Wc
Some implementations of the 'wc -l' program in Haskell, with an eye to C-like
performance. This illustrates the balance to be made between performance and
elegance, over several increasingly fast (and more complex) examples.
Baseline
The baseline is the C program 'wc'
$ du -hs /usr/share/dict/words 892K /usr/share/dict/words $ time wc -l /usr/share/dict/words 96030 /usr/share/dict/words wc -l /usr/share/dict/words 0.00s user 0.00s system 33% cpu 0.018 total
So the best we can probably hope to get is around 0.018s
Standard [Char]
main = print . length . lines =<< getContents
$ ghc -O wc.hs $ time ./a.out < /usr/share/dict/words 96030 ./a.out < /usr/share/dict/words 0.10s user 0.01s system 89% cpu 0.118 total
Ok. 0.118s. About 10x C, as to be expected with a list representation.
Data.PackedString
Ok, lets try the old Data.PackedString library.
My first attempt to directly use hGet failed, as hGet has a stack overflow for files > ~500k.
import Data.PackedString
import System.IO
main = print . length . linesPS =<< getit "/usr/share/dict/words"
where
getit f = do
h <- openFile f ReadMode
s <- hGetContents h
length s `seq` return ()
hClose h
return $! packString s
$ time ./a.out 96030 ./a.out 0.12s user 0.03s system 90% cpu 0.167 total
Hmm. Worse than [Char]. Unfortunately, this is not uncommon with Data.PackedString.
Data.ByteString
Try to improve performance a bit by using the new Data.ByteString library, a replacement for Data.PackedString. This uses packed byte arrays instead of heap-allocated [Char] to represent strings.
import qualified Data.ByteString as B
main = print . length . B.lines =<< B.getContents
$ ghc -O wc.hs -package fps $ time ./a.out < /usr/share/dict/words 96030 ./a.out < /usr/share/dict/words 0.01s user 0.00s system 42% cpu 0.025 total
0.025s, a bit less than 2x slower than C. Not too bad, and probably a satisfactory place to stop optimising in normal circumstances. Its also nice that the code is similarly concise.
Ptr hacking
ByteStrings give you access to the underlying pointers to bytes in memory, which can be used to optimise some particular code. So when the ByteString api doesn't provide what you want, you can step inside the ForeignPtr and go nuts.
This example also makes use of a cpp macro to force strictness on a function, via a seq guard case.
import Foreign
import Foreign.ForeignPtr
import System.Environment
import qualified Data.ByteString as B
#define STRICT4(f) f a b c d | a `seq` b `seq` c `seq` d `seq` False = undefined
main = head `fmap` getArgs >>= B.readFile >>= \(B.PS x _ l) ->
withForeignPtr x $ \p -> go p l 0 0
where go :: Ptr Word8 -> Int -> Int -> Int -> IO ()
STRICT4(go)
go p l n i | n >= l = print i
| otherwise = do (w::Word8) <- peek (p `plusPtr` n)
go p l (n+1) $ if w == 0x0a then (i+1) else i
$ ghc -O -package fps -fglasgow-exts -cpp wc.hs $ time ./a.out /usr/share/dict/words 96030 ./a.out /usr/share/dict/words 0.01s user 0.00s system 47% cpu 0.021 total
A little faster perhaps.
Use the FFI
Try and step around the inefficent need to inspect each character in Haskell, by using memchr(3), the C function to find each newline for us.
import Foreign
import Foreign.ForeignPtr
import Foreign.C.Types
import System.Environment
import qualified Data.ByteString as B
#define STRICT4(f) f a b c d | a `seq` b `seq` c `seq` d `seq` False = undefined
main = do
f <- head `fmap` getArgs
B.readFile f >>= \(B.PS x _ l) -> withForeignPtr x $ \p -> go p l 0 0
where
go :: Ptr Word8 -> Int -> Int -> Int -> IO ()
STRICT4(go)
go p l n i
| n >= l = print i
| otherwise = do
let p' = p `plusPtr` n
q = memchr p' 0x0a (fromIntegral (l-n))
if q == nullPtr
then print i
else do let k = q `minusPtr` p'
go p l (n+k+1) (i+1)
foreign import ccall unsafe "string.h memchr" memchr
:: Ptr Word8 -> CInt -> CSize -> Ptr Word8
$ ghc -O -package fps -cpp -ffi wc.hs $ time ./a.out /usr/share/dict/words 96030 ./a.out /usr/share/dict/words 0.00s user 0.01s system 70% cpu 0.020 total
Slowly inching forwards.
