编写一个并行数组Haskell表达式,在带有修复和加速的CPU和GPU上运行

Question

修复和加速API相似性

Haskell repa库用于在CPU上自动并行计算数组.加速库是GPU上的自动数据并行.API非常相似,具有相同的N维数组表示.人们甚至可以切换与加速,维修服务阵列fromRepa和toRepa在Data.Array.Accelerate.IO:

fromRepa :: (Shapes sh sh', Elt e) => Array A sh e -> Array sh' e
toRepa   :: Shapes sh sh'          => Array sh' e  -> Array A sh e

有多个后端用于加速,包括LLVM,CUDA和FPGA(参见http://www.cse.unsw.edu.au/~keller/Papers/acc-cuda.pdf的图2 ).虽然图书馆似乎没有得到维护,但我发现了一个加速的后端.鉴于修复和加速编程模型是相似的,我希望在它们之间有一种优雅的切换方式,即一次写入的函数可以用repa的R.computeP执行,或者用加速的后端执行,例如使用CUDA 运行函数.

两个非常相似的功能:修复和加速南瓜

采用简单的图像处理阈值功能.如果灰度像素值小于50,则将其设置为0,否则保留其值.这是它对南瓜的作用:

以下代码介绍了修复和加速实现:

module Main where

import qualified Data.Array.Repa as R
import qualified Data.Array.Repa.IO.BMP as R
import qualified Data.Array.Accelerate as A
import qualified Data.Array.Accelerate.IO as A
import qualified Data.Array.Accelerate.Interpreter as A

import Data.Word

-- Apply threshold over image using accelerate (interpreter)
thresholdAccelerate :: IO ()
thresholdAccelerate = do
  img <- either (error . show) id `fmap` A.readImageFromBMP "pumpkin-in.bmp"
  let newImg = A.run $ A.map evalPixel (A.use img)
  A.writeImageToBMP "pumpkin-out.bmp" newImg
    where
      -- *** Exception: Prelude.Ord.compare applied to EDSL types
      evalPixel :: A.Exp A.Word32 -> A.Exp A.Word32
      evalPixel p = if p > 50 then p else 0

-- Apply threshold over image using repa
thresholdRepa :: IO ()
thresholdRepa = do
  let arr :: IO (R.Array R.U R.DIM2 (Word8,Word8,Word8))
      arr = either (error . show) id `fmap` R.readImageFromBMP "pumpkin-in.bmp" 
  img <- arr
  newImg <- R.computeP (R.map applyAtPoint img)
  R.writeImageToBMP "pumpkin-out.bmp" newImg
  where
    applyAtPoint :: (Word8,Word8,Word8) -> (Word8,Word8,Word8)
    applyAtPoint (r,g,b) =
        let [r',g',b'] = map applyThresholdOnPixel [r,g,b]
        in (r',g',b')
    applyThresholdOnPixel x = if x > 50 then x else 0

data BackendChoice = Repa | Accelerate

main :: IO ()
main = do
  let userChoice = Repa -- pretend this command line flag
  case userChoice of
    Repa       -> thresholdRepa
    Accelerate -> thresholdAccelerate

问题:我只能写一次吗？

实现thresholdAccelerate和thresholdRepa非常相似.是否有一种优雅的方法来编写一次阵列处理功能,然后以编程方式选择多路CPU(修复)或GPU(加速)？我可以考虑选择我的导入,根据我是否需要CPU或GPU即导入Data.Array.Accelerate.CUDA或Data.Array.Repa执行类型的操作Acc a:

run :: Arrays a => Acc a -> a

或者,使用类型类,例如:

main :: IO ()
main = do
  let userChoice = Repa -- pretend this is a command line flag
  action <- case userChoice of
    Repa       -> applyThreshold :: RepaBackend ()
    Accelerate -> applyThreshold :: CudaBackend ()
  action

或者,对于我希望为CPU和GPU表达的每个并行阵列功能,我必须实现它两次 - 一次使用repa库,再一次使用加速库？

Answer 1

简短的回答是,目前,您不幸需要编写两个版本.

但是,我们正致力于对Accelerate的CPU支持,这将消除对代码的Repa版本的需求.特别是,Accelerate最近获得了一个新的基于LLVM的后端,同时针对GPU和CPU:https://github.com/AccelerateHS/accelerate-llvm

这个新的后端仍然是不完整的,错误的和实验性的,但我们正计划将其作为当前CUDA后端的可行替代方案.