<div>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. </div>
<div><br></div><div><div>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.</div>
<div><br><div>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...</div><div><br></div></div></div><div class="gmail_quote">On Sat, Dec 20, 2008 at 6:25 AM, Conal Elliott <span dir="ltr"><<a href="mailto:conal@conal.net">conal@conal.net</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">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 <i>before</i> some of the kill reports.<br>
<font color="#888888">
<br> - Conal</font><div><div></div><div class="Wj3C7c"><br><br><div class="gmail_quote">On Fri, Dec 19, 2008 at 11:17 AM, Conal Elliott <span dir="ltr"><<a href="mailto:conal@conal.net" target="_blank">conal@conal.net</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left:1px solid rgb(204, 204, 204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex">
Peter,<br><br>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.<br><br>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.<br>
<br>For example (removing messages other than "Killed"):<br><br> conal@compy-doble:~/Haskell/Misc$ rm Threads.o ; ghc Threads.hs -threaded -o Threads && ./Threads +RTS -N2<br> Killed ThreadId 5<br>
Killed ThreadId 4<br><br> conal@compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2<br> Killed ThreadId 5<br> Killed ThreadId 4<br> Killed ThreadId 7<br> Killed ThreadId 6<br><br> conal@compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2<br>
Killed ThreadId 5<br> Killed ThreadId 7<br> Killed ThreadId 4<br> Killed ThreadId 6<br><br> conal@compy-doble:~/Haskell/Misc$ ./Threads +RTS -N2<br> Killed ThreadId 5<br> Killed ThreadId 4<br><br> conal@compy-doble:~/Haskell/Misc$ <br>
<br>Simon -- does this behavior look like a GHC bug to you?<br><font color="#888888"><br> - Conal</font><div><div></div><div><br><br><div class="gmail_quote">On Fri, Dec 19, 2008 at 9:45 AM, Peter Verswyvelen <span dir="ltr"><<a href="mailto:bugfact@gmail.com" target="_blank">bugfact@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left:1px solid rgb(204, 204, 204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex"><div><div><div>I played a bit the the bracket function that timeout uses, but got strange results (both on Windows and OSX).</div>
<div><br></div><div>Ugly code fragment follows:</div><div><br></div><div>-%<-------------------------------------------------------------------------------------------------</div>
<div><br></div><div>import Prelude hiding (catch)</div><div><br></div><div>import Control.Concurrent</div><div>import Control.Concurrent.MVar</div><div>import Control.Exception</div><div>import System.IO</div><div>import Data.Char</div>
<div><br></div><div>withThread a b = bracket (forkIO a) kill (const b)</div><div> where</div><div> kill id = do</div><div> putStrLn ("Killing "++show id++"\n")</div><div> killThread id</div>
<div> putStrLn ("Killed "++show id++"\n")</div><div><br></div><div>race a b = do</div><div> v <- newEmptyMVar</div><div> let t x = x >>= putMVar v</div><div> withThread (t a) $ withThread (t b) $ takeMVar v</div>
<div>
<div><br></div><div>forkPut :: IO a -> MVar a -> IO ThreadId</div><div>forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler `catch` bhandler)</div><div> where</div><div> uhandler (ErrorCall "Prelude.undefined") = return ()</div>
<div> uhandler err = throw err</div><div> bhandler BlockedOnDeadMVar = return ()</div><div><br></div></div><div>sleep n = do</div><div> tid <- myThreadId</div><div> putStrLn ("Sleeping "++show n++" sec on "++show tid++"\n")</div>
<div> threadDelay (n*1000000)</div><div> putStrLn ("Slept "++show n++" sec on "++show tid++"\n")</div><div><br></div><div>f = sleep 2 `race` sleep 3</div><div><br></div><div>g = f `race` sleep 1</div>
<div><br></div><div>main = do</div><div> hSetBuffering stdout LineBuffering</div><div> g</div><div> </div><div>-%<-------------------------------------------------------------------------------------------------</div>
<div><br></div><div>Here's the output when running with GHCI:</div><div> </div><div>C:\temp>runghc racetest</div><div>Sleeping 1 sec on ThreadId 26</div><div>Sleeping 2 sec on ThreadId 27</div><div>Sleeping 3 sec on ThreadId 28</div>
<div>Slept 1 sec on ThreadId 26</div><div>Killing ThreadId 26</div><div>Killed ThreadId 26</div><div>Killing ThreadId 25</div><div>Killed ThreadId 25</div><div>Killing ThreadId 28</div><div>Killed ThreadId 28</div><div><br>
</div><div>Fine, all threads got killed. </div><div><br></div><div>Here's the output from an EXE compiled with GHC -threaded, but run without +RTS -N2</div><div><br></div><div>C:\temp> racetest</div><div>Sleeping 1 sec on ThreadId 5</div>
