[GHC] #13246: hPutBuf issues unnecessary empty write syscalls for large writes

[GHC]

#13246: hPutBuf issues unnecessary empty write syscalls for large writes
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        Reporter:  nh2               |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Runtime System    |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13245            |  Differential Rev(s):  Phab:D3117
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"805db96544111bd548c9a32488a9c97996cc2b49/ghc"
 805db965/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="805db96544111bd548c9a32488a9c97996cc2b49"
 Fix: hPutBuf issues unnecessary empty write syscalls for large writes
 (#13246)

 Until now, any `hPutBuf` that wrote `>= the handle buffer size` would
 trigger an unnecessary `write("")` system call before the actual write
 system call.

 This is fixed by making sure that we never flush an empty handle buffer:
 Only flush `when (w > 0)`.

 Reviewers: simonmar, austin, hvr, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D3119
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13246#comment:6>
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