[Haskell-cafe] Slow IO

Udo Stenzel u.stenzel at web.de
Tue Sep 12 16:26:22 EDT 2006

Daniel Fischer wrote:
> The programme consumed more and more memory (according to top),
> kswapd started to have a higher CPU-percentage than my programme,
> programme died, system yelling 'Speicherzugriffsfehler', top displays 
> 'kswapd<defunct>'.
> I believe that means my programme demanded more memory than I have available 
> (only 256MB RAM + 800MB swap). Is that a segfault or what is the correct 
> term?
> That is probably due to (apart from the stupidity of my IO-code) the large 
> overhead of Haskell lists.

Most certainly not.  I'm pretty sure this is to a bug in your code.
Something retains a data structure which is actually unneeded.  Probably
a case of "foldl" where "foldl'" should be used or a "try" in Parsec
code where it should be left out or a lot of "updateWiths" to a Map,
etc.  Or it could be a bad choice of data structure.  I bet, it's the
map you're using to represent the graph (which you don't even need to
represent at all, btw).

> So the chunk of the file which easily fits into my 
> RAM in ByteString form is too large as a list of ordinary Strings.

The chunk of file should never need to fit into RAM.  If that's a
problem, you also forgot to prime a crucial "foldl".

"Proof by analogy is fraud." -- Bjarne Stroustrup
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