richard Eisenberg rae at
Wed Nov 7 22:33:37 UTC 2018

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?


> On Oct 30, 2018, at 2:32 PM, Gabor Greif <ggreif at> wrote:
> Maybe a symptom of an AFPS bug?
> Just came across this, might be worth a look.
> Cheers,
>    Gabor
> On 10/26/18, Richard Eisenberg <rae at> wrote:
>> Hi devs,
>> I have a shiny, new iMac in my office. It's thus frustrating that it takes
>> my iMac longer to build GHC than my trusty 28-month-old laptop. Building
>> devel2 on a fresh checkout takes just about an hour. By contrast, my laptop
>> is done after 30 minutes of work (same build settings). The laptop has a
>> 2.8GHz Intel i7 running macOS 10.13.5; the desktop has a 3.5GHz Intel i5
>> running macOS 10.13.6. Both bootstrapped from the binary distro of GHC
>> 8.6.1.
>> Watching GHC build, everything is snappy enough during the stage-1 build.
>> But then, as soon as we start using GHC-produced executables, things slow
>> down. It's most noticeable in the rts_dist_HC phase, which crawls. Stage 2
>> is pretty slow, too.
>> So: is there anything anyone knows about recent Macs not liking locally
>> built executables? Or is there some local setting that I need to update? The
>> prepackaged GHC seems to work well, so that gives me hope that someone knows
>> what setting to tweak.
>> Thanks!
>> Richard
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