Am Donnerstag 25 Februar 2010 03:04:02 schrieb Chris Pettitt:
Hello Haskell-Beginners,
I'm having a fair amount of difficulty understanding the ST monad. In my basic experimentation I'm finding that I'm getting more heap allocated with STUArray than with a regular Array, which is not what I would expect.
And ordinarily you don't. The problem is that the STUArray suffers very badly from profiling. Compiling without -prof, the STUArray code allocates about half as much as the UArray code and takes ~42% of the time. With profiling, STUArray allocates ~40% more and takes ~50% longer than UArray. Two things: - profiling interacts badly with some optimisations, so what the profiling output says may deviate significantly from what your -O2 compiled production code actually does - some datatypes suffer more from profiling than others, so what the profiling output says for different choices of datatype may deviate significantly from how your -O2 compiled production code behaves Normally, -prof doesn't change the behaviour very much, but sometimes it does.
One additional point of confusion for me: when I run either function with +RTS -hc and use hp2ps I get an empty graph. I've seen these tools work quite well before, so I suspect I'm doing something wrong now.
Too little actual heap usage and too short running time. Change the upper limit to 10 million and you should get a graph with actual bands.
Thanks, Chris