[GHC] #698: allocaBytes does not actually free the memory after the computation

#698: allocaBytes does not actually free the memory after the computation
-------------------------------+--------------------------------------------
    Reporter:  guest           |        Owner:         
        Type:  bug             |       Status:  new    
    Priority:  normal          |    Milestone:         
   Component:  Compiler (FFI)  |      Version:  6.4.1  
    Severity:  normal          |     Keywords:         
          Os:  Linux           |   Difficulty:  Unknown
Architecture:  x86             |  
-------------------------------+--------------------------------------------
allocaBytes does not appear to free the memory after the computation has
 completed. For example, start ghci and run:

 {{{allocaBytes (100*1024*1024) $ \_ -> getLine}}}

 ghci's virtual memory usage will jump up by 100MB. When you press enter
 however, it does not drop back down.

 restart ghci and try:

 {{{bracket (mallocBytes 100*1024*1024) free $ \_ -> getLine}}}

 This time when you press enter, the memory usage will drop back down to
 its pre mallocBytes usage.

 This also happens for compiled programs.

 Note: the above test will only alloc virtual memory since nothing is ever
 read into the malloc'd memory. The following test program will actually
 force the pages to be mapped. It results in the same broken behaviour with
 the addition that the resident size of ghci gets stuck high.
 {{{
 module Main where

 import Control.Exception
 import Foreign.Marshal.Alloc
 import Foreign.Ptr
 import Data.Word
 import System.IO
 import System.Mem

 test :: Int -> IO ()
 test mb =
     let size = mb * 1024 * 1024 in
         do -- bracket (mallocBytes size) free $ wait size -- this version
 works
            allocaBytes size $ wait size  -- this version does not free the
 memory afterwards
            performGC

 wait :: Int -> Ptr Word8 -> IO ()
 wait size p =
     bracket (openBinaryFile "/dev/scsi/host0/bus0/target0/lun0/part1"
 ReadMode) (hClose) (\h -> hGetBuf h p size >> putStrLn "waiting..." >>
 getLine >> return ())

 main =
     do test 150
        putStrLn "is it free?"
        getLine
        return ()
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/698>
GHC <http://www.haskell.org/ghc/>
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