[Haskell-cafe] dph question

[Daniel Fischer]

On Friday 15 October 2010 14:59:18, Warren Harris wrote:
> I trying to learn a bit about data parallel haskell, and started from
> the wiki page here:
> http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell. Two
> questions:
>
> The examples express the dot product as:
>
> dotp_double xs ys = sumP [:x *
> <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.
>> y | x <- xs | y <- ys:]
>
> Unless I'm missing something, shouldn't this actually be:
>
> dotp_double xs ys = sumP [:x *
> <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.
>> y | x <- xs, y <- ys:]
>

No, it's supposed to be a parallel list comprehension, the dot product is

sum $ zipWith (*) xs ys

and the

{ blah x y | x <- xs | y <- ys }

syntax (where {, } stand in for [, ] in parallel list comprehensions and 
for [:, :] in parallel array comprehensions) means

{ blah x y | (x,y) <- zip xs ys }

> Second, when I run Main with the prescribed 10000 element array,
> everything seems to work quite nicely. The task takes about 2 seconds on
> my 4 processor x86_64, and threadscope shows all processors nicely
> utilized. However, when bumping this to 100000 elements, rather than
> taking 10x longer as I expected, the process never terminates. During
> one run I even lost control of my machine and needed to do a hard reset.
> Are there known limits to the array sizes that can be handled with dph,
> or can someone suggest what might be going wrong here? Thanks,
>
> Warren