[GHC] #15891: Excessive system time during execution of GHC-built executables on macOS

[GHC]

#15891: Excessive system time during execution of GHC-built executables on macOS
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           Reporter:  goldfire          |             Owner:  (none)
               Type:  bug               |            Status:  new
           Priority:  normal            |         Milestone:  8.6.3
          Component:  Compiler          |           Version:  8.6.2
           Keywords:                    |  Operating System:  MacOS X
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 It seems that the executables built by GHC on my configuration of macOS
 are slow, clocking in 80% of my machine's cycles in system time.

 Here is what I know.

 - Everything tested bootstrapping with both GHC 8.6.1 and GHC 8.6.2. There
 is no observable difference between these bootstrapping compilers. The
 detailed data reported is all from GHC 8.6.1.

 - When I try to build GHC, stage 1 builds snappily. But once stage-1 gets
 used, it slows to a crawl.

 - Activity Monitor reports that my computer is spending roughly 80% of its
 time in the System space, not the User space. (While building ghc-stage1
 itself, there is barely any system activity.)

 - I have sampled ghc-stage1 during a rts_dist_HC call. Samples are at
 https://gist.github.com/goldfirere/7316920ad37d776c25c15dbb0ed5996f

 - I then `dtruss`d the same, with output at
 https://gist.github.com/goldfirere/621ce9072e1204d30e96e7daecc80d02

 - System information:
  * macOS High Sierra, 10.13.6
  * Processor: 3.5 GHz Intel Core i5
  * Memory: 16 GB 2400 MHz DDR4
  * SSD main storage, formatted with APFS
  * happy 1.19.9
  * alex 3.2.4
  * XCode 10.1
  * gcc is clang-1000.11.45.5, Apple LLVM version 10.0.0, thread model:
 posix

 Others (see [https://mail.haskell.org/pipermail/ghc-
 devs/2018-November/016507.html thread]) have not been able to repro, but
 this is very reliable for me.

 Any ideas?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15891>
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