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(withdraw complaint (whatever))
(...and now the code snippet actually makes sense!)
 
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:not yet at least. try the #haskell irc channel on freenode which is usually manned by at least a few very helpfull people. alternatively try the [http://haskell.org/pipermail/haskell-cafe/ haskell-cafe] mailing list --[[User:JohannesAhlmann|Johannes Ahlmann]] 12:29, 22 January 2007 (UTC)
 
:not yet at least. try the #haskell irc channel on freenode which is usually manned by at least a few very helpfull people. alternatively try the [http://haskell.org/pipermail/haskell-cafe/ haskell-cafe] mailing list --[[User:JohannesAhlmann|Johannes Ahlmann]] 12:29, 22 January 2007 (UTC)
   
:: I tried the IRC channel. 300+ people and nobody speaking. Not very helpful. [[User:MathematicalOrchid|MathematicalOrchid]] 13:23, 22 January 2007 (UTC)
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== A question of speed ==
   
::: Try again. The channel is quite active, so active it's even distracting. If you don't believe me, check the [http://tunes.org/~nef/logs/haskell/ channel logs] for yourself. Besides, maybe now with conferences over, you'll have better luck. -- [[User:EricKow|kowey]] 19:12, 22 January 2007 (UTC)
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I have just performed a benchmark regarding the speed of <hask>(++)</hask> vs <hask>putStr</hask>, and received an extremely puzzling and counter-intuitive result. Perhaps somebody can explain?
   
=== reset breaks ===
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=== Test #1 ===
   
Not wishing to belabour the point, but I did spend over an hour in that channel. Several people came in and asked perfectly reasonably questions, but were greeted only by absolute silence... (Search the logs. My name is in there... somewhere.) [[User:MathematicalOrchid|MathematicalOrchid]] 22:10, 22 January 2007 (UTC)
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<haskell>
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writeFile "Test1.txt" $ concat $ replicate n "test"
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</haskell>
   
:When was this? -- [[User:EricKow|kowey]] 23:56, 22 January 2007 (UTC)
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For n = 10,000,000, that takes about 35 seconds wall-clock time and about 17 seconds CPU time on my test machine. (It also uses about 1.4 MB RAM.)
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=== Test #2 ===
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<haskell>
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writeFile "Test2.txt" $ concatMap (\x -> "test") [1..n]
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</haskell>
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For the same value of n, that takes about 43 seconds wall-clock time and about 20 seconds CPU time. (Uses 2.4 MB RAM for some reason...)
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=== Test #3 ===
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<haskell>
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writeFile "Test3.txt" $ build n ""
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build 0 x = x
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build n x = build (n-1) (x ++ "test")
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</haskell>
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This test does not finish. It simply consumes memory without limit, never writing anything to disk. (At 90 MB, Windoze warned me that 'the system is getting dangerously low on virtual memory'.)
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I added a couple of calls to <hask>seq</hask> in there - but this made no noticeable difference to anything.
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=== Test #4 ===
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And now the really interesting test:
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<haskell>
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do h <- openFile "Test4.txt" WriteMode ; mapM_ (\x -> hPutStr h "help") [1..n]
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</haskell>
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This takes 80 seconds wall-time and 70 seconds CPU time. (Memory usage appears to be 2.4 MB or less.)
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=== Summary ===
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I'm supprised that <hask>concatMap</hask> should be slower than <hask>concat</hask> and <hask>replicate</hask>. But then we're not talking about a huge speed difference.
  +
  +
I am absolutely astonished that <hask>(++)</hask> should be ''50% faster'' than the much more efficient I/O calls. Does anybody have the slightest clue how this can be? Currently the only think I can think of is that each I/O call has some kind of constant overhead, and so the ''number'' of I/O calls affects speed more than the amount of data processed. But even so... 50%??
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All this is with code compiled by GHC 6.6 - as if that makes any difference.

Latest revision as of 13:23, 5 February 2007

Contents

[edit] 1 IRC

Is this a good place to ask questions? MathematicalOrchid 15:30, 18 January 2007 (UTC)

not yet at least. try the #haskell irc channel on freenode which is usually manned by at least a few very helpfull people. alternatively try the haskell-cafe mailing list --Johannes Ahlmann 12:29, 22 January 2007 (UTC)

[edit] 2 A question of speed

I have just performed a benchmark regarding the speed of
(++)
vs
putStr
, and received an extremely puzzling and counter-intuitive result. Perhaps somebody can explain?

[edit] 2.1 Test #1

  writeFile "Test1.txt" $ concat $ replicate n "test"

For n = 10,000,000, that takes about 35 seconds wall-clock time and about 17 seconds CPU time on my test machine. (It also uses about 1.4 MB RAM.)

[edit] 2.2 Test #2

  writeFile "Test2.txt" $ concatMap (\x -> "test") [1..n]

For the same value of n, that takes about 43 seconds wall-clock time and about 20 seconds CPU time. (Uses 2.4 MB RAM for some reason...)

[edit] 2.3 Test #3

  writeFile "Test3.txt" $ build n ""
 
  build 0 x = x
  build n x = build (n-1) (x ++ "test")

This test does not finish. It simply consumes memory without limit, never writing anything to disk. (At 90 MB, Windoze warned me that 'the system is getting dangerously low on virtual memory'.)

I added a couple of calls to
seq
in there - but this made no noticeable difference to anything.

[edit] 2.4 Test #4

And now the really interesting test:

  do h <- openFile "Test4.txt" WriteMode ; mapM_ (\x -> hPutStr h "help") [1..n]

This takes 80 seconds wall-time and 70 seconds CPU time. (Memory usage appears to be 2.4 MB or less.)

[edit] 2.5 Summary

I'm supprised that
concatMap
should be slower than
concat
and
replicate
. But then we're not talking about a huge speed difference. I am absolutely astonished that
(++)
should be 50% faster than the much more efficient I/O calls. Does anybody have the slightest clue how this can be? Currently the only think I can think of is that each I/O call has some kind of constant overhead, and so the number of I/O calls affects speed more than the amount of data processed. But even so... 50%??

All this is with code compiled by GHC 6.6 - as if that makes any difference.