[GHC] #14414: Profiled program runs 2.5x faster than non-profiled

[GHC]

#14414: Profiled program runs 2.5x faster than non-profiled
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        Reporter:  Fuuzetsu          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by duog):

 Note that the entirety of the anomolous time is in the garbage collector.

 It looks to me like the program is running on only one core. If that's the
 case, then perhaps the garbage collector is spending entire time slices
 busy-waiting on a spin lock. As for why it doesn't happen while profiled,
 perhaps the non-profiled code has allocation-free loops, they can have
 poor interactions with garbage collection.

 Some things to try:
 * run the program inside `time` to verify that the numbers -s is reporting
 are correct
 * use `time` to prove that a non-haskell program is able to run on
 multiple cores. Sorry I don't have a one-liner for you...
 * try the profiled version, omitting -fprof-auto, this (should) give the
 same core as without -prof, and may exhibit the bad gc behaviour.
 * play with the -q* and -N RTS options to see if anything changes.
 * use https://wiki.haskell.org/ThreadScope to see exactly what the garbage
 collector is up to.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14414#comment:7>
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