#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 | Keywords:
Difficulty: Unknown | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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On the Clean mailing list, Philippos Apolinarius [[email protected]]
describes a small array program (using the ST monad) that runs 400x more
slowly in Haskell than Clean. I can understand a bit slower (e.g. maybe
they do a better job of array-bound checks), but 400x seems large. This
ticket is an invitation for someone to investigate.
He writes: I wrote a very simple program to check whether Haskell improved
its array processing libraries or not. Here is how to compile and run the
program arr.hs in Haskell (I have used the GHC compiler):
{{{
>ghc -O arr.hs -o arr.exe
$ time arr.exe +RTS -K32000000
2.8e8
real 0m3.938s
user 0m0.031s
sys 0m0.000s
}}}
The same program in Clean:
{{{
C:\Clean 2.2\exemplos\console>arraytest.exe
280000000
Execution: 0.01 Garbage collection: 0.01 Total: 0.03
C:\Clean 2.2\exemplos\console>arraytest.exe
280000000
Execution: 0.01 Garbage collection: 0.01 Total: 0.03
}}}
This means that Clean is 390 times faster than Haskell in this particular
problem. These results makes me worder whether Haskell is safer than
Clean. It turns out that Haskell checks index out of range at runtime,
exactly like Clean. Larger problems make the difference between Clean and
Haskell even worse. For instance, neural networks like the one described
in Schmidtt et al. run 400 times faster in Clean.
Below you will find the array examples. You can check that Clean is really
much faster than Haskell. I wonder why the Benchmarks Game site does not
report such a large difference between Haskell and Clean performances. I
believe that people who wrote Haskell benchmarks for the Benchmarks Game
site cheated in using foreign pointers to access arrays.
{{{
-- arr.hs
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)
}}}
Clean version
{{{
module arraytest
import StdEnv
fn i a acc | i<1 = acc
fn i a=:{[i]=b} acc
# a= {a&[i]= b+3.0}
# (c, a)= a![i]
= fn (i-1) a (c+acc)
Start= fn 2000000 vt 0.0
where
vt:: .{#Real}
vt = createArray 2000001 137.0
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3586>
GHC <http://www.haskell.org/ghc/>
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