The speculation package

[Tags: bsd3, library]

A framework for safe, programmable, speculative parallelism, loosely based on http://research.microsoft.com/pubs/118795/pldi026-vaswani.pdf

This package provides speculative function application and speculative folds. And speculative STM actions take the place of the transactional rollback machinery from the paper.

For example:

spec g f a evaluates f g while forcing a, if g == a then f g is returned, otherwise f a is evaluated and returned. Furthermore, if the argument has already been evaluated, we skip the f g computation entirely. If a good guess at the value of a is available, this is one way to induce parallelism in an otherwise sequential task. However, if the guess isn't available more cheaply than the actual answer, then this saves no work and if the guess is wrong, you risk evaluating the function twice.

The best-case timeline looks like:

 [---- f g ----]
    [----- a -----]
 [-- spec g f a --]

The worst-case timeline looks like:

 [---- f g ----]
    [----- a -----]
                  [---- f a ----]
 [------- spec g f a -----------]

Compare these to the timeline of f $! a:

 [---- a -----]
              [---- f a ----]

specSTM provides a similar time table for STM actions, but also rolls back side-effects.

Changes in 0.3.0:


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Versions0.0.0, 0.0.1, 0.0.2, 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.5.0, 0.5.1, 0.6.0, 0.7.0, 0.8.0, 0.8.0.1, 0.8.0.2, 0.8.1.0, 0.8.2.0, 0.9.0.0, 1.0.0.0, 1.1.0.0, 1.2.0.0, 1.2.0.1, 1.2.0.2, 1.3, 1.4, 1.4.1, 1.4.1.1, 1.4.1.2, 1.5, 1.5.0.1, 1.5.0.2
Change logNone available
Dependenciesbase (>=4 && <6), parallel (==2.2.*), stm (==2.1.*) [details]
LicenseBSD3
Copyright(c) 2010 Edward A. Kmett
AuthorEdward A. Kmett
MaintainerEdward A. Kmett <ekmett@gmail.com>
Stabilityexperimental
CategoryConcurrency
Home pagehttp://github.com/ekmett/speculation
UploadedSun Jun 27 15:04:52 UTC 2010 by EdwardKmett
DistributionsLTSHaskell:1.5.0.2, NixOS:1.5.0.2, Stackage:1.5.0.2
Downloads4360 total (217 in last 30 days)
Votes
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StatusDocs uploaded by user
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Readme for speculation-0.3.0

speculation

This package provides speculative evaluation primitives for Haskell, very loosely based on the paper "Safe Programmable Speculative Parallelism" by Prabhu, Ramalingam, and Vaswani.

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.4622

Combinators

speculative function application

spec

spec :: Eq a => a -> (a -> b) -> a -> b

spec g f a evaluates f g while forcing a, if g == a then f g is returned. Otherwise f a is evaluated.

Furthermore, if the argument has already been evaluated, we avoid sparking the parallel computation at all.

If g is a good guess at the value of a, this is one way to induce parallelism in an otherwise sequential task.

However, if g isn't available more cheaply than a, then this saves no work, and if g is wrong, you risk evaluating the function twice. spec a f a = f $! a

The best-case timeline looks like: [---- f g ----] [----- a -----] [-- spec g f a --]

The worst-case timeline looks like: [---- f g ----] [----- a -----] [---- f a ----] [------- spec g f a -----------]

Compare these to the timeline of @f $! a@: [---- a -----] [---- f a ----]

specSTM

specSTM provides a similar compressed timeline for speculated STM actions, but also rolls back side-effects.

folds

A number of speculative folds are also provided.

These take an extra argument which is a function that guesses the result of of the fold up to a given point.

specFoldr

specFoldr :: (Foldable f, Eq b) => (Int -> b) -> (a -> b -> b) -> b -> f a -> b

Given a valid estimator g, 'specFoldr g f z xs yields the same answer as foldr' f z xs.

g n should supply an estimate of the value returned from folding over the /last/ n elements of the container.

As with spec, if the guess g n is accurate a reasonable percentage of the time and faster to compute than the fold, then this can provide increased opportunities for parallelism.

specFoldl

specFoldl :: (Foldable f, Eq b) => (Int -> b) -> (b -> a -> b) -> b -> f a -> b

specFoldl works similarly to foldl', except that g n should provide an estimate for the /first/ n elements.