Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
Is the choice of whether or not to open/close with each chunk read something that we can reasonably hide from the I/O API's user? There is at least one way in which is semantically distinct -- that old trick of opening a tempfile and then unlinking it to hide it. It may be the sort of thing that you do on demand, too -- we have a file handle pool and as we run out of handles we switch to opening/closing. For a single really long read, opening/closing every 4k is just churn; if your doing thousands of long reads at once, though, it can't be helped. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
2008/09/18 o...@okmij.org: Operationally, the code does not open more than one file at a time. More importantly, the code *never* reads more than 4096 characters at a time. A block of the file is read, split into words, counted, and only then another chunk is read. After one file is done, it is closed, and another file is processed. One can see that only one file is being opened at a time by enabling traces. The processing is fully incremental. It opens and closes each file in turn; but it would it be unwise to open and close each file as we'd read a chunk from it? This would allow arbitrary interleaving. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
I hate to say it; but you know you can tweak the OS to allow excessive file handle usage. I once wrote a Haskell script to empty a very, vary large S3 bucket. On Linux, I had to put it in a shell while loop to keep it going, due to file handle exhaustion. On my Mac it ran without incident. :; ulimit unlimited Turns out the `ulimit` on my Mac is pretty high. -- Jason Dusek |...tweak the OS...| http://www.kegel.com/c10k.html#limits.filehandles ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
Oh, curses. I didn't run it with the right option. :; ulimit -a core file size (blocks, -c) 0 data seg size (kbytes, -d) 6144 file size (blocks, -f) unlimited max locked memory (kbytes, -l) unlimited max memory size (kbytes, -m) unlimited open files (-n) 256 pipe size(512 bytes, -p) 1 stack size (kbytes, -s) 8192 cpu time (seconds, -t) unlimited max user processes (-u) 266 virtual memory (kbytes, -v) unlimited So now I'm not sure why it worked on my Mac. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
It depends on the underlying file control used by ghc. if it's the FILE stream pointer, some implementations suffer from a 255 file limit. If it's a standard file descriptor (open instead of fopen), then it's limited by ulimit. On Sun, Apr 5, 2009 at 10:35 AM, Jason Dusek jason.du...@gmail.com wrote: Oh, curses. I didn't run it with the right option. :; ulimit -a core file size (blocks, -c) 0 data seg size (kbytes, -d) 6144 file size (blocks, -f) unlimited max locked memory (kbytes, -l) unlimited max memory size (kbytes, -m) unlimited open files (-n) 256 pipe size(512 bytes, -p) 1 stack size (kbytes, -s) 8192 cpu time (seconds, -t) unlimited max user processes (-u) 266 virtual memory (kbytes, -v) unlimited So now I'm not sure why it worked on my Mac. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- We can't solve problems by using the same kind of thinking we used when we created them. - A. Einstein ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
It opens and closes each file in turn; but it would it be unwise to open and close each file as we'd read a chunk from it? This would allow arbitrary interleaving. If I understand you correctly, you are proposing processing several files in parallel, so to interleave IO. If the `files' in question are communication pipes, or if KAIO (kernel asynchronous IO) is available, it is indeed a good strategy. The last example in the file http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs (called test_driver_mux) demonstrates how to interleave IO with Iteratees. Iteratees of course do not care how the source data have been obtained, with or without interleaving. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
oleg:
Given the stance against top-level mutable variables, I have not
expected to see this Lazy IO code. After all, what could be more against
the spirit of Haskell than a `pure' function with observable side
effects. With Lazy IO, one indeed has to choose between correctness
and performance. The appearance of such code is especially strange
after the evidence of deadlocks with Lazy IO, presented on this list
less than a month ago. Let alone unpredictable resource usage and
reliance on finalizers to close files (forgetting that GHC does not
guarantee that finalizers will be run at all).
Is there an alternative?
Hi Oleg!
I'm glad you joined the thread at this point.
Some background: our best solutions for this problem using lazy IO, are
based on chunk-wise lazy data structures, typically lazy bytestrings.
Often we'll write programs like:
import qualified Data.ByteString.Lazy.Char8 as B
import System.Environment
main = do
[f] - getArgs
s - B.readFile f
print (B.count '\n' s)
Which are nicely efficient
$ ghc -O2 A.hs --make
$ du -hs data
100M data
$ time ./A data
11078540
./A data 0.17s user 0.04s system 100% cpu 0.210 total
And we know from elsewhere the performance is highlycompetitive:
http://shootout.alioth.debian.org/gp4/benchmark.php?test=sumcollang=all
Now, enumerators are very promising, and there's a lot of interest at
the moment, (e.g. just this week, Johan Tibell gave an inspiring talk at
Galois about this approach to IO,
http://www.galois.com/blog/2008/09/12/left-fold-enumerators-a-safe-expressive-and-efficient-io-interface-for-haskell/
and we spent the day sketching out an enumerator bytestring design,
But there are some open questions. Perhaps you have some answers?
