RE: [Haskell-cafe] Implementing computations with timeout
On 07 January 2005 20:00, Sebastian Sylvan wrote: > On Fri, 7 Jan 2005 20:56:42 +0100, Sebastian Sylvan > <[EMAIL PROTECTED]> wrote: >> On Fri, 07 Jan 2005 15:31:10 +0200, Einar Karttunen >> wrote: >>> Hello >>> >>> What is the best way of doing an computation with a timeout? >> >> I like the approach taken in "Tackling the ackward squad": >> > > I should also state that this isn't safe when it comes to asynchronous > exceptions. > If one were to raise an exception in a timeout'd computation it would > simply abort the takeMVar which means the two child processes won't > get killed. Yes, making a timeout combinator that (a) can be nested and (b) is invisible with respect to asynchronous exceptions is quite a challenge. I'm not sure that we managed to do it. STM will almost certainly make it easier, though. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Implementing computations with timeout
On 07 January 2005 22:17, Tomasz Zielonka wrote: > On Fri, Jan 07, 2005 at 11:00:27PM +0100, Tomasz Zielonka wrote: >> Hmmm, TMVar's seem to be significantly (ie. 6 times) faster than >> MVars in some simple tests :) >> >> Is it expected? > > If it is, we can reimplement MVars using STM, when STM becomes stable > :) No, TMVars are quite a bit slower than MVars if you use the basic takeMVar/putMVar ops. However, if you use the safer withMVar/modifyMVar combinators, then you'll probably find that performance is about the same. A simple test I did comparing TChan with Chan came up with roughly the same performance, and STM isn't really tuned yet. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, Jan 07, 2005 at 11:00:27PM +0100, Tomasz Zielonka wrote: > Hmmm, TMVar's seem to be significantly (ie. 6 times) faster than MVars > in some simple tests :) > > Is it expected? If it is, we can reimplement MVars using STM, when STM becomes stable :) Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, Jan 07, 2005 at 04:47:12PM +0100, Tomasz Zielonka wrote: > On Fri, Jan 07, 2005 at 04:55:05PM +0200, Einar Karttunen wrote: > > if we use IO as the signature then using the TMVar has few advantages over > > using an MVar. > > Yes, I think you are right here. Hmmm, TMVar's seem to be significantly (ie. 6 times) faster than MVars in some simple tests :) Is it expected? Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, 7 Jan 2005 20:56:42 +0100, Sebastian Sylvan <[EMAIL PROTECTED]> wrote: > On Fri, 07 Jan 2005 15:31:10 +0200, Einar Karttunen > wrote: > > Hello > > > > What is the best way of doing an computation with a timeout? > > I like the approach taken in "Tackling the ackward squad": > I should also state that this isn't safe when it comes to asynchronous exceptions. If one were to raise an exception in a timeout'd computation it would simply abort the takeMVar which means the two child processes won't get killed. /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, 07 Jan 2005 15:31:10 +0200, Einar Karttunen wrote: > Hello > > What is the best way of doing an computation with a timeout? I like the approach taken in "Tackling the ackward squad": First a funcion which will "race" two IO computations against each other, returning the "winning" result. This is accomplished by simply spawning a thread for each computation which does nothing but evalute the IO computation and putting the result in an MVar. Then the MVar is read (this will lock until there is something in the MVar to read), when finally there is something to read it will obviously contain the result of the IO computation that completed first. Then both of the spawned threads are killed (one of them is already dead at this point) via throwTo. parIO :: IO a -> IO a -> IO a parIO a1 a2 = do m <- newEmptyMVar ; c1 <- forkIO (child m a1) ; c2 <- forkIO (child m a2) ; r <- takeMVar m ; throwTo c1 Kill ; throwTo c2 Kill ; return r where child m a = do r <- a putMVar m r Next we simply race the IO computation to be "timed out" against a thread which delays and then returns Nothing. timeout :: Int -> IO a -> IO (Maybe a) timeout n a = parIO a1 a2 where a1 = do r <-a return (Just r) a2 = do threadDelay n return Nothing /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, Jan 07, 2005 at 04:55:05PM +0200, Einar Karttunen wrote: > if we use IO as the signature then using the TMVar has few advantages over > using an MVar. Yes, I think you are right here. Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, Jan 07, 2005 at 04:55:05PM +0200, Einar Karttunen wrote: > > Isn't this buggy if fun just keeps working without throwing an exception > or using retry? I meant wholly inside STM There is not that much that you can do inside STM. This may be a problem if you want to wait for a genuine IO action. Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, Jan 07, 2005 at 02:57:19PM +0100, Tomasz Zielonka wrote: > My guess is it would be something like this, however you may want to do it > differently to get better compositionality (withTimeout returns an IO action, > not a STM action): Maybe this will suffice, but I don't know if the delay thread will be garbage collected. import Control.Concurrent import Control.Concurrent.STM import Monad (when) makeDelay :: Int -> IO (STM ()) makeDelay time = do v <- atomically (newTVar False) forkIO $ do threadDelay time atomically (writeTVar v True) return $ readTVar v >>= \b -> when (not b) retry withTimeout :: Int -> STM a -> IO (Maybe a) withTimeout time fun = do delay <- makeDelay time atomically (fmap Just fun `orElse` (delay >> return Nothing)) Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
Tomasz Zielonka <[EMAIL PROTECTED]> writes: > import Control.Concurrent (forkIO, threadDelay) > import Control.Concurrent.STM > > withTimeout :: Int -> STM a -> IO (Maybe a) > withTimeout time fun = do > mv <- atomically newEmptyTMVar > tid <- forkIO $ do > threadDelay time > atomically (putTMVar mv ()) > x <- atomically (fmap Just fun `orElse` (takeTMVar mv >> return > Nothing)) > killThread tid > return x Isn't this buggy if fun just keeps working without throwing an exception or using retry? I meant wholly inside STM - if we use IO as the signature then using the TMVar has few advantages over using an MVar. - Einar Karttunen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing computations with timeout
On Fri, Jan 07, 2005 at 03:31:10PM +0200, Einar Karttunen wrote: > Hello > > What is the best way of doing an computation with a timeout? > > A naive implementation using two threads is easy to create - but > what is the preferred solution? > > withTimeout :: forall a. Int -> IO a -> IO (Maybe a) > btw How would I do the same with the new STM abstraction? My guess is it would be something like this, however you may want to do it differently to get better compositionality (withTimeout returns an IO action, not a STM action): import Control.Concurrent (forkIO, threadDelay) import Control.Concurrent.STM withTimeout :: Int -> STM a -> IO (Maybe a) withTimeout time fun = do mv <- atomically newEmptyTMVar tid <- forkIO $ do threadDelay time atomically (putTMVar mv ()) x <- atomically (fmap Just fun `orElse` (takeTMVar mv >> return Nothing)) killThread tid return x PS. STM is cool! :) Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
