[Haskell-cafe] [GHC 7.8.4] IO Monad leak space?

[Clinton Mead]

I recall having this issue also, and I eventually ending up concluding that
GHC didn't optimise away the list in "mapM_". Try with GHC 7.10 though, it
seems to be better with optimising lists into simple loops.

On Fri, Jul 3, 2015 at 6:18 PM, Baojun Wang <wangbj at gmail.com> wrote:

> Hi,
>
> First of all, I found it interesting that
>
> loopM_ f k n s = when (k <= n) (f k >> loopM_ f (s+k) n s)
>
> loopM_ seems faster than mapM_    ( mapM_ f [k, k+s..n]))
>
> I think mapM_ is used very commonly, why it's performance is even lower
> than a hand-written loop function?
>
> 2nd, even I replace mapM_ with loopM_ from above, when chain IO action, it
> still can leak space. ( Because IO Monad (>>) need keep ``RealWorld s''
> updated so that I/O actions can be done in-order? )
>
> Consider below function:
>
> f3 :: UArray Int Int -> IOUArray Int Int64 -> Int -> IO ()
> f3 u r i = let !v = u ! i
>             in go (f31 v) i i
>   where f31 v j = readArray r j >>= \v1 ->
>           writeArray r j (v1 + (fromIntegral i) * (fromIntegral v))
>         f31 :: Int -> Int -> IO ()
>         go g k s = when (k <= maxn) (
>           g k >> go g (s+k) s )
>
> When call f3:
>
>   loopM_ (f3 uu res) 1 1 1000000
>
> Which will have blow profiling output:
>
> individual inherited
> COST CENTRE MODULE no. entries %time %alloc %time %alloc
>
>
> ...
> loopM_ Main 104 4000002 7.4 10.1 100.0 99.3
> f3 Main 113 1000000 1.0 2.0 70.2 69.1
> f3.go Main 116 14970034 32.7 67.1 68.8 67.1
> f3.f31 Main 117 13970034 34.5 0.0 36.1 0.0
> f3.f31.\ Main 118 13970034 1.7 0.0 1.7 0.0
> f3.f31 Main 114 0 0.3 0.0 0.4 0.0
> f3.f31.\ Main 115 0 0.1 0.0 0.1 0.0
> ...
> Why f3.go consumes so much space (67.1%)? The only reason I can think of
> is IO Monad chain (>>) isn't space free as I thought.
>
> Did I get something fundamentally wrong?
>
> Thanks
> baojun
>
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