[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:  normal            |            Milestone:  8.2.1
       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, #8224       |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonmar):

 Yes, perhaps we should default to -qb0 when -A is larger than some
 threshold.

 The mutator parallelism looks not too bad (3.24 out of 4).  Was there
 enough parallelism in the program you were compiling to do better than
 that?

 The `MVar`s are probably just the compilation manager: it fires up a
 thread for every module, and they wait on the results of compiling the
 modules they depend on.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:56>
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