[Haskell-cafe] Question about STRef

[Robin Palotai]

Alternatively, just use "return $! ", which has the same effect as seq-ing,
and is generally a good practice (unless you want to be explicitly lazy).
See http://johantibell.com/files/haskell-performance-patterns.html.

the-thing +RTS -s     (with GHC 8)

  3,965,758,088 bytes allocated in the heap
      46,006,592 bytes copied during GC
         207,792 bytes maximum residency (5 sample(s))             <----
down to 200K from 1.5G
         182,864 bytes maximum slop
               8 MB total memory in use (0 MB lost due to fragmentation)

Robin

2017-01-26 12:42 GMT+01:00 Johannes Waldmann <
johannes.waldmann at htwk-leipzig.de>:

> Thanks for pointing out this (Knuth's) nice test case.
> I should use this as an exam question ...
>
> Allocation and run-time can be reduced
> by replacing  return (x3' + x4')
> with  let x = (x3' + x4') in x `seq` return x
> And, modifySTRef' *does* help.
>
> I *do* notice a regression
> (for the seq-ed and primed version)
>
> ghc-8.0.2 : 2.6 GB alloc, 2.3 sec
> ghc-6.10.4: 2.0 GB alloc, 1.7 sec
>
> (measured on X5365  @ 3.00GHz )
>
> Numbers are for total allocation,
> residency is small  (it runs with  +RTS -H10k -A10k
> but then the number of GCs goes up)
>
> - J.
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