* Can we write a Data.ByteString.Enumerator that has matching or
better performance than its dual, the existing chunk-wise lazy
stream type?
* Is there a translation from
data ByteString = Empty | Chunk {-# UNPACK #-} !S.ByteString ByteString
and functions on this type,
foldlChunks :: (a - S.ByteString - a) - a - ByteString - a
foldlChunks f z = go z
where
go !a Empty= a
go !a (Chunk c cs) = go (f a c) cs
to an enumerator implementation?
* Can we compose enumerators as we can stream functions?
* Can we do fusion on enumerators? Does that make composition easier?
(Indeed, is there an encoding of enumerators analogous to stream
fusion control?)
Any thoughts?
-- Don
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO
[EMAIL PROTECTED] writes: It is interesting to compare the above main function with the corresponding lazy IO: Minor point I know, but aren't you really comparing it with the corresponding *strict* IO? main'' = do names - getArgs files - mapM readFile names print $ length $ words (concat files) This works nicely if you replace the middle line with a lazy version, e.g.: files - mapM (unsafeInterleaveIO . B.readFile) names -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
oleg-30 wrote: I have not expected to see this Lazy IO code. After all, what could be more against the spirit of Haskell than a `pure' function with observable side effects. What could even be more against the spirit of Haskell than the original theme of this thread, i.e. code that makes us cry? Lennart's piece fudges purity, agreed, but it reads nicely as idiomatic Haskell, swift on the eyes if not on the machine. Consider if readFile's semantics were modified, i.e. not lazy, at least not always. In the ideal world, a smart enough compiler would just do the right thing, i.e. the IO String returned would be strict, or better yet, it would automatically chunkify the read to obtain constant space usage. Lazy IO is indeed a nasty can of worms, not unrelated to the issue of monadic IO as a gigantic sin bin. We could avoid it entirely, or we could sort out and algebraize the different interactions into a happier marriage of the pair. -- View this message in context: http://www.nabble.com/Lazy-vs-correct-IO--Was%3A-A-round-of-golf--tp19567128p19573538.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
I agree that lazy IO is a can with some worms in it. But it's not that strange.
The readFile operation is in the IO monad, so it has an effect on the world.
This effect is not finished when readFile returns, and from the world
point of view
it's not entirely deterministic.
On Fri, Sep 19, 2008 at 7:51 AM, [EMAIL PROTECTED] wrote:
Lennart Augustsson wrote
main = do
name:_ - getArgs
file - readFile name
print $ length $ lines file
Given the stance against top-level mutable variables, I have not
expected to see this Lazy IO code. After all, what could be more against
the spirit of Haskell than a `pure' function with observable side
effects. With Lazy IO, one indeed has to choose between correctness
and performance. The appearance of such code is especially strange
after the evidence of deadlocks with Lazy IO, presented on this list
less than a month ago. Let alone unpredictable resource usage and
reliance on finalizers to close files (forgetting that GHC does not
guarantee that finalizers will be run at all).
Is there an alternative?
-- Counting the lines in a file
import IterateeM
count_nl = liftI $ IE_cont (step 0)
where
step acc (Chunk str) = liftI $ IE_cont (step $! acc + count str)
step acc stream = liftI $ IE_done acc stream
count [] = 0
count ('\n':str) = succ $! count str
count (_:str) = count str
main = do
name:_ - getArgs
IE_done counter _ - unIM $ enum_file name . enum_eof == count_nl
print counter
The function count_nl could have been in the library, but I'm a
minimalist. It is written in a declarative rather than imperative
style, and one easily sees what it does. The above code as well as the
IterateeM library is Haskell98. It does not use any unsafe Haskell
functions whatsoever.
time wc -l /usr/share/dict/words
235882 /usr/share/dict/words
real0m0.024s
user0m0.022s
sys 0m0.000s
time ~/Docs/papers/DEFUN08/Wc /usr/share/dict/words
235882
real0m0.141s
user0m0.126s
sys 0m0.008s
To compare with lazy IO, the code using readFile gives
time ~/Docs/papers/DEFUN08/Wc /usr/share/dict/words
235882
real0m0.297s
user0m0.262s
sys 0m0.023s
So, choosing correctness does not mean losing in performance; in fact,
one may even gain.
