Gwern Branwen wrote:
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So, the Hint library was recently updated and I was editing Mueval to run with (i386) 6.10, when I discovered that for some reason, DoS'ing expressions were succeeding in rendering my machine unusable. Eventually, I discovered that my watchdog thread didn't seem to be running. But with +RTS -N2 -RTS all my tests did pass!
Here's a simple example of what I mean; the following is basically a very lightly adapted version of the actual Mueval code:
- ---------------- import Control.Concurrent (forkIO, killThread, myThreadId, threadDelay, throwTo, yield, ThreadId) import System.Posix.Signals (sigXCPU, installHandler, Handler(CatchOnce)) import Control.OldException (Exception(ErrorCall))
main :: IO () main = do tid ThreadId -> IO () watchDog tout tid = do installHandler sigXCPU (CatchOnce $ throwTo tid $ ErrorCall "Time limit exceeded.") Nothing forkIO $ do threadDelay (tout * 100000) -- Time's up. It's a good day to die. throwTo tid (ErrorCall "Time limit exceeded") yield -- give the other thread a chance killThread tid -- Die now, srsly. error "Time expired" return () -- Never reached. Either we error out in -- watchDog, or the evaluation thread finishes.
This particular example illustrates a bug in 6.10.1 that we've since fixed: http://hackage.haskell.org/trac/ghc/ticket/2783 However in general you can still write expressions that don't allocate anything (e.g. nfib 1000), and your watchdog thread won't get a chance to run unless there's a spare CPU available (+RTS -N2). Cheers, Simon