[Haskell-cafe] ANNOUNCE: parallel-188.8.131.52
ekmett at gmail.com
Wed Nov 18 17:09:09 EST 2009
I love the new Eval Applicative!
Out of idle curiosity, can parListN be generalized to parTraverseN similar
to how parList was generalized to parTraverse? Similarly, parListChunk?
On Wed, Nov 18, 2009 at 9:21 AM, Simon Marlow <marlowsd at gmail.com> wrote:
> I've just uploaded parallel-184.108.40.206 to Hackage. If you're using Strategies
> at all, I'd advise updating to this version of the parallel package. It's
> not completely API compatible, but if you're just using the supplied
> Strategies such as parList, the changes should be relatively minor.
> The Haddock docs include a full description of the changes, reproduced
> below. Haddock docs are also here, until Hackage catches up:
> The Strategy-using programs I've tried so far go faster. Enjoy!
> Version 1.x
> The original Strategies design is described in
> and the code was written by
> Phil Trinder, Hans-Wolfgang Loidl, Kevin Hammond et al.
> Version 2.x
> Later, during work on the shared-memory implementation of
> parallelism in GHC, we discovered that the original formulation of
> Strategies had some problems, in particular it lead to space leaks
> and difficulties expressing speculative parallelism. Details are in
> the paper \"Runtime Support for Multicore Haskell\" <
> This module has been rewritten in version 2. The main change is to
> the 'Strategy a' type synonym, which was previously @a -> Done@ and
> is now @a -> a at . This change helps to fix the space leak described
> in \"Runtime Support for Multicore Haskell\". The problem is that
> the runtime will currently retain the memory referenced by all
> sparks, until they are evaluated. Hence, we must arrange to
> evaluate all the sparks eventually, just in case they aren't
> evaluated in parallel, so that they don't cause a space leak. This
> is why we must return a \"new\" value after applying a 'Strategy',
> so that the application can evaluate each spark created by the
> The simple rule is this: you /must/ use the result of applying
> a 'Strategy' if the strategy creates parallel sparks, and you
> should probably discard the the original value. If you don't
> do this, currently it may result in a space leak. In the
> future (GHC 6.14), it will probably result in lost parallelism
> instead, as we plan to change GHC so that unreferenced sparks
> are discarded rather than retained (we can't make this change
> until most code is switched over to this new version of
> Strategies, because code using the old verison of Strategies
> would be broken by the change in policy).
> The other changes in version 2.x are:
> * Strategies can now be defined using a convenient Applicative
> type Eval. e.g. parList s = unEval $ traverse (Par . s)
> * 'parList' has been generalised to 'parTraverse', which works on
> any 'Traversable' type.
> * 'parList' and 'parBuffer' have versions specialised to 'rwhnf',
> and there are transformation rules that automatically translate
> e.g. @parList rwnhf@ into a call to the optimised version.
> * 'NFData' is deprecated; please use the @DeepSeq@ class in the @deepseq@
> package instead. Note that since the 'Strategy' type changed, 'rnf'
> is no longer a 'Strategy': use 'rdeepseq' instead.
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