[Haskell-cafe] Barrier implementation

[Lemmih]

On 12/16/05, Peter Eriksen <s022018 at student.dtu.dk> wrote:
> Greeting,
>
> Something is not working for me, and I could use some more eyes on this.
> What I'm trying to accomplish is to implement a simpel barrier for ten
> worker threads (id = 0..9) using STM.  With or without the barrier, the
> program produces an unordered interleaving of the output from the
> workers.  Here's what I get with the program below:
>
> $ ghc --make Main.lhs
> $ a.out
> 0134568027913457896012579026813423904671238455702468159367839684012570279134685049137825901642375689134057892610462578903156012389473268457910267801345923924567801304689235714013679458256702465913878...
>
> And here's what I get without the line "atomically $ barrier tv id":
>
> $ a.out
> 1249056782934567210845619720538461975203698469175203469850123485076912348579406123894625738942106381592740631859274163841092315768491302578416930728254169302785693024917853029640217390856490...
>
> The first run should've been something like:
> 012345678901234567890123456789012345...
> since each worker thread 0..9 should write its id out once per
> iteration,
> and the workers should iterate in sync.
>
> Here's the code:
>
> > module Main where
> >
> > import Control.Concurrent
> > import Control.Concurrent.STM
> > import System.Random
> >
> > worker :: Int -> TVar Int -> IO ()
> > worker id tv = do
> >       sleepingTime <- randomRIO (0, 50000)
> >       threadDelay sleepingTime
> >       putStr $ show id

You're printing the ID after a random sleep. Shouldn't be a big
surprise that the output will be shuffled.

> >       atomically $ barrier tv id

If you move 'putStr $ show id' down below the barrier then it'll
behave like you want it to.

> >       worker id tv
>
> Each worker sleeps for some time, then outputs its id and waits at
> the barrier for all the other workers to finish their sleep+output.
>
> > barrier :: TVar Int -> Int -> STM ()
> > barrier tv id = do
> >       passed <- readTVar tv
> >       if (passed `mod` 10 == id)
> >               then writeTVar tv (passed+1)
> >               else retry
>
> The barrier is simply a global variable, tv, which holds the number of
> times any worker passed the barrier.  Now, a worker may only pass the
> barrier iff the worker with an id one less just passed, or else it
> should block.
>
> > main :: IO ()
> > main = do
> >       tv <- atomically $ newTVar 0
> >       for [0..9] $ \i -> forkIO $ worker i tv
> >       threadDelay (10*10^6)
> >
> > for = flip mapM_


--
Friendly,
  Lemmih