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

[GHC]

#8972: Investigate adding fast compare-and-swap Int type/primops
-------------------------------------+------------------------------------
        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:
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Comment (by tibbe):

 I've added a little benchmark that shows that even calling out to C can
 speed things up a lot.

 Using `IORef`:
 {{{
 $ ghc -O2 Bench.hs counter.c -DOLD=1
 ...
 $ time ./Bench

 real    0m1.698s
 user    0m1.670s
 sys     0m0.020s
 }}}

 Calling out to C:
 {{{
 $ ghc -O2 Bench.hs counter.c
 ...
 $ time ./Bench

 real    0m0.108s
 user    0m0.090s
 sys     0m0.010s
 }}}

 That's a ~16x speed-up.

 Things look similar with the threaded RTS:

 Using `IORef`:
 {{{
 $ ghc -O2 Bench.hs counter.c -DOLD=1 -threaded
 ...
 $ time ./Bench

 real    0m1.998s
 user    0m1.980s
 sys     0m0.010s
 }}}

 Calling out to C:
 {{{
 $ ghc -O2 Bench.hs counter.c -threaded
 $ time ./Bench

 real    0m0.117s
 user    0m0.100s
 sys     0m0.010s

 }}}

 With `+RTS -N6` (real hardware cores on the test machine):

 Using `IORef`:
 {{{
 $ time ./Bench +RTS -N6

 real    1m22.565s
 user    1m23.850s
 sys     1m11.240s
 }}}

 Calling out to C:
 {{{
 $ time ./Bench +RTS -N6

 real    0m0.247s
 user    0m1.340s
 sys     0m0.010s
 }}}

 Atrocious results for the `IORef` solution! It seems like contended
 `IORef`s don't work well at all.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8972#comment:1>
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