[Haskell-cafe] nbody (my own attempt) and performance problems
westondan at imageworks.com
Wed Nov 28 14:18:18 EST 2007
Just out of curiosity...
> --some getter functions
> pVel !(_,vel,_) = vel
> pPos !(pos,_,_) = pos
> pMass !(!_,!_,!mass) = mass
What does the !(...) buy you? I thought tuples were already strict by
default in patterns (you'd need ~(...) to make them irrefutable), so
isn't the above equivalent to:
--some getter functions
pVel (_,vel,_) = vel
pPos (pos,_,_) = pos
pMass (!_,!_,!mass) = mass
And why in any case are the tuple components for pMass strict but for
pVel and pPos non-strict? Is is that mass is always used but position
and velocity are not?
Ryan Dickie wrote:
> I sat down tonight and did tons of good learning (which was my goal).
> Yes, the variable names in the unrolling is a little "ugly" but it helps
> to read the C++ version for context. There are two for loops (advN is
> each inner one unrolled). the other function names match the C++
> version. It was my goal to implement an unrolled version of that.
> Fortunately, my performance is excellent now. It is only 4x slower than
> the C++ version and 2x slower than the Haskell one listed (which uses
> pointer trickery). I am sure there could be more done but I am at my
> limit of comprehension. But if I may guess, I would say that any speed
> issues now are related to a lack of in place updating for variables and
> I'd also like to thank everyone for their help so far. I have attached
> my latest version.
> On Nov 27, 2007 7:14 PM, Sterling Clover < s.clover at gmail.com
> <mailto:s.clover at gmail.com>> wrote:
> 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.
> 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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