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POIO over big files in haskell: Performance issue
 

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  1. POIO over big files in haskell: Performance issue
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    <p>I'm trying to work over big files using Haskell. I'd like to browse an input file byte after byte, and to generate an output byte after byte. Of course I need the IO to be buffered with blocks of reasonable size (a few KB). I can't do it, and I need your help please.</p> <pre><code>import System import qualified Data.ByteString.Lazy as BL import Data.Word import Data.List main :: IO () main = do args &lt;- System.getArgs let filename = head args byteString &lt;- BL.readFile filename let wordsList = BL.unpack byteString let foldFun acc word = doSomeStuff word : acc let wordsListCopy = foldl' foldFun [] wordsList let byteStringCopy = BL.pack (reverse wordsListCopy) BL.writeFile (filename ++ ".cpy") byteStringCopy where doSomeStuff = id </code></pre> <p>I name this file <code>TestCopy.hs</code>, then do the following:</p> <pre class="lang-none prettyprint-override"><code>$ ls -l *MB -rwxrwxrwx 1 root root 10000000 2011-03-24 13:11 10MB -rwxrwxrwx 1 root root 5000000 2011-03-24 13:31 5MB $ ghc --make -O TestCopy.hs [1 of 1] Compiling Main ( TestCopy.hs, TestCopy.o ) Linking TestCopy ... $ time ./TestCopy 5MB real 0m5.631s user 0m1.972s sys 0m2.488s $ diff 5MB 5MB.cpy $ time ./TestCopy 10MB real 3m6.671s user 0m3.404s sys 1m21.649s $ diff 10MB 10MB.cpy $ time ./TestCopy 10MB +RTS -K500M -RTS real 2m50.261s user 0m3.808s sys 1m13.849s $ diff 10MB 10MB.cpy $ </code></pre> <p>My problem: There is a huge difference between a 5MB and a 10 MB file. I'd like the performances to be linear in the size of the input file. Please what am i doing wrong, and how can I achieve this? I don't mind using lazy bytestrings or anything else as long as it works, but it has to be a standard ghc library.</p> <p>Precision: It's for a university project. And I'm not trying to copy files. The <code>doSomeStuff</code> function shall perform compression/decompression actions that I have to customize.</p>
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    1. COByteString's `pack` and `unpack` are very costly operations. Can't you `doSomeStuff` directly with ByteString? NB: lazy ByteString is 'buffered' internally which might be adequate for your task
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    2. COI just tried it without pack and unpack, working directly on bytestrings, and the result is even longer, and still i have this big difference between 5MB and 10MB.
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    3. COMaybe you could post somewhere a complete working example of your code demonstrating the problem?
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