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

[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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