I made a few modifications to your code, and found that replacing `return (x3' + x4')` with `return $! x3' + x4'` reduced maximum residency down to 64kb. This forces evaluation earlier. You can see the progression here:

https://gist.github.com/snoyberg/6a48876aedb9b19c808a0c53e86109ac

I took it one step further, and used the mutable-containers package to use an unboxed reference instead of the boxed STRef type. In other words: it avoids allocating a heap object. Here's that version of the code:

#!/usr/bin/env stack

-- stack --resolver lts-7.14 --install-ghc exec --package mutable-containers -- ghc -O2 -with-rtsopts=-s

import Control.Monad.ST

import Data.Mutable

a :: Int

-> ST s Int

a k x1 x2 x3 x4 x5 =

do kk <- fmap asURef $ newRef k

let b = do k0 <- readRef kk

let k1 = k0 - 1

writeRef kk k1

a k1 b x1 x2 x3 x4

if k <= 0 then do x3' <- x3; x4' <- x4; return $! x3' + x4'

else do x5' <- x5; b' <- b; return $! x5' + b'

main = print (runST (a 22 (return 1) (return (-1)) (return (-1)) (return 1) (return 0)))

It knocked down total allocations from 2.7GB to 1.8GB, which is an improvement, but I think there's still some more low hanging fruit here.

On Thu, Jan 26, 2017 at 12:26 PM, Antoine Rimlet <antoine.rimlet@gmail.com> wrote:

Hi list,

I try to get the following little program (a slightly modified "Man or boy", it prints -14254067) work "as expected", that is, without consuming lots of memory:

import Control.Monad.ST
import Data.STRef
a k x1 x2 x3 x4 x5 =
do kk <- newSTRef k
let b = do k <- modifySTRef kk pred >> readSTRef kk; a k b x1 x2 x3 x4
if k <= 0 then do x3' <- x3; x4' <- x4; return (x3' + x4')
else do x5' <- x5; b' <- b; return (x5' + b')
main = print (runST (a 22 (return 1) (return (-1)) (return (-1)) (return 1) (return 0)))

I use GHC 7.8.4, and executing this program uses about 2.5 GB of memory (or about 3.5 GB with runghc). However, the "equivalent" program in OCaml only needs 4 MB (or 9 MB with the interpreter):

let rec a k x1 x2 x3 x4 x5 =
let kk = ref k in let rec b () = begin decr kk; a !kk b x1 x2 x3 x4 end
in if k <= 0 then let v = x3 () in v + x4 () else let v = x5 () in v + b ();;
print_int (a 22 (fun () -> 1) (fun () -> -1) (fun () -> -1) (fun () -> 1) (fun () -> 0));; print_newline ();;

Therefore I suspect I'm doing something wrong, but I can't see what. I did try to use the strict version modifySTRef' as indicated in the manual, but with no visible improvement.

Thanks,

Antoine

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