[GHC] #8972: Investigate adding fast compare-and-swap Int type/primops

[GHC]

#8972: Investigate adding fast compare-and-swap Int type/primops
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        Reporter:  tibbe             |            Owner:  tibbe
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  8157, 7883
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Comment (by jberryman):

 FWIW...

 Replying to [comment:1 tibbe]:
 >
 > Atrocious results for the `IORef` solution! It seems like contended
 `IORef`s don't work well at all.

 This is my experience. The behavior I've seen is interesting; check out
 the distribution of samples of a test of 100K concurrent
 `atomicModifyIORef`s, for 2 cores:

     [http://htmlpreview.github.io/?https://github.com/jberryman/chan-
 benchmarks/blob/master/reports/ghc-7.6/bench-multi.vars.html#b2]
 ...and 8 cores:
     [http://htmlpreview.github.io/?https://github.com/jberryman/chan-
 benchmarks/blob/master/reports/ghc-7.6_8-core/bench-multi.vars.html#b2]

 The `atomicModifyIORefCAS` from atomic-primops doesn't display that nasty
 spread; with the primitive CAS one thread always wins, but it seems like
 whatever GHC is doing is causing vicious retry loops, which sometimes stay
 in phase for awhile.

 And the fetch-and-add-based counter from `atomic-primops` handles
 contention far and away better than anything else available. From what I
 remember I couldn't even measure contention effects going from 2 to 8
 contending threads (although I did do both tests on two different
 machines, so...).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8972#comment:5>
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