[Haskell-cafe] nbody (my own attempt) and performance problems

[Sterling Clover]

The first step would be profiling -- i.e. compiling with -prof -auto- 
all to tag each function as a cost center, then running with +RTS -p  
to generate a cost profile. The problem here is you've got massive  
amounts of unrolling done already, so it's sort of hard to figure out  
what's doing  what, and the names you've given the unrolled functions  
are... less than helpful. (first rule of optimization: optimize  
later.)  The use of tuples shouldn't be a problem per se in terms of  
performance, but it really hurts readability to lack clear type  
signatures and types. You'd probably be better off constructing a  
vector data type as does the current Haskell entry -- and by forcing  
it to be strict and unboxed (you have nearly no strictness  
annotations I note -- and recall that $! only evaluates its argument  
to weak head normal form, which means that you're just checking if  
the top-level constructor is _|_) you'll probably get better  
performance to boot. In any case, declaring type aliases for the  
various units you're using would also help readability quite a bit.

--S

On Nov 27, 2007, at 5:41 PM, Ryan Dickie wrote:

> I thought it would be a nice exercise (and a good learning  
> experience) to try and solve the nbody problem from the debian  
> language shootout. Unfortunately, my code sucks. There is a massive  
> space leak and performance is even worse. On the bright side, my  
> implementation is purely functional. My twist: I manually unrolled  
> a few loops from the C++ version.
>
> I believe that most of my performance problems stem from my abuse  
> of tuple. The bodies are passed as a tuple of planets, a planet is  
> a tuple of (position, velocity, mass) and the vectors position and  
> velocity are also tuples of type double. My lame justification for  
> that is to make it nice and handy to pass data around.
>
> Any tips would be greatly appreciated.
>
> --ryan
> <nbody3.hs>
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