#3586: Slow array code
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Reporter: simonpj | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.4
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Comment (by dons):
Isn't the problem here that you're using unboxed arrays in Clean, but
boxed arrays in Haskell?
Secondly, there's a space leak in the fn function, and a bunch of things
that aren't in the Clean code. Fixing things:
Correcting the space leak:
{{{
{-# LANGUAGE BangPatterns #-}
import Control.Monad.ST
import Data.Array.ST
main = print $ runST
(do arr <- newArray (1,2000000)
137.0 :: ST s
(STArray s
Int Double)
a <- readArray arr 1
b <- readArray arr 1
fn 2000000 arr 0.0 )
fn i !a !acc | i < 1 = do (return acc)
fn i a acc= do
b <- readArray a i
writeArray a i (b+3.0)
c <- readArray a i
fn (i-1) a (acc+c)
}}}
Results in:
{{{
$ time ./B +RTS -sstderr
./B +RTS -sstderr
2.8e8
./B +RTS -sstderr 2.95s user 0.07s system 99% cpu 3.034 total
}}}
3.034s
Where 98.6% of the time is spent in GC (this is why you don't use boxed
mutable arrays when you can use unboxed ones.
Increasing the GC thresholds, improves performance 20 fold:
{{{
$ time ./B +RTS -sstderr -A200M
./B +RTS -sstderr -A200M
2.8e8
./B +RTS -sstderr -A200M 0.06s user 0.12s system 100% cpu 0.176 total
}}}
0.176s using boxed arrays. Switching to an STUArray, and adding some type
annotations:
{{{
{-# LANGUAGE BangPatterns #-}
{-# OPTIONS -fexcess-precision #-}
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Base
main = print $ runST
(do arr <- newArray (1,2000000) 137.0 :: ST s (STUArray s Int
Double)
fn 2000000 arr 0.0 )
fn :: Int -> STUArray s Int Double -> Double -> ST s Double
fn i !a !acc | i < 1 = return acc
fn i a acc = do
b <- unsafeRead a i
unsafeWrite a i (b+3.0)
c <- unsafeRead a i
fn (i-1) a (c + acc)
./B +RTS -sstderr 0.26s user 0.01s system 97% cpu 0.283 total
}}}
So still not great. Perhaps someone can look into it further?
The original bug report is a duplicate of the boxed, mutable arrays
ticket.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3586#comment:1>
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