[petsc-dev] major changes to DA in PETSc-dev

Barry Smith bsmith at mcs.anl.gov
Mon Oct 18 08:23:03 CDT 2010

On Oct 18, 2010, at 8:08 AM, Lisandro Dalcin wrote:

> On 18 October 2010 14:26, Jed Brown <jed at 59a2.org> wrote:
>> On Mon, Oct 18, 2010 at 03:25, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>> DAXXX() routines are now either DMXXX() or DMDAXXX() see petscdm.h for
>>> details
>> I'm a little concerned about this because all of these have implicit
>> downcasting with no RTTI, so they are all totally unsafe.  When DA was a
>> distinct type, you couldn't call DAGetCorners with a DM that was not also a
>> DA.  Now the user has a generic DM, but almost every call they will make is
>> a memory error if their DM is not DMDA.  It could be made runtime-safe by
>> putting RTTI guards at the top of every public DMDA* function, but runtime
>> type checking is not as good as compile-time if the latter is reasonably
>> achievable.
>> Note that PETSc itself (for multigrid, DD, etc) works almost exclusively
>> with the DM API and should not need to call the DMDA API directly
>> (specialization is via DMOps).  In contrast, user code is specialized to a
>> particular implementation of DM, and pretty much always uses the specialized
>> API.  The old API was safer because the API that the user called was
>> explicitly specialized, rather than implicitly downcasted from DM.
>> Of course I agree that DM organization was messy, but I'm not convinced that
>> this change is an improvement for the user since it gives up a lot of safety
>> and offers little in return.
>> Jed
> In a private chat discussion, Jed said this:
> With DMs, the user will never have a discretization that works with a
> generic DM, they will always reference explicitly which DM they are
> using.
> Because DA is so different from an unstructured mesh or a particle
> method, they have to choose which method they are using.
> Very different from matrices where they don't have to choose until runtime.
> I totally agree.

  You are both wrong on this issue. In 1995 most people thought the flexibility was totally unneeded and actually harmful for matrices; they were wrong. In five years you will realize that you were wrong on the issue for DMs in 2010. Just wait, this is going to be damn powerful.


> -- 
> Lisandro Dalcin
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