[Haskell-cafe] Why is my mappend so slow?

[Atze van der Ploeg]

Maybe it is due to using lists and ++? Thats a well know inefficiency.
On Nov 19, 2015 9:06 PM, "martin" <martin.drautzburg at web.de> wrote:

> I just tried both strict pairs and seq, and it didn't change anything.
> Also, wouldn't then THUNKS consume a lot of
> memory in my heap profile? I forgot to mention that this is not the case.
> Max heap is around 35k and the top-consumer
> is ARR_WORDS. THUNK is below 1k.
>
> I am going though 10,000,000 iterations and if anything would pile up, it
> would consume at least one byte per iteration,
> wouldn't it? But I can't see 10 MBytes anywhere. It looks as if the time
> is really spent on *computing* something.
>
> Am 11/19/2015 um 07:15 PM schrieb Roman Cheplyaka:
> > My guess is that you have accumulating thunks inside your (Int,a) tuple.
> > Be sure to force them (by using a strict pair type, bang patterns, or
> > however else).
> >
> > On 11/19/2015 07:18 PM, martin wrote:
> >> Hello all,
> >>
> >> I wrote a Logger which, under certain conditions, prepends log-entries
> to a log and a Monoid instance of it. But as soon
> >> as I mappend two Loggers my performance drops by 50%. This even happens
> when I mappend  mempty as shown below in --<2--.
> >> I understand that the system has to do *something*, but it seems to
> cost a bit much. Without the strictness annotation
> >> in --<1-- the performance degradation is even more dramatic (orders of
> magnitude).
> >>
> >> The profile tells me that more that 50% of the time is spent in mappend.
> >>
> >> COST CENTRE  MODULE    %time %alloc
> >>
> >> mappend.\    Logger     50.6   35.8
> >> logCount'.f  Logger     18.7   40.3
> >> logCount'    Logger      5.4    0.0
> >>
> >> Why is that so, and can I do anything about it?  I am willing to change
> the overall design if required.
> >>
> >>
> >> This is the code
> >>
> >> -- | A writer does the formatting
> >> newtype Wtr a log = Wtr {runWtr :: a -> log}
> >>
> >> -- | A looger is a writer plus an internal state
> >> data Logger a log = Lgr {runLogger :: a -> log -> (log, Logger a log)}
> >>
> >> instance Monoid (Logger a log) where
> >>         mempty = Lgr (\_ l -> (l,mempty))
> >>         mappend lgr1 lgr2 = Lgr $ \a l -> let !(log1',!lgr1') =
> runLogger lgr1 a l      --<1--
> >>                                               !(log2',!lgr2') =
> runLogger lgr2 a log1'  --<1--
> >>                                           in (log2', mappend lgr1'
> lgr2')
> >>
> >>
> >> and this is how I test it
> >>
> >> -- | Count calls __s__ and write log when s has reached nxt and then
> every dn calls
> >> logCount' :: Monoid log => Int -> Int -> Int ->  Wtr (Int,a) log ->
> Logger a log
> >> logCount' dn nxt s wtr = Lgr f
> >>         where
> >>             f a l = if s == nxt
> >>                        then (runWtr wtr (s,a)  <> l, logCount'  dn
> (nxt+dn) (s+1) wtr)
> >>                        else (l,                      logCount'  dn nxt
>     (s+1) wtr)
> >>
> >>
> >> -- | Count calls and write log every dn calls
> >> logCount dn = logCount' dn dn 0
> >>
> >>
> >> -- testLogger :: Logger Int Int [String] -> [String]
> >> testLogger lgr xs = fst $ foldl' f ([],lgr) xs
> >>         where
> >>             f (log', lgr') x = runLogger lgr' x log'
> >>
> >> ex_wtr :: Wtr (Int,a) [String]
> >> ex_wtr = Wtr $ \(x,_) -> ["Counted to " ++ (show x)]
> >>
> >> ex_wtr2 :: Wtr Int [String]
> >> ex_wtr2 = Wtr $ \x -> ["Counted to " ++ (show x)]
> >>
> >> ex_inputs :: [Int]
> >> ex_inputs = [1..10000000]
> >>
> >> ex_logger = mempty <> logCount 300000 ex_wtr <> mempty
>  --<2--
> >> -- ex_logger = logCount 300000 ex_wtr
> >>
> >>
> >> ex_main = do
> >>     timeIt $ putStrLn $ ppShow $ testLogger ex_logger ex_inputs
> >>
> >> main = ex_main
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> >
> >
> >
> >
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