[Haskell-cafe] Re: Haskell Speed

[Branimir Maksimovic]

>From: Isaac Gouy <igouy at yahoo.com>
>To: haskell-cafe at haskell.org
>Subject: [Haskell-cafe] Re: Haskell Speed
>Date: Thu, 29 Dec 2005 13:00:15 -0800 (PST)
>
>--- Isaac Gouy <igouy at yahoo.com> wrote:
> > We'll be happy to also show a Haskell program that
> > uses Data.HashTable - first, someone needs to
> > contribute that program.
>
>Someone did:  k-nucleotide Haskell GHC #2
>http://shootout.alioth.debian.org/gp4/benchmark.php?test=knucleotide&lang=all
>
>
To comment some observation on this program.
Most of the pressure now is on Data.HashTable.
I've susspected such large memory usage on substring from array conversions,
so mad version with data MyString = MakeMyStrinf { buf :: Ptr Char, size :: 
Int }
and there was no substrings in map or anywhere else, but memory
consumption remains.
So after eliminating inserts and updates into HashTable memory was ok.
Finally I've realized that updates into hash table actually increase
memory usage to large extent instead to keep memory same
on average. So I guess this is bug in HashTable?
Second in this test, hash function needs to be very strong,
as even with following I got longest chain of 16 elements.
myHashString = fromIntegral . ff''' . ff'' . ff' . foldr f 0
      where f c m = f'' $ f' (ord c + m)
            f' m = m + (m `shiftL` 10)
            f'' m = m `xor` (m `shiftR` 6)
            ff' m = m + (m `shiftL` 3)
            ff'' m = m `xor` (m `shiftR` 11)
            ff''' m = m + (m `shiftL` 15)
Default hashString has longestChain of 18 elements.
Perhaps someone can explain if such a memory leaks from HashTable updates
are normal or are bugs?
All in all functional version consumes less memory and is twice as
fast.

Greetings, Bane.

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