[Haskell-cafe] Re: Strange memory consumption problems in something that should be tail-recursive

[Jefferson Heard]

Didn't think it was overly slow, just that I could do better :-).


On Tuesday 13 February 2007 16:30, apfelmus at quantentunnel.de wrote:
> Jefferson Heard wrote:
> > Argh, bitten by the scheme bug! Right -- NO tail recursion...  So that
> > leaves me with some rather non-intuitive strategies for achieving
> > execution time efficiency.  Anyone care to point me in the direction of a
> > document on efficiency in Haskell?
>
> Besides, proper tail recursion in Haskell needs strictness annotations,
> but the best way is to forget the two words "tail recursive" altogether :)
>
> It always helps to do a rough calculation of how much time you have to
> expect it to run. Processing 1TB with a 1GHz processor and 16=2^4
> machine instruction in the inner loop (must be quite short, the loop) takes
>
>  2^40 / (2^30 / 16) = 2^14 seconds ~ 4.5 hours
>
> Of course, these 4.5 hours are quite sensitive to the 2^4 factor and
> might well be 3 or 9 hours. Assuming that you ran alex on a String, the
> reported 36 hours are entirely reasonable, in the sense of alex not
> being overly slow.
>
> Regards,
> apfelmus
>
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