ekmett/speculation

Transcript

1. ekmett/speculation
   Mitsutoshi Aoe
   @ma0e
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2. ekmett/speculation
   Safe Programmable Speculative Parallelism, PLDI 2010
   http://hackage.haskell.org/package/speculation/
   https://github.com/ekmett/speculation
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3. Motivating examples
   Inherently sequential algorithms
   Lexing
   Huffman decoding
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4. Timeline - the best-case
   a
   f g
   spec g f a
   g == a
   spec :: Eq a 㱺 a → (a → b) → a → b)
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5. Timeline - the worst-case
   a
   f g
   spec g f a
   g /= a
   spec :: Eq a 㱺 a → (a → b) → a → b)
   f a
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6. Timeline - under high load
   a
   spec g f a
   g == a
   spec :: Eq a 㱺 a → (a → b) → a → b)
   f a
   f $! a
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7. Speculation in Haskell
   spec :: Eq a 㱺 a ! (a ! b) ! a ! b
   spec guess f a = speculation `par`
   if guess == a
   then speculation
   else f a
   where
   speculation = f guess
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8. Speculation in Haskell
   spec :: Eq a 㱺 a ! (a ! b) ! a ! b
   spec guess f a =
   | unsafeIsEvaluated a = f a
   | otherwise = speculation `par`
   if guess == a
   then speculation
   else f a
   where
   speculation = f guess
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9. Speculation in Haskell
   spec :: Eq a 㱺 a ! (a ! b) ! a ! b
   spec guess f a =
   | unsafeIsEvaluated a = f a
   | numCapabilities == 1 = f $! a
   | otherwise = speculation `par`
   if guess == a
   then speculation
   else f a
   where
   speculation = f guess
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10. Speculating STM
    specSTM :: Eq a
    㱺 STM a ! (a ! STM b) ! a ! STM b
    specSTM mguess f a = a `par` do
    guess ← mguess
    result ← f guess
    unless (guess == a) retry
    return result
    `orElse`
    f a
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11. Speculating STM
    specSTM :: Eq a
    㱺 STM a ! (a ! STM b) ! a ! STM b
    specSTM mguess f a = do
    sparks ← unsafeIOToSTM numSparks
    if sparks < numCapabilities
    then a `par` do
    guess ← mguess
    result ← f guess
    unless (guess == a) retry
    return result
    `orElse` f a
    else f $! a
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12. C.C.Speculation.Foldable
    foldr :: (Foldable f, Eq b)
    㱺 (Int ! b) ! (a ! b ! b)
    ! b ! f a ! b
    foldr g f z =
    extractAcc . F.foldr mf (Acc 0 z)
    where
    mf a (Acc n b) = Acc
    (n + 1)
    (spec (g n) (f a) b)
    data Acc a = Acc {-# UNPACK #-} !Int a
    extractAcc (Acc _ a) = a
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13. C.C.Speculation.Traversable
    traverse
    :: (Traversable t, Applicative f, Eq a)
    㱺 (Int ! a) ! (a ! f b) ! t a ! f (t b)
    traverse g f xs =
    runAccT (T.traverse go xs) 0
    where
    go a = AccT $ λi# ! acc
    (i# +# 1#)
    (spec (g (I# i)) f a)
    newtype AccT m a = AccT
    { runAccT :: Int# ! Acc (m a) }
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14. C.C.Speculation.List
    scanr :: Eq b 㱺 (Int ! b)
    ! (a ! b ! b) ! b ! [a] ! [b]
    scanr g f z
    = map extractAcc
    . L.scanr mf (Acc 0 z)
    where
    mf a (Acc n b) =
    let n’ = n + 1
    in Acc n’ (spec (g n’) (f a) b)
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15. C.C.Speculation.Class
    class MonadSpec m where
    specByM
    :: (a ! a ! Bool) ! a ! a ! m b
    specByM’
    :: (a ! a ! Bool) ! a ! a ! m b
    specM :: (MonadSpec m, Eq a)
    㱺 a ! a ! m a
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16. Speed optimizations
    unsafeIsEvaluated :: a ! Bool
    ekmett/tag-bits
    numSparks#
    :: State# s ! (# State# s, Int# #)
    since GHC 7 (#4167)
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17. Demo
    Speculative lexing from “Parallelizing the Web Browser”
    https://gist.github.com/maoe/5279239
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