[GHC] #9321: Support for waiting on multiple MVars

[GHC]

#9321: Support for waiting on multiple MVars
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              Reporter:  schyler     |             Owner:  simonmar
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.8.3
             Component:  Runtime     |          Keywords:  mvar
  System                             |  Operating System:  Unknown/Multiple
            Resolution:              |   Type of failure:  None/Unknown
Differential Revisions:              |         Test Case:
          Architecture:              |          Blocking:
  Unknown/Multiple                   |
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:              |
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Comment (by schyler):

 And finally, the motivation. STM gives us many-to-one waiting.

 Disadvantages of STM:
 * Does not scale well because it's not fair

 Advantage of MVar:
 * It's fair
 * It's fast

 Disadvantages of MVar:
 * To wait on N channels, have to use N extra forks, 2 MVar puts and N+1
 total MVar read operations

 Advantages of MVar multiple waiting support:
 * To wait on N channels, have to use no extra forks, 1 MVar put and 1 MVar
 read operation (with N-1 waiter ignores).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9321#comment:2>
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