Why Haskell function execution time measurment differs from ghc timings?

Question

I want to measure time which Haskell spent to execute some function and use TimeIt package(also i tried these recommendations). But showed time differs from actual time application spent (I've ran application with +RTS -sstderr option):

CPU time:   4.85s
...  
  INIT    time    0.00s  (  0.00s elapsed)
  MUT     time    0.98s  ( 61.69s elapsed)
  GC      time    0.22s  (  0.19s elapsed)
  EXIT    time    0.00s  (  0.00s elapsed)
  Total   time    1.20s  ( 61.89s elapsed)

Application source:

import qualified Data.ByteString.Lazy.Char8 as LBS
import System.Environment
import Data.Char
import Data.Int
import System.TimeIt

readChunks :: Int64 -> LBS.ByteString -> Int64
readChunks size str 
                            | LBS.null str = 0
                            | otherwise = let (chunk, rest) = LBS.splitAt size str
                                                 in do
                                                        let len = LBS.length chunk
                                                        len `seq` len + readChunks size rest



processFile :: String -> IO()
processFile name = do
    putStrLn name
    content <- LBS.readFile name
    let 
        (recNumStr, rest) = LBS.span (not.isControl) content
        recNum = LBS.readInt recNumStr
    case recNum of
        Nothing -> putStrLn "can't parse"
        Just (value, rest) -> print (value)
    let chunkSize = 100*1024*1024   

    timeIt $ print (readChunks chunkSize rest)

UPDATE: I've found that Chronograph package shows right execution time (information taken from this question).

Answer 1

Well you are doing significant work that isn't being timed - it seems reasonable that this work makes up the difference, namely:

putStrLn name
content <- LBS.readFile name
let 
    (recNumStr, rest) = LBS.span (not.isControl) content
    recNum = LBS.readInt recNumStr
case recNum of
    Nothing -> putStrLn "can't parse"
    Just (value, rest) -> print (value)

If you time that as well then you'll probably find most the difference. Also note there are other operations before you even enter main (which is true even for C programs).