dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Jan 26, 2010 at 12:26:42PM +0000, Garth N. Wells wrote:
>
>
> Anders Logg wrote:
> > I think I found a solution for the const Array<const double> thing.
> > It involves const_cast but with a little twist that I think makes it a
> > good const_cast.
> >
> > I first tried copying data instead of copying the array but that had a
> > noticeable impact on run-time. I've been running the electrodynamics
> > demo to check.
> >
> > When copying data, the timing went from
> >
> >   real 0m48.910s
> >   user 0m46.150s
> >   sys  0m2.070s
> >
> > to
> >
> >   real 0m49.756s
> >   user 0m47.540s
> >   sys  0m2.180s
> >
> > This timing is from using a loop over the array. Using memcpy instead
> > was even worse:
> >
> >   real 0m50.502s
> >   user 0m48.490s
> >   sys  0m1.990s
> >
> > I've now implemented another fix using const_cast and it runs even
> > faster than before which is strange:
> >
> >   real 0m47.761s
> >   user 0m45.800s
> >   sys  0m1.920s
> >
>
> We use boost::shared_ptr in Array, which may have non-negligible
> creation overhead when handling small arrays. I can't remember if we
> create Arrays repeatedly in the eval process, but if we do it might that
> the shared_ptr creation dominates and that this hides the cost of copying.
>
> I was planning at some point to check the performance of Array using
> shared and plain pointers.
>
> > Anyway, the suggested fix is the following:
> >
> > 1. Make the x member variable const in Data.h:
> >
> >   const Array<double> x;
> >
> > 2. Use a double const_cast when copying the array:
> >
> >   const_cast<Array<double>*>(&(this->x))->update(gdim, const_cast<double*>(x));
> >
> > This is an honest const_cast. The member variable x is const so it
> > "can't" be changed so the input data double* x remains const in
> > practice.
> >
> > If there are no objections, I can push the change which will change
> > the x variable in all eval functions from
> >
>
> Go ahead.
>
> Garth

ok.

--
Anders

Attachment: signature.asc
Description: Digital signature