[GHC] #3989: Parallel GC with more -N than physical processors gives dramatic slowdown (75 times)

[GHC]

#3989: Parallel GC with more -N than physical processors gives dramatic slowdown
(75 times)
---------------------------------+------------------------------------------
    Reporter:  NeilMitchell      |       Owner:                
        Type:  bug               |      Status:  new           
    Priority:  normal            |   Component:  Runtime System
     Version:  6.12.1            |    Keywords:                
          Os:  Windows           |    Testcase:                
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown  
---------------------------------+------------------------------------------
 Running hlint 1.6.21 (http://hackage.haskell.org/package/hlint) with GHC
 6.12.1 on a Windows laptop with two cores, I get:

 {{{
 timer hlint src +RTS -N1 -qg = 1.344 seconds
 timer hlint src +RTS -N2 -qg = 1.000 seconds
 timer hlint src +RTS -N3 -qg = 0.984 seconds
 timer hlint src +RTS -N4 -qg = 1.016 seconds
 timer hlint src +RTS -N1 = 1.344 seconds
 timer hlint src +RTS -N2 = 0.969 seconds
 timer hlint src +RTS -N3 = 76.563 seconds
 }}}

 At -N1, -qg has no effect (as expected)

 At -N2, -qg has a small positive effect (I repeated the benchmarks many
 times, so the effect is there)

 At -N3, -qg is essential or it takes forever

 The result seems to be that if you overschedule your garbage collector it
 goes totally crazy. People often use -N with a higher number than their
 processors, since it nicely allows IO and computation to be interleaved.
 GC should probably drop that -N down if it gets lots of contention.

 This is a performance regression from GHC 6.10.4, where HLint worked fine
 with +RTS -N3. I only caught this regression as my test suite started to
 take forever to run.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3989>
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