Thunk memory leak as a result of map function

Question

I am writing a multi-threaded program that makes quite a use of mvars; in this case I have a thread that periodically changes list inside an mvar. Unfortunately, there is a thunk memory leak. There seems to be aproblem that the 'map id' (in real program I use something else than id) function leaks. I just cannot find a way how to avoid that - I was playing with 'seq' with no result. What is the right way to fix the leak?

upgraderThread :: MVar [ChannelInfo] -> IO ()
upgraderThread chanMVar = forever job
    where
        job = do
            threadDelay 1000
            vlist <- takeMVar chanMVar
            let reslist = map id vlist
            putMVar chanMVar reslist

score 3 · Accepted Answer · answered Jul 08 '11 at 22:11

3

After a few more tries, this one seems to work:

upgraderThread chanMVar = forever job
    where
        job = do
            threadDelay 1000
            vlist <- takeMVar chanMVar
            let !reslist = strictList $ map id vlist
            putMVar chanMVar reslist

        strictList xs = if all p xs then xs else []
            where p x = x `seq` True

answered Jul 08 '11 at 22:11

ondra

9,122
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6

The pattern you're using in `strictList` is generalized by `deepseq`; see http://hackage.haskell.org/packages/archive/deepseq/latest/doc/html/Control-DeepSeq.html – acfoltzer Jul 08 '11 at 23:10

score 3 · Answer 2 · answered Jul 12 '11 at 15:42

3

Besides the space leak, the earlier version may also have a "time leak" in that the unevaluated thunks placed into the mvar may get evaluated by the receiving thread instead of the sending one, possibly destroying any intended parallelism.

answered Jul 12 '11 at 15:42

solrize

31
1

Thunk memory leak as a result of map function

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