#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>
GHC <http://www.haskell.org/ghc/>
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