[Haskell-cafe] Concurrency question

Donald Bruce Stewart dons at cse.unsw.edu.au
Sun Sep 4 21:37:55 EDT 2005

> Donald Bruce Stewart wrote:
> >Maybe your loop does no allocations, so the scheduler can't get in and do a
> >context switch.  You could put the computation in an external program, and 
> >run
> >it over a fork, using unix signals in the external program to kill the
> >computation after a period of time. 
> I thought about doing that, but function is closely connected with the 
> rest of the program. Running it in another process would require some 
> parsing of its arguments and I want circumvent these difficulties.

Ah, I've found another example. This function attempts to run an
expensive computation. If it doesn't return within a given time, a cheap
function is used instead. This was mostly written by Stefan Wehr:

    watchdogIO :: Int  -- milliseconds
             -> IO a   -- expensive computation
             -> IO a   -- cheap computation
             -> IO a

    watchdogIO millis expensive cheap = do
           mvar <- newEmptyMVar
           tid1 <- forkIO $ do x <- expensive
                               x `seq` putMVar mvar (Just x)
           tid2 <- forkIO $ do threadDelay (millis * 1000)
                               putMVar mvar Nothing
           res <- takeMVar mvar
           case res of
             Just x -> do
                    info ("EXPENSIVE was used")
                    killThread tid2 `catch` (\e -> warn (show e))
                    return x
             Nothing -> do
                    info ("WATCHDOG after " ++ show millis ++ " milliseconds")
                    killThread tid1 `catch` (\e -> warn (show e))

-- Don

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