DA question

[Barry Smith]

On Apr 29, 2008, at 2:27 PM, Amit.Itagi at seagate.com wrote:

> Barry,
>
> Can this be achieved using SDA ? I am working with regular arrays, and
> doing only explicit updates.

    Yes. SDA actually uses the DA, it just hides the Vec concept from  
the user.

    Barry

>
>
> Thanks
>
> Rgds,
> Amit
>
> owner-petsc-users at mcs.anl.gov wrote on 04/29/2008 01:28:17 PM:
>
>>
>>   If you are running a true explicit scheme then you have no need
>> to ever have a "global representation" at each time step. In this
>> case you can use DALocalToLocalBegin() then DALocalToLocalEnd()
>> and pass the same vector in both locations. This will update the  
>> ghost
>> points but WILL NOT do any copy of the local data since it is already
>> in the correct locations.
>>
>>   Barry
>>
>> On Apr 29, 2008, at 8:54 AM, Amit.Itagi at seagate.com wrote:
>>
>>> Hi,
>>>
>>> I spent some more time understanding DA's, and how DA's should serve
>>> my
>>> purpose. Since in the time domain calculation, I will have to
>>> scatter from
>>> the global vector to the local vector and vice-versa at every
>>> iteration
>>> step, I have some follow-up questions.
>>>
>>> 1) Does the scattering involve copying the part stored on the local
>>> node as
>>> well (i.e. part of the local vector other than the ghost values), or
>>> is the
>>> local part just accessed by reference ? In the first scenario, this
>>> would
>>> involve allocating twice the storage for the local part. Also, does
>>> the
>>> scattering of the local part give a big hit in terms of CPU time ?
>>>
>>> 2) In the manual, it says "In most cases, several different vectors
>>> can
>>> share the same communication information (or, in other words, can
>>> share a
>>> given DA)" and "PETSc currently provides no container for multiple
>>> arrays
>>> sharing the same distributed array communication; note, however,
>>> that the
>>> dof parameter handles many cases of interest". I am a bit confused.
>>> Suppose
>>> I have two arrays having the same layout on the regular grid, can I
>>> store
>>> the first array data on one vector, and the second array data on the
>>> second
>>> vector (and have a DA with dof=1, instead of a DA with dof=2), and
>>> be able
>>> to scatter and update the first vector without scattering/updating  
>>> the
>>> second vector ?
>>>
>>> Thanks
>>>
>>> Rgds,
>>> Amit
>>>
>>> owner-petsc-users at mcs.anl.gov wrote on 04/09/2008 04:09:59 PM:
>>>
>>>> Hi Amit,
>>>>
>>>> Why do you need two staggered grids? I do EM finite difference
>>>> frequency
>>>> domain modeling on a staggered grid using just one DA. Works
>>>> perfectly
>>> fine.
>>>> There are some grid points that are not used, but you just set them
>>>> to
>>> zero
>>>> and put a 1 on the diagonal of the coefficient matrix.
>>>>
>>>>
>>>> Randy
>>>>
>>>>
>>>> Amit.Itagi at seagate.com wrote:
>>>>> Hi Berend,
>>>>>
>>>>> A detailed explanation of the finite difference scheme is given
>>>>> here :
>>>>>
>>>>> http://en.wikipedia.org/wiki/Finite-difference_time-domain_method
>>>>>
>>>>>
>>>>> Thanks
>>>>>
>>>>> Rgds,
>>>>> Amit
>>>>>
>>>>>
>>>>>
>>>>>
>>>
>>>>>           Berend van Wachem
>>>
>>>>>           <berend at chalmers.
>>>
>>>>>           se>
>>> To
>>>>>           Sent by:                  petsc-users at mcs.anl.gov
>>>
>>>>>           owner-petsc-users
>>> cc
>>>>>           @mcs.anl.gov
>>>
>>>>>           No Phone Info
>>> Subject
>>>>>           Available                 Re: DA question
>>>
>>>>>
>>>
>>>>>
>>>
>>>>>           04/09/2008 02:59
>>>
>>>>>           PM
>>>
>>>>>
>>>
>>>>>
>>>
>>>>>           Please respond to
>>>
>>>>>           petsc-users at mcs.a
>>>
>>>>>                nl.gov
>>>
>>>>>
>>>
>>>>>
>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Dear Amit,
>>>>>
>>>>> Could you explain how the two grids are attached?
>>>>> I am using multiple DA's for multiple structured grids glued
>>>>> together.
>>>>> I've done the gluing with setting up various IS objects. From the
>>>>> multiple DA's, one global variable vector is formed. Is that what
>>>>> you
>>>>> are looking for?
>>>>>
>>>>> Best regards,
>>>>>
>>>>> Berend.
>>>>>
>>>>>
>>>>> Amit.Itagi at seagate.com wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Is it possible to use DA to perform finite differences on two
>>> staggered
>>>>>> regular grids (as in the electromagnetic finite difference time
>>>>>> domain
>>>>>> method) ? Surrounding nodes from one grid are used to update the
>>>>>> value
>>> in
>>>>>> the dual grid. In addition, local manipulations need to be done
>>>>>> on the
>>>>>> nodal values.
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> Rgds,
>>>>>> Amit
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>