<div>Sleeping 3 sec on ThreadId 7</div><div>Sleeping 2 sec on ThreadId 6</div><div>Slept 1 sec on ThreadId 5</div><div>Killing ThreadId 5</div><div>Killed ThreadId 5</div><div>Killing ThreadId 4</div><div>Killed ThreadId 4</div>
<div>Killing ThreadId 7</div><div><br></div><div>So "Killed ThreadId 7" is not printed here. What did I do wrong?</div><div><br></div><div>Here's the output from an EXE compiled with GHC -threaded, but run with +RTS -N2</div>
<div><br></div><div>C:\temp> racetest +RTS -N2</div><div>Sleeping 1 sec on ThreadId 5</div><div>Sleeping 3 sec on ThreadId 7</div><div>Sleeping 2 sec on ThreadId 6</div><div>Slept 1 sec on ThreadId 5</div><div><br></div>
<div>Killing ThreadId 5</div><div>Killed ThreadId 5</div><div>Killing ThreadId 4</div><div>Killed ThreadId 4</div><div>Killing ThreadId 7</div><div>Killed ThreadId 7</div><div><br></div><div>This works again. </div><div>
<br>
</div><div>Is this intended behavior?</div><div><br></div><div>Cheers,</div><div>Peter Verswyvelen</div><font color="#888888"><div>CTO - Anygma</div></font><div><div></div><div><div><br></div><div class="gmail_quote">
On Fri, Dec 19, 2008 at 10:48 AM, Simon Marlow <span dir="ltr"><<a href="mailto:marlowsd@gmail.com" target="_blank">marlowsd@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left:1px solid rgb(204, 204, 204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex">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 ;-)<br>
<br>
For a good example of how to do this sort of thing, see<br>
<br>
<a href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout.html" target="_blank">http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout.html</a><br>
<br>
the docs are sadly missing the source links at the moment, I'm not sure why, but you can find the source in<br>
<br>
<a href="http://darcs.haskell.org/packages/base/System/Timeout.hs" target="_blank">http://darcs.haskell.org/packages/base/System/Timeout.hs</a><br>
<br>
Cheers,<br>
Simon<br>
<br>
Conal Elliott wrote:<br>
<blockquote class="gmail_quote" style="border-left:1px solid rgb(204, 204, 204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex"><div>
(I'm broadening the discussion to include haskell-cafe.)<br>
<br>
Andy -- What do you mean by "handling all thread forking locally"?<br>
<br>
- Conal<br>
<br></div><div><div></div><div>
On Thu, Dec 18, 2008 at 1:57 PM, Andy Gill <<a href="mailto:andygill@ku.edu" target="_blank">andygill@ku.edu</a> <mailto:<a href="mailto:andygill@ku.edu" target="_blank">andygill@ku.edu</a>>> wrote:<br>
<br>
Conal, et. al,<br>
<br>
I was looking for exactly this about 6~9 months ago. I got the<br>
suggestion to pose it as a challenge<br>
to the community by Duncan Coutts. What you need is thread groups,<br>
where for a ThreadId, you can send a signal<br>
to all its children, even missing generations if needed. <br>
I know of no way to fix this at the Haskell level without handling<br>
all thread forking locally. <br>
Perhaps a ICFP paper about the pending implementation :-) but I'm<br>
not sure about the research content here.<br>
<br>
Again, there is something deep about values with lifetimes. <br>
Andy Gill<br>
<br>
<br>
On Dec 18, 2008, at 3:43 PM, Conal Elliott wrote:<br>
<br>
</div></div><blockquote class="gmail_quote" style="border-left:1px solid rgb(204, 204, 204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex"><div><div></div><div>
I realized in the shower this morning that there's a serious flaw<br>
in my unamb implementation as described in<br>
<a href="http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice" target="_blank">http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice</a>. I'm looking for ideas for fixing the flaw. Here's the code for<br>
racing computations:<br>
<br>
race :: IO a -> IO a -> IO a<br>
a `race` b = do v <- newEmptyMVar<br>
ta <- forkPut a v<br>
tb <- forkPut b v<br>
x <- takeMVar v<br>
killThread ta<br>
killThread tb<br>
return x<br>
<br>
forkPut :: IO a -> MVar a -> IO ThreadId<br>
forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler<br>
`catch` bhandler)<br>
where<br>
uhandler (ErrorCall "Prelude.undefined") = return ()<br>
uhandler err = throw err<br>
bhandler BlockedOnDeadMVar = return ()<br>
<br>
The problem is that each of the threads ta and tb may have spawned<br>
other threads, directly or indirectly. When I kill them, they<br>
don't get a chance to kill their sub-threads.<br>
<br>
Perhaps I want some form of garbage collection of threads, perhaps<br>
akin to Henry Baker's paper "The Incremental Garbage Collection of<br>
Processes". As with memory GC, dropping one consumer would<br>
sometimes result is cascading de-allocations. That cascade is<br>
missing from my implementation.<br>
<br>
Or maybe there's a simple and dependable manual solution,<br>
enhancing the method above.<br>
<br>
Any ideas?<br>
<br>
- Conal<br>
<br>
<br>
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