Yeah, that's right. `bytestring` tries to minimize copies as much as
possible so when you slice into a `ByteString` it just updates an offset
and length field. It's internal representation is:
data ByteString = PS {-# UNPACK #-} !(ForeignPtr Word8) -- payload
{-# UNPACK #-} !Int -- offset
{-# UNPACK #-} !Int -- length
On 8/22/2015 6:49 AM, Alexey Raga wrote:
Oh, I have found what was the problem.
It wasn't in fold, it was in a bytestring. Even if I use only a part
of it in a map, the whole bytestring is hanging in memory, probably
because BS.split doesn't really split it but returns portions of the
original one.
I assume something similar happens when using "pipes-csv" library
because that's how I started this investigation.
Creating a copy before pushing it to the map helps.
Cheers,
Alexey.
On Saturday, August 22, 2015 at 6:27:17 PM UTC+10, Alexey Raga wrote:
I was trimming everything else from my program and so far I think
that it is the Map thing, since if I don't do any "M.insert" in
this code it works fine and don't consume any memory at all.
In "real life" I use "pipes-csv", but here are the implementations
of "bslines" and "toVector":
import Data.ByteString.Char8 as BS
import Pipes.ByteString as PBS
import Control.Foldl as F (Fold(..), purely, mconcat)
bslines :: (MonadIO m) => Producer ByteString m ()
bslines = purely folds F.mconcat . view PBS.lines $ PBS.stdin
toVector a = V.fromList $ BS.splitWith (==',') a
I will soon try to profile the code as you suggested.
Cheers,
Alexey.
On Saturday, August 22, 2015 at 3:15:07 PM UTC+10, Gabriel
Gonzalez wrote:
Yes. It's definitely strict in the accumulator and your `Map`
fold looks correct to me. As a side, note you should probably
be building a `Set` instead of a `Map`.
Are you sure that it's the map that is leaking space? You
should profile your heap using the instructions here:
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/prof-heap.html
<https://downloads.haskell.org/%7Eghc/latest/docs/html/users_guide/prof-heap.html>
Also, how are `bslines` and `toVector` implemented?
On 8/21/2015 9:33 PM, Alexey Raga wrote:
Sorry for another beginner's question, but is P.fold strict
or lazy on its accumulator?
In the documentation it says that it is "Strict fold of the
elements of a 'Producer'", but here is what I see:
I have a big CSV file (50M rows), and one of the columns
contains about 4.5M unique values. I fold these values into a
Data.Map.Strict:
names :: Producer ByteString IO ()
names = bslines
>-> P.map toVector
>-> P.map (V.! 25)
main :: IO ()
main = do
m <- P.fold (\s a -> if M.member a s then s else
M.insert a 1 s) M.empty id names
Prelude.print $ M.size m
Prelude.print $ M.findMax m
Prelude.print "ok"
The output suggests that the map is of the right size, and
the max element is correct. This takes ~6.4GB of RAM.
Now I extract these 4.5M unique values into a separate file
and run the same code again (only the column index is
changing, nothing else).
The output is the same (same size, same max element), except
that now it only takes 1.2GB of RAM to run.
Am I right suspecting that laziness causes this issue? But
where and how can I fix it?
Cheers,
Alexey.
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