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

Oh -- I think the problem here was simply that the process itself exited before all of the threads had a chance to get killed. When I add a short sleep to the end of main, or even just a 'yield', I see that all threads reported as killed. What clued me in was finally paying attention to the observation that under ghci I get the new prompt *before* some of the kill reports. - Conal On Fri, Dec 19, 2008 at 11:17 AM, Conal Elliott wrote:

Peter,

Thanks for digging. In your results below, I see only three out of four threads killed even in the best case. Each time, there is no report of the 'sleep 2' thread being killed.

When I run your code on Linux (Ubuntu 8.10), everything looks great when run under ghci. If compiled, with and without -threaded and with and without +RTS -N2, I sometimes get four kill messages and sometimes fewer. In the latter case, I don't know if the other threads aren't getting killed or if they're killed but not reported.

For example (removing messages other than "Killed"):

conal@compy-doble:~/Haskell/Misc$ rm Threads.o ; ghc Threads.hs -threaded -o Threads && ./Threads +RTS -N2 Killed ThreadId 5 Killed ThreadId 4

conal@compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2 Killed ThreadId 5 Killed ThreadId 4 Killed ThreadId 7 Killed ThreadId 6

conal@compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2 Killed ThreadId 5 Killed ThreadId 7 Killed ThreadId 4 Killed ThreadId 6

conal@compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2 Killed ThreadId 5 Killed ThreadId 4

conal@compy-doble:~/Haskell/Misc$

Simon -- does this behavior look like a GHC bug to you?

- Conal

On Fri, Dec 19, 2008 at 9:45 AM, Peter Verswyvelen wrote:

...

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 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.ht...

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 > 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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