[GHC] #15938: Hadrian's recompilation check is extremely slow

[GHC]

#15938: Hadrian's recompilation check is extremely slow
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Build System      |              Version:  8.6.2
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by alpmestan):

 @snowleopard: Ben reported the following to me in private:

 - when re-running hadrian the second time with `-VVV`, Ben saw:

     {{{
      | Building Stage1 libraries: binary, Cabal, ghc, ghc-boot, ghc-boot-
 th, ghc-heap, ghci, hpc, mtl, parsec, template-haskell, text,
 transformers, terminfo, array, base, bytestring, containers, deepseq,
 directory, filepath, ghc-compact, ghc-prim, haskeline, integer-gmp,
 pretty, process, rts, stm, time, xhtml, libiserv, unix
      | Building Stage1 programs : ghc, ghc-pkg, hsc2hs, unlit, hpc, ghc-
 iserv, runghc
      # _build/stage1/lib/package.conf.d/ghc-8.7.conf
      # _build/stage1/lib/package.conf.d/Cabal-2.4.0.1.conf
      # _build/stage1/lib/package.conf.d/ghci-8.7.conf
      # _build/stage2/bin/ghctags
      # _build/stage2/bin/haddock
      # _build/stage1/lib/package.conf.d/text-1.2.3.1.conf
      # _build/stage1/lib/package.conf.d/parsec-3.1.13.0.conf
      # _build/stage1/lib/bin/ghc-iserv
      # _build/stage1/bin/hsc2hs
      # _build/stage1/bin/ghc
      # _build/stage1/lib/package.conf.d/haskeline-0.7.4.3.conf
      # _build/stage1/lib/package.conf.d/libiserv-8.7.1.conf
      # _build/stage1/bin/ghc-pkg
      # _build/stage0/lib/package.conf.d/directory-1.3.1.5.conf
     }}}

     and it just pauses there for ~15 seconds before it finally starts
 issuing build commands.

 - using -j16 or -j1 doesn't seem to affect the "pause" time

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15938#comment:4>
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