dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Aug 25, 2011 at 07:45:49AM -0400, Garth N. Wells wrote:

> > Sure, but I would claim SAX scales better.
>
> In terms of memory, yes.
>
> It is not sufficiently scalable to be a total solution. Plus it's too slow.

Agree.

> > Wouldn't it be better to
> > just use one of DOM or SAX?
>
> Maybe. A SAX implementation is considerably more complex. The new
> implementation reserves this complexity for a possibly critical case and
> localises the complexity of the code. The old code was very complex and
> less localised.
>
> The locality means that it's no big deal to have a simple DOM
> implementation for the majority of cases next to a more complex SAX
> implementations for special cases. There is no point in the size and
> complexity of a SAX parser for simple cases, e.g. reading parameter files.

Agree, but see below.

> > Either we use SAX all the way if it gives
> > better performance than DOM,
>
> It doesn't give better performance. We discussed this before. Without
> checking the archive, I recall that the DOM implementation was about 50
> times faster for large data sets than the old SAX implementation.
>
> > or we use DOM all the way as a solution
> > for medium sized problems and complement with HDF5 for large scale
> > problems. Having DOM + SAX + HDF5 seems messy.
> >
>
> This may happen, but the fact is that we don't have HDF5 in place yet.
>
> >>>>> What is the difference between XMLLocalMeshData and
> >>>>> XMLLocalMeshDataDistributed etc.
> >>>>>
> >>>>
> >>>> Initially I planned to use DOM for all, but as outlined above decided
> >>>> after some testing to retain SAX for meshes (but update to SAX2, since
> >>>> the libxml2 SAX parser is deprecated and has memory leaks). Hence,
> >>>> XMLLocalMeshData uses DOM and XMLLocalMeshDataDistributed uses SAX. So
> >>>> far I've kept the DOM version since it's easy to code and could be
> >>>> useful when reading non-distributed meshes on each process (which may
> >>>> differ on different processes).
> >>>
> >>> I don't understand the difference between XMLLocalMeshData and
> >>> XMLLocalMeshDataDistributed. Is XMLLocalMeshDataDistributed doing now
> >>> what XMLLocalMeshData did before?
> >>>
> >>
> >> Yes, but updated to SAX2 (which was very painful).
> >>
> >> The 'new' XMLLocalMeshData is a DOM version. It could be removed.
> >
> > Or kept if we will add HDF5 anyway as a more scalable solution.
> >
>
> Again, it may be desirable to keep a SAX parser for reading meshes in
> parallel since a mesh is the most likely large data structure to be
> created externally, and the most complex. HDF5 would require a user to
> supply a binary mesh file rather than an XML file. Most other large data
> sets are created internally, and the read and written. In this case,
> HDF5 will be fine.

How about using DOM everywhere and reserve the use of SAX for an
XML->HDF5 converter?

--
Anders