[Haskell-beginners] Thread Blocking

[Dean Herington & Elizabeth Lacey]

There's no basis on which to assume both alternative schedules are 
equally likely.  I would guess that, in your first example, there's 
enough start-up cost for a new thread that the main thread 
consistently gets to its putMVar first, whereas in the second 
example, main is waylaid with its putStrLn, allowing the forked 
thread to get there first consistently.

Dean

At 12:46 AM +0530 9/5/12, mukesh tiwari wrote:
>Hi Eugene
>Thank you for reply.
>
>On Wed, Sep 5, 2012 at 12:32 AM, Eugene Perederey 
><<mailto:eugene.perederey at gmail.com>eugene.perederey at gmail.com> 
>wrote:
>
>Why do you think main should block more than once?
>I see only two possible scenarios: the fun thread puts to mvar first
>thus blocking main,
>
>
>So at least in this case I should get thread blocked indefinitely in 
>an MVar operation
>
>
>or 10 is put into mvar in main, blocking the other thread indefinitely.
>
>
>or main will execute and fun thread will die. There are 50 - 50 
>chance for this ( assuming both are equally likely ). I did some 
>modification in my code and Now I am  consistently getting 
>"Concurrent: thread blocked indefinitely in an MVar operation"
>
>import Data.List
>import Control.Concurrent
>
>fun m = do
>    putMVar m 10
>    return ()
>
>
>main = do
>   m <- newEmptyMVar 
>   forkIO $ fun m
>   putStrLn "I am inside main"
>   putMVar m 10
>   return ()
>
>[mukesh.tiwari@ Programming]$ ghc-7.4.1 -threaded -fforce-recomp 
>Concurrent.hs
>[1 of 1] Compiling Main             ( Concurrent.hs, Concurrent.o )
>Linking Concurrent ...
>[mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>I am inside main
>Concurrent: thread blocked indefinitely in an MVar operation
>[mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>I am inside main
>Concurrent: thread blocked indefinitely in an MVar operation
>[mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>I am inside main
>Concurrent: thread blocked indefinitely in an MVar operation
>[mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>I am inside main
>Concurrent: thread blocked indefinitely in an MVar operation
>[mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>I am inside main
>Concurrent: thread blocked indefinitely in an MVar operation
>
>My question is why the outcome in first case is deterministic. Every 
>time the code executing  ( at least half the time main thread should 
>be blocked ) .
>
>Regards
>Mukesh Tiwari
>
>
>
>On 4 September 2012 11:54, mukesh tiwari 
><<mailto:mukeshtiwari.iiitm at gmail.com>mukeshtiwari.iiitm at gmail.com> 
>wrote:
>>  Hello All
>>  I was going trough Real World Haskell and it says "If we try to put a value
>>  into an MVar that is already full, our thread is put to sleep until another
>>  thread takes the value out". I wrote a simple code to block main
>>
>>  import Data.List
>>  import Control.Concurrent
>>
>>  fun m = do
>>     putMVar m 10
>>     return ()
>>
>>
>>  main = do
>>    m <- newEmptyMVar
>>    forkIO $ fun m
>>    putMVar m 10
>>    return ()
>>
>>  What I am expecting that main should be blocked at least couple of times
>>  but its behaving more deterministically.
>>  [mukesh.tiwari@ Programming]$ ghc-7.4.1 -threaded -fforce-recomp
>>  Concurrent.hs
>>  [1 of 1] Compiling Main             ( Concurrent.hs, Concurrent.o )
>>  Linking Concurrent ...
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>  > [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$ ./Concurrent  +RTS -N2
>>  [mukesh.tiwari@ Programming]$
>>
>>  I am expecting to get thread blocked indefinitely on MVar at least half the
>>  time. Could some one please tell me why this deterministic behavior ?
>>  Regards
>>  Mukesh Tiwari
>>
>>
>
>  > _______________________________________________
>>  Beginners mailing list
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>> 
>><http://www.haskell.org/mailman/listinfo/beginners>http://www.haskell.org/mailman/listinfo/beginners
>>
>
>
>
>--
>Best,
>Eugene Perederey
>
>
>
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