[Haskell-cafe] Re: [reactive] problem with unamb -- doesn't kill
enough threads
Peter Verswyvelen
bugfact at gmail.com
Sat Dec 20 06:00:20 EST 2008
I see. Of course, how silly of me, killThread is asynchronous, it only waits
until the exception is raised in the receiving thread, but it does not wait
until the thread is really killed. The documentation does not seem to
mention this explicitly.
Now, what would be the clean way to make sure all threads are indeed killed
before the process quits? I tried to add another MVar that gets set after
the thread handles uncatched exceptions (so something like bracket (forkIO
a) (putMVar quit ()) return) and the code that calls killThread then does
takeMVar quit, but this did not solve the problem.
A yield didn't do it for me on Windows, I had a to put in a rather large
threadDelay of 1/10th of a second...
On Sat, Dec 20, 2008 at 6:25 AM, Conal Elliott <conal at conal.net> wrote:
> 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 <conal at conal.net> 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 at compy-doble:~/Haskell/Misc$ rm Threads.o ; ghc Threads.hs
>> -threaded -o Threads && ./Threads +RTS -N2
>> Killed ThreadId 5
>> Killed ThreadId 4
>>
>> conal at compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2
>> Killed ThreadId 5
>> Killed ThreadId 4
>> Killed ThreadId 7
>> Killed ThreadId 6
>>
>> conal at compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2
>> Killed ThreadId 5
>> Killed ThreadId 7
>> Killed ThreadId 4
>> Killed ThreadId 6
>>
>> conal at compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2
>> Killed ThreadId 5
>> Killed ThreadId 4
>>
>> conal at 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 <bugfact at gmail.com>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 <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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>>>>> Haskell-Cafe mailing list
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>>>>>
>>>>
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>>>
>>>
>>
>
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