[GHC] #8224: Excessive system time -- new IO manager problem?

[GHC]

#8224: Excessive system time -- new IO manager problem?
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        Reporter:  rrnewton          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  high              |        Milestone:  7.8.1
       Component:  Runtime System    |          Version:  7.7
      Resolution:                    |         Keywords:  IO Manager,
Operating System:  Linux             |  System Time
 Type of failure:  Runtime           |     Architecture:  x86_64 (amd64)
  performance bug                    |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by AndreasVoellmy):

 I ran with mutrace and did see any obvious explanations.

 Is it possible that by having more safe foreign calls RTS threads spend
 more time in some spin locks? If so, which spin locks would those be?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8224#comment:16>
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