As far as I lnow the function 'unsafeIOToSTM' is not transactional in nature
- IO actions will be performed immediately and are not rolled back, and are
then re-performed on retry.
On Oct 25, 2011 12:49 PM, "Ben Franksen"
I have an application in mind where concurrent access to large arrays (up to millions of elements) of mostly small elements (Int or Double) is common. Typical access patterns would be chunk-wise, i.e. reads or writes from index n up to index m. Imagine stuff like images, scientific data, etc.
All this suggests that Control.Concurrent.STM.TArray, in its current implementation, is not appropriate. Quoting the docs:
"It is currently implemented as Array ix (TVar e), but it may be replaced by a more efficient implementation in the future (the interface will remain the same, however)."
An array of TVars is certainly *much* too inefficient for what I have in mind w.r.t. both memory and cpu time. In fact I had already decided to use Data.Vector.Unboxed from the vector package.
I see that Data.Vector.Unboxed.Mutable provides
slice :: Unbox a => Int -> Int -> MVector s a -> MVector s a Yield a part of the mutable vector without copying it.
which is almost what I need... Can I use this together with unsafeIOToSTM internally inside a library to provide shared transactional access to an IOArray? The docs warn that using unsafeIOToSTM is (obviously) "highly dangerous", but for what I have in mind the listed problems are not an issue:
* running the IO code multiple times is ok * aborting is ok, too * inconsistent views are ok, too
The main question is: does the STM transaction actually "see" that I changed part of the underlying array, so that the transaction gets re-tried? Or do I have to implement this manually, and if yes: how?
Has anyone done something like that before? (If you tried this and found it doesn't work, please tell me, it would save me a lot of work repeating the effort).
Is someone working on providing a more efficient version of TArray?
Would it help if I said I'd be a happy user of a better TArray? ;-)
If what I sketched above is infeasible (I would not be surprised if it was, I haven't yet spent much effort trying it out), what other options do I have? Is there an internal API for the STM stuff, i.e. a C header file or something which would make it possible to add efficient TArrays?
Or should I use a high-level approach, something like a Data.Sequence.Seq of medium sized chunks (TVar (IOVector e))?
Any comments are highly appreciated!
Cheers Ben
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