[GHC] #10414: Buggy behavior with threaded runtime (-N1 working, -N2 getting into <<loop>>)

[GHC]

#10414: Buggy behavior with threaded runtime (-N1 working, -N2 getting into
<<loop>>)
-------------------------------------+-------------------------------------
        Reporter:  exio4             |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by michaelt):

 I simplified the problem a little. The first command line argument is for
 the chunk size given to `parListChunk`:

 {{{#!hs

 import           Control.Parallel.Strategies
 import           System.Environment

 type Pair = (Int, [Int])

 loop' :: Pair ->  [Pair]
 loop' (size,qns) = go 1
     where go n | n > size  = []
                | otherwise =  (size, n:qns) : (go (n+1))

 worker :: Int -> Int -> Pair -> [Pair]
 worker chunksize =  go
     where go 1  = loop'
           go n  = withStrategy (parListChunk chunksize rseq)
                  . concatMap (go (n-1))
                  . loop'

 main :: IO ()
 main = do
     chunksize:n:_ <- fmap (map read) getArgs
     print $ length (worker chunksize n (n,[]))

 }}}

 With this i get, e.g.:

 {{{
 $ ghc -O2 -threaded -rtsopts -fforce-recomp threads.hs
 $ time ./threads 8 7 +RTS -N1
 823543

 real    0m2.133s
 user    0m2.025s
 sys     0m0.097s
 $ time ./threads 8 7 +RTS -N2
 threads: <<loop>>

 real    0m0.368s
 user    0m0.074s
 sys     0m0.016s

 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10414#comment:4>
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