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

[GHC]

#8623: Strange slowness when using async library with FFI callbacks
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        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):

 But {{{Control.Concurrent.yield}}} does not help, which I would have
 thought would also cause the thread to yield.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8623#comment:3>
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