[GHC] #10818: GHC 7.10.2 takes much longer to compile some packages

[GHC]

#10818: GHC 7.10.2 takes much longer to compile some packages
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        Reporter:  kazu-yamamoto     |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by j.waldmann):

 Here is some more timing info for travis:

 I added "time" measurements around the cabal commands
 https://travis-ci.org/jwaldmann/haskell-tpdb/builds/109678365

 So the timing they are showing is indeed wallclock.

 Still, sometimes "user time" was larger than "real time". I suspect this
 is because cabal uses `-j $ncpus` by default. I think for `cabal install
 --dependencies` this means to compile them in parallel, which is OK, but
 for `cabal build` this means to pass `-j $ncpus` to ghc, which decreases
 performance, and sometimes drastically so.

 So, I put explicit `-j 1` for these commands,
 https://travis-ci.org/jwaldmann/haskell-tpdb/builds/109683703
 and now the timings look at least somewhat reasonable.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10818#comment:15>
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