[Haskell-cafe] dph question

[Warren Harris]

Got it - thanks. Any idea about the run-away process problem? Thanks,

Warren

On Fri, Oct 15, 2010 at 9:32 AM, Daniel Fischer <daniel.is.fischer at web.de>wrote:

> 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
>
>
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