Read the Core
While we're here, we can check whether the strictness on the 'go' function makes any difference, by reading the GHC Core:
$ ghc -O -package fps -cpp -ffi wc.hs -ddump-simpl | less
Search for the 'go' function:
Main.$wgo :: GHC.Prim.Addr# -> GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.IOBase.IO ()
And without the strictness:
Main.$wgo :: GHC.Ptr.Ptr GHC.Word.Word8 -> GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Base.Int -> GHC.IOBase.IO ()
So GHC is helpfully unboxing the Ptr Word8 into a raw machine Addr#.
Avoid some code
The guard that checks the length is uneeded, since memchr takes a length argument anyway. It also calculates the next pointer for us, so avoid recalculating it.
Checking the Core, 'go' is now:
Main.$wgo :: GHC.Prim.Addr# -> GHC.Prim.Word# -> GHC.Prim.Int# -> GHC.IOBase.IO ()
import Foreign
import Foreign.ForeignPtr
import Foreign.C.Types
import System.Environment
import qualified Data.ByteString as B
#define STRICT3(f) f a b c | a `seq` b `seq` c `seq` False = undefined
main = do
f <- head `fmap` getArgs
B.readFile f >>= \(B.PS x _ l) -> withForeignPtr x $ \p -> go p (fromIntegral l) 0
where
go :: Ptr Word8 -> CSize -> Int -> IO ()
STRICT3(go)
go p l i
| otherwise = do
let q = memchr p 0x0a l
if q == nullPtr
then print i
else do let k = fromIntegral $ q `minusPtr` p
go (q `plusPtr` 1) (l - k) (i+1)
foreign import ccall unsafe "string.h memchr" memchr
:: Ptr Word8 -> CInt -> CSize -> Ptr Word8
The code is certainly a bit simpler, at least.
$ ghc -O -package fps -cpp -ffi wc.hs $ time ./a.out /usr/share/dict/words 96030 ./a.out /usr/share/dict/words 0.00s user 0.00s system 59% cpu 0.020 total
But we can't seem to squeeze any more out, at least on data this size.
Using mmap
The same program as above, but use mmap(2) instead of readFile.
import Foreign
import Foreign.ForeignPtr
import Foreign.C.Types
import System.Environment
import qualified Data.ByteString as B
#define STRICT3(f) f a b c | a `seq` b `seq` c `seq` False = undefined
main = do
f <- head `fmap` getArgs
B.mmapFile f >>= \(B.PS x _ l) -> withForeignPtr x $ \p -> go p (fromIntegral l) 0
where
go :: Ptr Word8 -> CSize -> Int -> IO ()
STRICT3(go)
go p l i
| otherwise = do
let q = memchr p 0x0a l
if q == nullPtr
then print i
else do let k = fromIntegral $ q `minusPtr` p
go (q `plusPtr` 1) (l - k) (i+1)
foreign import ccall unsafe "string.h memchr" memchr
:: Ptr Word8 -> CInt -> CSize -> Ptr Word8
$ time ./a.out /usr/share/dict/words 96030 ./a.out /usr/share/dict/words 0.00s user 0.00s system 36% cpu 0.019 total
A little faster again.
Going via C
We reach a point where I can't think of any more tricks, so we can always code up a little C and call into that, for this tight loop. Sometimes we just have to do this, and that's what the ffi is for, after all.
-- wc.hs
import Foreign
import System.Environment
import qualified Data.ByteString as B
main = do
f <- head `fmap` getArgs
B.mmapFile f >>= \(B.PS x _ l) -> withForeignPtr x $ \p -> print (c_wc p l)
foreign import ccall unsafe "wc.h wc" c_wc :: Ptr Word8 -> Int -> Int
-- wc_c.c
#include <sys/types.h>
#include <unistd.h>
int wc(char *p, int len) {
int c;
for (c = 0; len--; ++p)
if (*p == '\n')
++c;
return c;
}
-- wc.h
int wc(char *p, int len);
$ time ./a.out /usr/share/dict/words 96030 ./a.out /usr/share/dict/words 0.00s user 0.00s system 51% cpu 0.017 total
And we are done. Note that the tight C loop didn't give us much in the end over the naive ByteString code, which is a satisfying result.