While profiling is important, I think there is a lot of low hanging fruit
to be had. Any time you have a loop like
let doStep step =
This is 90% of space leaks in any program. Look at the step variable, is
it ever fully evaluated? It gets oApply run on it, then a little later it
gets oEval on it. If neither of those are strict, then that will leak. It
is extremely easy to just go
let doStep !step =
run it again, and look at its max memory usage.
Another example is
monteCarloEval nExper struct =
I think whether nExpr and struct will be fully evaluated is dependent on
what r ends up being. If r is IO it will be fully evaluated, I think. In
any case, it is really easy to just go
monteCarloEval !nExper !struct =
and find out. Then if you are still having trouble, profiling is always an
option. There is a point where excessive strictness annotations can
degrade performance, but honestly, I've never gotten to that point.
On Sun, Sep 11, 2016 at 6:49 PM, Dennis Raddle
I'm a Haskell semi-beginner, and I've implemented a crude Monte Carlo search optimization algorithm, and I want to know how to use strict evaluation to prevent it from consuming too much memory.
I am willing to learn new things to do this---if you want to point me in the right direction and maybe give some references, I'll take it from there. However, if there is something simple that can take care of the whole problem, please let me know.
I have not run this with the profiler so I don't actually know precisely what it is doing now. I have not run out of memory using it in small cases, but I hope to use it on much larger cases.
Here is the code so far:
import qualified Data.List as L import Control.Monad import Data.Function
-- Suppose that we do backtracking search for an optimal arrangement of -- elements in some kind of "structure". The "structure" is built one step at -- a time. -- -- An example would be searching for an optimal arrangement of furniture. The -- structure is a represenation of the room and all items that have been -- placed in it so far. We could list the options (or "steps") available to us -- at any point in building the structure, that is a list of furniture items -- and locations. -- -- We have the notion that the structure, at some point in adding steps, -- becomes complete. We have an evaluation function providing a "goodness -- score" on either a partially built or complete structure. -- -- The search will optimize the goodness score over all possible complete -- structures. (Or perhaps with Monte Carlo search, an approximate optimal -- value.) -- -- Class Opt (for "optimization") defines a structure type "struct" and a step -- type "step".
class Opt struct step | struct -> step where -- number of steps chosen so far in current state of 'struct': oSize :: struct -> Int -- list all the available steps to choose next. If this list is null, then -- the structure is complete. oList :: struct -> [step] -- apply a step to the structure to create a new structure: oApply :: struct -> step -> struct -- evaluate the "goodness" of the current state of a structure. Higher is -- better. oEval :: struct -> Double
-- Implement a kind of Monte Carlo search. (I have a vague idea of the -- literature on Monte Carlo; this algorithm is my guess at something that -- does the job). We work in a state monad of class -- "RandMonad" which holds the StdGen data and provides several methods for -- accessing it. The only method we need from RandMonad is -- -- rChooseItem :: RandMonad m => [a] -> m a -- -- which chooses a random item from a non-null list. -- -- The basic algorithm is this: at each point in building the structure, we -- have a structure S and a list of next steps step_list. We apply each step -- in step_list to S, in turn. After applying a step x to S, call the result -- S_x. We evaluate the "monte carlo fitness" of S_x by making random -- completions of it---that is, choosing a bunch of additional steps -- randomly---doing 'nExper' completions (nExper might be 1000 to 10000). The -- fitness as measured by oEval of the very best random completion becomes -- the "monte carlo fitness" of S_x. We then choose the step from step_list, -- x, that maximizes the "monte carlo fitness" of S_x. -- -- We then repeat this process until S is complete.
-- Function monteCarlo will take a partially complete structure, and optimize -- it over an investigation of 'nExper' possible "completions".
monteCarlo :: (RandMonad r, Opt a b) => Int -> a -> r a monteCarlo nExper struct = case oList struct of -- If structure is complete, then it is its own optimization. [] -> return struct -- Otherwise find optimal step according to "monte carlo fitness" and -- recursively call 'monteCarlo' steps -> do let doStep step = do let newStruct = oApply struct step score <- monteCarloEval nExper newStruct return (score,newStruct) (_,winner) <- L.maximumBy (compare `on` fst) `liftM` mapM doStep steps monteCarlo nExper winner
-- monteCarloEval -- -- Evaluate the "monte carlo fitness" of a structure 'struct' by completing it -- in nExper random ways (that is, make all remaining choices in purely random -- way) and finding the maximum value of the evaluated final state among all -- nExper ways. -- monteCarloEval :: (RandMonad r, Opt a b) => Int -> a -> r Double monteCarloEval nExper struct = case oList struct of [] -> return $ oEval struct _ -> do scores <- replicateM nExper (randomComplete struct) return . maximum . map oEval $ scores
-- Make random choices of steps until a structure 's' is complete. randomComplete :: (RandMonad r, Opt a b) => a -> r a randomComplete s = case oList s of [] -> return s steps -> rChooseItem steps >>= (randomComplete . oApply s)
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners