[GHC] #8733: I/O manager causes unnecessary syscalls in send/recv loops

[GHC]

#8733: I/O manager causes unnecessary syscalls in send/recv loops
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       Reporter:  tibbe             |             Owner:  simonmar
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Runtime System    |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Network applications often call send followed by recv, to send a message
 and then read an answer. This causes syscall traces like this one:

 {{{
 recvfrom(13, )                -- Haskell thread A
 sendto(13, )                  -- Haskell thread A
 recvfrom(13, ) = -1 EAGAIN    -- Haskell thread A
 epoll_ctl(3, )                -- Haskell thread A (a job for the IO
 manager)
 recvfrom(14, )                -- Haskell thread B
 sendto(14, )                  -- Haskell thread B
 recvfrom(14, ) = -1 EAGAIN    -- Haskell thread B
 epoll_ctl(3, )                -- Haskell thread B (a job for the IO
 manager)
 }}}

 The recvfrom call always fails, as the response from the partner we're
 communicating with won't be available right after we send the request.

 We ought to consider descheduling the thread as soon as sending is "done".
 The hard part is to figure out when that is.

 See http://www.yesodweb.com/blog/2014/02/new-warp for a real world
 example.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8733>
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