On Wednesday 23 March 2011 03:32:16, Tim Docker wrote:
On Mon, Mar 21, 2011 at 9:59 AM, I wrote:
My question on the ghc heap profiler on stack overflow:
http://stackoverflow.com/questions/5306717/how-should-i-interpret-the- output-of-the-ghc-heap-profiler
remains unanswered :-( Perhaps that's not the best forum. Is there someone here prepared to explain how the memory usage in the heap profiler relates to the "Live Bytes" count shown in the garbage collection statistics?
I've made a little progress on this. I've simplified my program down to a simple executable that loads a bunch of data into an in-memory map, and then writes it out again. I've added calls to `seq` to ensure that laziness is not causing excessing memory consumption. When I run this on my sample data set, it takes ~7 cpu seconds, and uses ~120 MB of vm An equivalent python script, takes ~2 secs and ~19MB of vm :-(.
The code is below. I'm mostly concerned with the memory usage rather than performance at this stage. What is interesting, is that when I turn on garbage collection statistics (+RTS -s), I see this:
10,089,324,996 bytes allocated in the heap 201,018,116 bytes copied during GC 12,153,592 bytes maximum residency (8 sample(s)) 59,325,408 bytes maximum slop 114 MB total memory in use (1 MB lost due to fragmentation)
Generation 0: 19226 collections, 0 parallel, 1.59s, 1.64selapsed Generation 1: 8 collections, 0 parallel, 0.04s, 0.04selapsed
INIT time 0.00s ( 0.00s elapsed) MUT time 5.84s ( 5.96s elapsed) GC time 1.63s ( 1.68s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 7.47s ( 7.64s elapsed)
%GC time 21.8% (22.0% elapsed)
Alloc rate 1,726,702,840 bytes per MUT second
Productivity 78.2% of total user, 76.5% of total elapsed
This seems strange. The maximum residency of 12MB sounds about correct
for my data. But what's with the 59MB of "slop"? According to the ghc docs: | The "bytes maximum slop" tells you the most space that is ever wasted | due to the way GHC allocates memory in blocks. Slop is memory at the | end of a block that was wasted. There's no way to control this; we | just like to see how much memory is being lost this way.
There's this page also:
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/Slop
but it doesn't really make things clearer for me.
Is the slop number above likely to be a significant contribution to net memory usage?
Yes, absolutely.
Are there any obvious reasons why the code below could be generating so much?
I suspect packing a lot of presumably relatively short ByteStrings would generate (the lion's share of) the slop. I'm not familiar with the internals, though, so I don't know where GHC would put a newPinnedByteArray# (which is where your ByteString contents is), what alignement requirements those have.
The data file in question has 61k lines, and is <6MB in total.
Thanks,
Tim
-------- Map2.hs --------------------------------------------
module Main where
import qualified Data.Map as Map import qualified Data.ByteString.Char8 as BS import System.Environment import System.IO
type MyMap = Map.Map BS.ByteString BS.ByteString
foldLines :: (a -> String -> a) -> a -> Handle -> IO a foldLines f a h = do eof <- hIsEOF h if eof then (return a) else do l <- hGetLine h let a' = f a l a' `seq` foldLines f a' h
undumpFile :: FilePath -> IO MyMap undumpFile path = do h <- openFile path ReadMode m <- foldLines addv Map.empty h hClose h return m where addv m "" = m addv m s = let (k,v) = readKV s in k `seq` v `seq` Map.insert k v m
readKV s = let (ks,vs) = read s in (BS.pack ks, BS.pack vs)
It might be better to read the file in one go and construct the map in pure code (foldl' addv Map.empty $ lines filecontents). Also, it will probably be better to do everything on ByteStrings. The file format seems to be ("key","value") on each line, with possible whitespace and empty lines. If none of the keys or values may contain a '\"', undumpFile path = do contents <- BS.readFile path return $! foldl' addv Map.empty (BS.lines contents) where addv m s | BS.null s = m | otherwise = case BS.split '"' s of (_ : k : _ : v : _) -> Map.insert k v m _ -> error "malformed line" should perform much better. If a key or value may contain '"', it's more complicated, using a regex library to split might be a good option then.
dump :: [(BS.ByteString,BS.ByteString)] -> IO () dump vs = mapM_ putV vs where putV (k,v) = putStrLn (show (BS.unpack k, BS.unpack v))
main :: IO () main = do args <- getArgs case args of [path] -> do v <- undumpFile path dump (Map.toList v) return ()