blocking parallel program

[Facundo Domínguez]

In case anyone wants to contribute to it, I have submitted a bug report [1].

Best,
Facundo

[1] https://ghc.haskell.org/trac/ghc/ticket/8521

On Mon, Oct 21, 2013 at 9:03 AM, Facundo Domínguez
<facundo.dominguez at parsci.com> wrote:
>> Oh I see; the problem is the GHC RTS is attempting to shut down,
>> and in order to do this it needs to grab all of the capabilities.
>
> Thanks, again. However, the program doesn't seem to be blocking when
> the main thread finishes, but rather in the "takeMVar mv1" line. I'm
> copying the modified version with a print call that never manages to
> print in the console for me.
>
> btw, I'm using ghc 7.6.3.
>
> Best,
> Facundo
>
> -----
>
> import Control.Concurrent
> import Control.Monad
> import System.Environment
>
> main :: IO ()
> main = do
>   y <- getArgs
>   mv0 <- newEmptyMVar
>   mv1 <- newEmptyMVar
>   forkIO $ do
>     takeMVar mv0
>     putMVar mv1 ()
>     loop (y == ["yield"])
>   putMVar mv0 ()
>   takeMVar mv1
>      >>= print
>
> loop :: Bool -> IO ()
> loop cooperative = go
>   where
>     go = when cooperative yield >> go
>
> On Sun, Oct 20, 2013 at 2:37 AM, Edward Z. Yang <ezyang at mit.edu> wrote:
>> Oh I see; the problem is the GHC RTS is attempting to shut down,
>> and in order to do this it needs to grab all of the capabilities. However,
>> one of them is in an uninterruptible loop, so the program hangs (e.g.
>> if you change the program as follows:
>>
>>     main :: IO ()
>>     main = do
>>       forkIO $ do
>>         loop (y == ["yield"])
>>       threadDelay 1000
>> )
>>
>> With a sufficiently recent version of GHC, if you compile with -fno-omit-yields,
>> that should fix the problem.
>>
>> Edward
>>
>> Excerpts from Facundo Domínguez's message of Sat Oct 19 16:05:15 -0700 2013:
>>> Thanks. I just tried that. Unfortunately, it doesn't seem to help.
>>>
>>> Facundo
>>>
>>> On Sat, Oct 19, 2013 at 8:47 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
>>> > Hello Facundo,
>>> >
>>> > The reason is that you have compiled the program to be multithreaded, but it
>>> > is not running with multiple cores. Compile also with -rtsopts and then
>>> > pass +RTS -N2 to the program.
>>> >
>>> > Excerpts from Facundo Domínguez's message of Sat Oct 19 15:19:22 -0700 2013:
>>> >> Hello,
>>> >>    Below is a program that seems to block indefinitely with ghc in a
>>> >> multicore machine. This program has a loop that does not produce
>>> >> allocations, and I understand that this may grab one of the cores. The
>>> >> question is, why can't the other cores take the blocked thread?
>>> >>
>>> >> The program was compiled with:
>>> >>
>>> >> $ ghc --make -O -threaded test.hs
>>> >>
>>> >> and it is run with:
>>> >>
>>> >> $ ./test
>>> >>
>>> >> Program text follows.
>>> >>
>>> >> Thanks,
>>> >> Facundo
>>> >>
>>> >> --------
>>> >>
>>> >> import Control.Concurrent
>>> >> import Control.Monad
>>> >> import System.Environment
>>> >>
>>> >> main :: IO ()
>>> >> main = do
>>> >>   y <- getArgs
>>> >>   mv0 <- newEmptyMVar
>>> >>   mv1 <- newEmptyMVar
>>> >>   forkIO $ do
>>> >>     takeMVar mv0
>>> >>     putMVar mv1 ()
>>> >>     loop (y == ["yield"])
>>> >>   putMVar mv0 ()
>>> >>   takeMVar mv1
>>> >>
>>> >> loop :: Bool -> IO ()
>>> >> loop cooperative = go
>>> >>   where
>>> >>     go = when cooperative yield >> go
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