[Haskell-cafe] Re: [reactive] problem with unamb -- doesn't kill enough threads

[Peter Verswyvelen]

I played a bit the the bracket function that timeout uses, but got strange
results (both on Windows and OSX).

Ugly code fragment follows:

-%<-------------------------------------------------------------------------------------------------

import Prelude hiding (catch)

import Control.Concurrent
import Control.Concurrent.MVar
import Control.Exception
import System.IO
import Data.Char

withThread a b = bracket (forkIO a) kill (const b)
    where
      kill id = do
        putStrLn ("Killing "++show id++"\n")
        killThread id
        putStrLn ("Killed "++show id++"\n")

race a b = do
    v <- newEmptyMVar
    let t x = x >>= putMVar v
    withThread (t a) $ withThread (t b) $ takeMVar v

forkPut :: IO a -> MVar a -> IO ThreadId
forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler `catch`
bhandler)
 where
   uhandler (ErrorCall "Prelude.undefined") = return ()
   uhandler err                             = throw err
   bhandler BlockedOnDeadMVar               = return ()

sleep n = do
  tid <- myThreadId
  putStrLn ("Sleeping "++show n++" sec on "++show tid++"\n")
  threadDelay (n*1000000)
  putStrLn ("Slept "++show n++" sec on "++show tid++"\n")

f = sleep 2 `race` sleep 3

g = f `race` sleep 1

main = do
  hSetBuffering stdout LineBuffering
  g

-%<-------------------------------------------------------------------------------------------------

Here's the output when running with GHCI:

C:\temp>runghc racetest
Sleeping 1 sec on ThreadId 26
Sleeping 2 sec on ThreadId 27
Sleeping 3 sec on ThreadId 28
Slept 1 sec on ThreadId 26
Killing ThreadId 26
Killed ThreadId 26
Killing ThreadId 25
Killed ThreadId 25
Killing ThreadId 28
Killed ThreadId 28

Fine, all threads got killed.

Here's the output from an EXE compiled with GHC -threaded, but run without
+RTS -N2

C:\temp> racetest
Sleeping 1 sec on ThreadId 5
Sleeping 3 sec on ThreadId 7
Sleeping 2 sec on ThreadId 6
Slept 1 sec on ThreadId 5
Killing ThreadId 5
Killed ThreadId 5
Killing ThreadId 4
Killed ThreadId 4
Killing ThreadId 7

So "Killed ThreadId 7" is not printed here. What did I do wrong?

Here's the output from an EXE compiled with GHC -threaded, but run with +RTS
-N2

C:\temp> racetest +RTS -N2
Sleeping 1 sec on ThreadId 5
Sleeping 3 sec on ThreadId 7
Sleeping 2 sec on ThreadId 6
Slept 1 sec on ThreadId 5

Killing ThreadId 5
Killed ThreadId 5
Killing ThreadId 4
Killed ThreadId 4
Killing ThreadId 7
Killed ThreadId 7

This works again.

Is this intended behavior?

Cheers,
Peter Verswyvelen
CTO - Anygma

On Fri, Dec 19, 2008 at 10:48 AM, Simon Marlow <marlowsd at gmail.com> wrote:

> Sounds like you should use an exception handler so that when the parent
> dies it also kills its children.  Be very careful with race conditions ;-)
>
> For a good example of how to do this sort of thing, see
>
>
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout.html
>
> the docs are sadly missing the source links at the moment, I'm not sure
> why, but you can find the source in
>
> http://darcs.haskell.org/packages/base/System/Timeout.hs
>
> Cheers,
>        Simon
>
> Conal Elliott wrote:
>
>> (I'm broadening the discussion to include haskell-cafe.)
>>
>> Andy -- What do you mean by "handling all thread forking locally"?
>>
>>  - Conal
>>
>> On Thu, Dec 18, 2008 at 1:57 PM, Andy Gill <andygill at ku.edu <mailto:
>> andygill at ku.edu>> wrote:
>>
>>    Conal, et. al,
>>
>>    I was looking for exactly this about 6~9 months ago. I got the
>>    suggestion to pose it as a challenge
>>    to the community by Duncan Coutts. What you need is thread groups,
>>     where for a ThreadId, you can send a signal
>>    to all its children, even missing generations if needed.
>>    I know of no way to fix this at the Haskell level without handling
>>    all thread forking locally.
>>    Perhaps a ICFP paper about the pending implementation :-) but I'm
>>    not sure about the research content here.
>>
>>    Again, there is something deep about values with lifetimes.
>>    Andy Gill
>>
>>
>>    On Dec 18, 2008, at 3:43 PM, Conal Elliott wrote:
>>
>>     I realized in the shower this morning that there's a serious flaw
>>>    in my unamb implementation as described in
>>>
>>> http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice.
>>>    I'm looking for ideas for fixing the flaw.  Here's the code for
>>>    racing computations:
>>>
>>>        race :: IO a -> IO a -> IO a
>>>        a `race` b = do v  <- newEmptyMVar
>>>                        ta <- forkPut a v
>>>                        tb <- forkPut b v
>>>                        x  <- takeMVar  v
>>>                        killThread ta
>>>                        killThread tb
>>>                        return x
>>>
>>>        forkPut :: IO a -> MVar a -> IO ThreadId
>>>        forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler
>>>    `catch` bhandler)
>>>         where
>>>           uhandler (ErrorCall "Prelude.undefined") = return ()
>>>           uhandler err                             = throw err
>>>           bhandler BlockedOnDeadMVar               = return ()
>>>
>>>    The problem is that each of the threads ta and tb may have spawned
>>>    other threads, directly or indirectly.  When I kill them, they
>>>    don't get a chance to kill their sub-threads.
>>>
>>>    Perhaps I want some form of garbage collection of threads, perhaps
>>>    akin to Henry Baker's paper "The Incremental Garbage Collection of
>>>    Processes".  As with memory GC, dropping one consumer would
>>>    sometimes result is cascading de-allocations.  That cascade is
>>>    missing from my implementation.
>>>
>>>    Or maybe there's a simple and dependable manual solution,
>>>    enhancing the method above.
>>>
>>>    Any ideas?
>>>
>>>       - Conal
>>>
>>>
>>>    _______________________________________________
>>>    Reactive mailing list
>>>    Reactive at haskell.org <mailto:Reactive at haskell.org>
>>>    http://www.haskell.org/mailman/listinfo/reactive
>>>
>>
>>
>>
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>>
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