#7436: Derived Foldable and Traversable instances become extremely inefficient
due
to eta-expansion
---------------------------------+------------------------------------------
Reporter: shachaf | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Runtime performance bug
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------+------------------------------------------
Comment(by shachaf):
The two instances I gave work as as they are without deriving (no Core
necessary -- the code is a simplified version of what `-ddump-deriv`
generated). Here's a simpler version without type classes (or lists):
{{{
data Nat = Z | S Nat
mkNat :: Int -> Nat
mkNat 0 = Z
mkNat n = S (mkNat (n-1))
unNat :: Nat -> ()
unNat Z = ()
unNat (S n) = unNat n
fast :: (b -> b) -> b -> Nat -> b
fast s z Z = z
fast s z (S n) = s (fast s z n)
slow :: (b -> b) -> b -> Nat -> b
slow s z Z = z
slow s z (S n) = s (slow (\e -> s e) z n)
main :: IO ()
--main = print $ unNat . fast S Z . mkNat $ n
main = print $ unNat . slow S Z . mkNat $ n
where n = 100000
}}}
And the `+RTS -s` output for both:
* Fast:
{{{
shachaf@carbon:~/9$ ghc -rtsopts -O2 C.hs && time ./C +RTS -s
[1 of 1] Compiling Main ( C.hs, C.o )
Linking C ...
()
9,651,768 bytes allocated in the heap
10,904 bytes copied during GC
44,416 bytes maximum residency (2 sample(s))
21,120 bytes maximum slop
1 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max
pause
Gen 0 17 colls, 0 par 0.00s 0.00s 0.0000s
0.0000s
Gen 1 2 colls, 0 par 0.00s 0.00s 0.0001s
0.0002s
INIT time 0.00s ( 0.00s elapsed)
MUT time 0.00s ( 0.00s elapsed)
GC time 0.00s ( 0.00s elapsed)
EXIT time 0.00s ( 0.00s elapsed)
Total time 0.00s ( 0.00s elapsed)
%GC time 0.0% (9.8% elapsed)
Alloc rate 2,412,942,000 bytes per MUT second
Productivity 100.0% of total user, 116.6% of total elapsed
real 0m0.005s
user 0m0.004s
sys 0m0.000s
}}}
* Slow:
{{{
shachaf@carbon:~/9$ ghc -rtsopts -O2 C.hs && time ./C +RTS -s
[1 of 1] Compiling Main ( C.hs, C.o )
Linking C ...
()
11,251,768 bytes allocated in the heap
4,122,872 bytes copied during GC
1,223,248 bytes maximum residency (3 sample(s))
528,816 bytes maximum slop
4 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max
pause
Gen 0 19 colls, 0 par 0.00s 0.00s 0.0002s
0.0002s
Gen 1 3 colls, 0 par 0.00s 0.00s 0.0006s
0.0016s
INIT time 0.00s ( 0.00s elapsed)
MUT time 8.23s ( 8.25s elapsed)
GC time 0.00s ( 0.01s elapsed)
EXIT time 0.00s ( 0.00s elapsed)
Total time 8.24s ( 8.25s elapsed)
%GC time 0.0% (0.1% elapsed)
Alloc rate 1,366,747 bytes per MUT second
Productivity 100.0% of total user, 99.8% of total elapsed
real 0m8.253s
user 0m8.237s
sys 0m0.000s
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7436#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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