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@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@ku.edu <mailto:
andygill@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
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