[GHC] #8623: Strange slowness when using async library with FFI callbacks

[GHC]

#8623: Strange slowness when using async library with FFI callbacks
--------------------------------------------+------------------------------
        Reporter:  JohnWiegley              |            Owner:  simonmar
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  Runtime System           |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  MacOS X                  |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by JohnWiegley):

 I tried using just `forkIO` and `MVar`, but it does not exhibit the
 problem:

 {{{
 main = do
     let test = mk'speed_test_cb (return ()) >>= speed_test
     test
     m <- newEmptyMVar
     _ <- forkIO $ test >> putMVar m ()
     takeMVar m
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8623#comment:4>
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