[Haskell-cafe] trying to understand out of memory exceptions
cgaebel at uwaterloo.ca
Tue Apr 16 20:32:26 CEST 2013
Have you tried the lazy bytestring version?
On Tuesday, April 16, 2013, Anatoly Yakovenko wrote:
> unfortunately read file tries to get the file size
> readFile :: FilePath -> IO ByteStringreadFile f = bracket (openFile f ReadMode) hClose (\h -> hFileSize h >>= hGet h . fromIntegral)
> which wont work on a special file, like a socket. which is what i am trying to simulate here.
> > wrote:
>> See the comment for hGetContents:
>> "This function reads chunks at a time, doubling the chunksize on each
>> read. The final buffer is then realloced to the appropriate size. For files
>> > half of available memory, this may lead to memory exhaustion. Consider
>> using readFile<http://hackage.haskell.org/packages/archive/bytestring/0.9.2.1/doc/html/Data-ByteString.html#v:readFile> in
>> this case."
>> Maybe try lazy bytestrings?
>> - Clark
>> On Tuesday, April 16, 2013, Anatoly Yakovenko wrote:
>>> -- So why does this code run out of memory?
>>> import Control.DeepSeq
>>> import System.IO
>>> import qualified Data.ByteString.Char8 as BS
>>> scanl' :: NFData a => (a -> b -> a) -> a -> [b] -> [a]
>>> scanl' f q ls = q : (case ls of
>>>  -> 
>>> x:xs -> let q' = f q x
>>> in q' `deepseq` scanl' f q' xs)
>>> main = do
>>> file <- openBinaryFile "/dev/zero" ReadMode
>>> chars <- BS.hGetContents file
>>> let rv = drop 100000000000 $ scanl' (+) 0 $ map fromEnum $ BS.unpack
>>> print (head rv)
>>> -- my scanl' implementation seems to do the right thing, because
>>> main = print $ last $ scanl' (+) (0::Int) [0..]
>>> -- runs without blowing up. so am i creating a some thunk here? or is
>>> hGetContents storing values? any way to get the exception handler to print
>>> a trace of what caused the allocation?
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