[GHC] #7429: Unexplained performance boost with +RTS -h

[GHC]

#7429: Unexplained performance boost with +RTS -h
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    Reporter:  simonmar          |       Owner:                         
        Type:  bug               |      Status:  new                    
    Priority:  normal            |   Milestone:  7.8.1                  
   Component:  Compiler          |     Version:  7.6.1                  
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
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Changes (by tomberek):

 * cc: tomberek@… (added)


Comment:

 I'm not sure how to insert an IO () into the ST/STO monad, either that
 sounds "unsafe" or I lift everything to IO? I tried using:

 {{{
 lift $ unsafeIOToST performGC
 }}}

 in Main2.hs

 This fixes the problem when the circular list is of size 2 (using function
 twoItemList).  The problem creeps back in when using threeItemList or when
 the list is produced by nlist. Perhaps my generation of the circular list
 is suspect. Perhaps too lazy? The inner loop in Core is unchanged from
 previous versions.

 Regarding "small data structure": This effect is actually more pronounced
 the larger the circular list is.  I've tested this up to n = 2 million and
 it seems that the GC overhead of profiling catches up around 1 million,
 but the non-GC time difference becomes very pronounced.

 So the "GC fix" works for an explicitly constructed circular list of
 length 2, but nothing else. And still, working in only ST is always slower
 than in a superfluous StateT over ST.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7429#comment:7>
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