#7556: build/fold causes with ByteString unpack causes huge memory leak
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Reporter: glguy | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Runtime performance bug | Blockedby:
Blocking: | Related:
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Changes (by shachaf):
* cc: shachaf@… (added)
Comment:
The issue is
[http://hackage.haskell.org/packages/archive/bytestring/0.10.2.0/doc/html/src
/Data-ByteString.html#unpackFoldr here]:
{{{
unpack ps = build (unpackFoldr ps)
{-# INLINE unpack #-}
--
-- Have unpack fuse with good list consumers
--
-- critical this isn't strict in the acc
-- as it will break in the presence of list fusion. this is a known
-- issue with seq and build/foldr rewrite rules, which rely on lazy
-- demanding to avoid bottoms in the list.
--
unpackFoldr :: ByteString -> (Word8 -> a -> a) -> a -> a
unpackFoldr (PS fp off len) f ch = withPtr fp $ \p -> do
let loop q n _ | q `seq` n `seq` False = undefined -- n.b.
loop _ (-1) acc = return acc
loop q n acc = do
a <- peekByteOff q n
loop q (n-1) (a `f` acc)
loop (p `plusPtr` off) (len-1) ch
{-# INLINE [0] unpackFoldr #-}
{-# RULES
"ByteString unpack-list" [1] forall p .
unpackFoldr p (:) [] = unpackBytes p
}}}
When we use `foldr`, `foldr/build` fusion turns the whole expression into
an application of `unpackFoldr`, which is tail recursive and therefore not
sufficiently lazy -- but also not strict in the accumulator, so it builds
up a big thunk. In `example1`, fusion doesn't happen, so the fold happens
over `unpackBytes` instead, which generates list in small chunks that can
be processed lazily.
This looks like a `bytestring` bug to me.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7556#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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