Re: [Haskell-cafe] Proof of a multi-threaded application

If you have multiple agents interacting with some structure and you want it to look like each of them is accessing it serialized, then STM is exactly what you want. The performance is always "in comparison to what"; you will likely get worse performance than the "best possible" implementation of your algorithm using locks, but that algorithm may be very difficult to write. My intuition is that if you spend put two programmers on a task, one using STM and one using locks, that the STM solution will perform better given the same amount of implementation time for any non-trivial problem. In addition, you can choose the level of "fine-grained-ness" in STM just as you can with locks, by choosing what things to make into TVars and what things to make pure. For example, this structure:

type STMMap a = TVar (Map k a)

will have much different performance than

type STMMap k a = TVar (STMNode a) data STMNode a = Tip | Branch !k a (STMMap k a) (STMMap k a)

and either one could be better depending on what you are doing. The former is like a "global lock" on the map, whereas the latter is analgous to "fine-grained" locks. But writing an algorithm using the "fine grained locks" version is a lot simpler to get right in STM than doing so with locks directly. In addition, STM's "optimistic concurrency" gives you the equivalent of multiple-reader single-writer locks for free; writing code with those directly is even more difficult to get right! -- ryan On Mon, Nov 17, 2008 at 10:35 PM, Ketil Malde wrote:

"Tim Docker" writes:

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My apologies for side-tracking, but does anybody have performance numbers for STM? I have an application waiting to be written using STM, boldly parallelizing where no man has parallelized before, but if it doesn't make it faster,

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Faster than what?

Faster than an equivalent non-concurrent program. It'd also be nice if performance was comparable lock-based implementation.

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Are you considering using STM just to make otherwise pure code run in parallel on multiple cores?

No, I have a complex structure that must be updated in non-predictable ways. That's what STM is for, no?

-k -- If I haven't seen further, it is by standing in the footprints of giants _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe