[Haskell-cafe] GHC, odd concurrency space leak

[Jason Dagit]

On Wed, Apr 14, 2010 at 3:13 PM, Daniel Fischer <daniel.is.fischer at web.de>wrote:

> Am Mittwoch 14 April 2010 23:49:43 schrieb Jason Dagit:
> > > It will be interesting to hear what fixes this!
> > >
> > >
> > > forever' m = do _ <- m
> > >                 forever' m
> >
> > When I define that version of forever, the space leak goes away.
>
> Not with optimisations.
>

Thanks for pointing that out.  I forgot to say so in my email.

Here are two reduced versions of the original program:

Good version, ghc --make Terminate.hs:
\begin{code}
{-# OPTIONS -O0 #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Main where

import Control.Monad (forever)

import Control.Concurrent
import Control.Concurrent.STM

spawn :: IO a -> IO ThreadId
spawn io = forkIO (io >> return ())

forever' m = do _ <- m
                forever' m

startp4 :: IO ThreadId
startp4 = spawn (forever' (return ()))

startp3 :: IO ThreadId
startp3 = spawn (forever $
                 do startp4
                    putStrLn "Delaying"
                    threadDelay (3 * 1000000))

main = do
  putStrLn "Main thread starting"
  startp3
  threadDelay (1 * 1000000)
\end{code}

The bad version, ghc --make NonTermination.hs:
\begin{code}
{-# OPTIONS -O2 #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

-- Note:  Change the optimization to -O1 to get a terminating version
-- that uses much more memory than it should.

module Main where

import Control.Monad (forever)

import Control.Concurrent
import Control.Concurrent.STM

spawn :: IO a -> IO ThreadId
spawn io = forkIO (io >> return ())

startp4 :: IO ThreadId
startp4 = spawn (forever (return ()))

startp3 :: IO ThreadId
startp3 = spawn (forever $
                 do startp4
                    putStrLn "Delaying"
                    threadDelay (3 * 1000000))

main = do
  putStrLn "Main thread starting"
  startp3
  threadDelay (1 * 1000000)
\end{code}

Can some core expert please look at these and explain the difference?

Thanks!
Jason
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