[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 slyfox):

 I think it's expected if you have GC work imbalance. Some GC work-stealing
 threads
 sit in yield loop at:
 http://git.haskell.org/ghc.git/blob/HEAD:/rts/sm/GC.c#l981

 Imbalance can be seen seen as:
 '''+RTS -sstderr''' shows it as '''Parallel GC work balance: 80.03%
 (serial 0%, perfect 100%)'''.

 and can be tuned by heap size or allocation area ('''-A''').

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:70>
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