[GHC] #9221: (super!) linear slowdown of parallel builds on 40 core machine

[GHC]

#9221: (super!) linear slowdown of parallel builds on 40 core machine
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        Reporter:  carter            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.8.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #910              |  Differential Revisions:
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Comment (by ezyang):

 I was chatting with one of my colleagues about this problem recently, and
 they said something very provocative: if GHC is not scaling because there
 is some global mutable state (e.g. the NameCache) which all the threads
 are hitting, it doesn't make sense to try to fix the compiler to scale;
 you should just run multiple processes of the compiler in parallel (like
 GHC's build does). Guaranteed scaling!

 Do people agree with this viewpoint? Disagree? If we take this viewpoint
 seriously, we should spend more time improving GHC's ability to output the
 dependency analysis for other tools to then build.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:23>
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