Can enumerators compose? Well, we already seen the example above
(enum_file name . enum_eof)
where the operation (.)
e1 . e2 = (==) e2 . e1
is a flipped composition if monadic bind were considered a flipped
application.
Here is a more interesting example: count words in all the files whose
names are given on the command line. There may be many files given,
thousands of them.
-- Count the stream. Again, could have been in the library
stream_count :: Monad m = IterateeGM el m Int
stream_count = liftI $ IE_cont (step 0)
where
step acc (Chunk []) = liftI $ IE_cont (step acc)
step acc (Chunk [_]) = liftI $ IE_cont (step $! succ acc)
step acc (Chunk ls) = liftI $ IE_cont (step $! acc + length ls)
step acc stream = liftI $ IE_done acc stream
main = do
names - getArgs
let enumerators = foldr (\name - (enum_file name .)) enum_eof names
IE_done (IE_done counter _) _ - unIM $ enumerators ==
enum_words stream_count
print counter
We notice that the composition of enumerators corresponds to the
`concatenation' of their sources. Declaratively, the meaning of the
above code is:
-- all the given files are concatenated
-- the resulting stream of characters is converted to a stream
of words
-- the stream of words is counted.
Operationally, the code does not open more than one file at a
time. More importantly, the code *never* reads more than 4096
characters at a time. A block of the file is read, split into words,
counted, and only then another chunk is read. After one file is done,
it is closed, and another file is processed. One can see that only one
file is being opened at a time by enabling traces. The processing is
fully incremental.
/usr/local/share/doc/ghc6 find . -name \*.html -print | time xargs
~/Docs/papers/DEFUN08/Wc
3043421
16.99 real15.83 user 0.71 sys
BTW, the program has counted words in 1169 files.
It is interesting to compare the above main function with the
corresponding lazy IO:
main'' = do
names - getArgs
files - mapM readFile names
print $ length $ words (concat files)
The number of lines is comparable. The execution is not. If we try to
run the lazy IO code, we get:
/usr/local/share/doc/ghc6 find . -name \*.html -print | time xargs
~/Docs/papers/DEFUN08/Wc
Wc: ./libraries/Win32/Graphics-Win32-GDI-Path.html:
openFile: resource exhausted (Too many open files)
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
On Fri, 2008-09-19 at 16:30 +0100, Lennart Augustsson wrote: I agree that lazy IO is a can with some worms in it. But it's not that strange. The readFile operation is in the IO monad, so it has an effect on the world. This effect is not finished when readFile returns, and from the world point of view it's not entirely deterministic. On operating systems that fail to maintain file system consistency. Blaming an effect of an *operating system* misfeature on a *language* feature is somewhat perverse. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy vs correct IO [Was: A round of golf]
On Fri, Sep 19, 2008 at 2:51 AM, [EMAIL PROTECTED] wrote:
Lennart Augustsson wrote
main = do
name:_ - getArgs
file - readFile name
print $ length $ lines file
Given the stance against top-level mutable variables, I have not
expected to see this Lazy IO code. After all, what could be more against
the spirit of Haskell than a `pure' function with observable side
effects. With Lazy IO, one indeed has to choose between correctness
and performance. The appearance of such code is especially strange
after the evidence of deadlocks with Lazy IO, presented on this list
less than a month ago. Let alone unpredictable resource usage and
reliance on finalizers to close files (forgetting that GHC does not
guarantee that finalizers will be run at all).
Is there an alternative?
-- Counting the lines in a file
import IterateeM
count_nl = liftI $ IE_cont (step 0)
where
step acc (Chunk str) = liftI $ IE_cont (step $! acc + count str)
step acc stream = liftI $ IE_done acc stream
count [] = 0
count ('\n':str) = succ $! count str
count (_:str) = count str
main = do
name:_ - getArgs
IE_done counter _ - unIM $ enum_file name . enum_eof == count_nl
print counter
The function count_nl could have been in the library, but I'm a
minimalist. It is written in a declarative rather than imperative
style, and one easily sees what it does. The above code as well as the
IterateeM library is Haskell98. It does not use any unsafe Haskell
functions whatsoever.
Is the IterateeM library available on-line anywhere? I'm familiar
enough with your earlier work on enumerators that I can guess what
most of what that code is doing, but I'd like a better idea of what
== does.
--
Dave Menendez [EMAIL PROTECTED]
http://www.eyrie.org/~zednenem/
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
