[petsc-users] I suggest to create a demo code for the problem of "Get a value from global vector to a local processor"

[leejearl]

Hi, Matt,



On Thu, 2019-01-17 at 21:20 -0500, Matthew Knepley wrote:
> On Thu, Jan 17, 2019 at 9:04 PM leejearl <leejearl at mail.nwpu.edu.cn>
> wrote:
> > Hi Matt,
> > 	Thanks for your reply.  We have got to a consensus. For such a
> > problem, I have sought the mail list for helps and found a great 
> > many misleading answers. I think some of answers may work for the
> > DMDA. Now I have overcome this 
> > by creating a PetscSF and PetscSFBcast, and the procedures are
> > described as follow.
> > 
> > 
> > 1. Using DMGLobaltoLocal to update the local vector(the ghost cell
> > mighb get the wrong result)
> > 2. Creating a PetscSF which matches the local ghost cell and the
> > global donor cell across the processes
> > 3. Using PetscSFBCast to update the local ghost cell
> > 
> > Now, my code can work. Can you give me a more scalable procedure?
> 
> That is a scalable procedure. I do not do it that way because it
> requires more steps than just meshing the structure directly.

You are right. You have said that the more efficient way is to meshing
the structure directly. It means that we have better to generate 
a mesh with periodic topology structure. Is it right?

>   Thanks,
>     Matt 
> > Thanks.
> > 
> > On Thu, 2019-01-17 at 06:43 -0500, Matthew Knepley wrote:
> > > On Thu, Jan 17, 2019 at 3:34 AM leejearl via petsc-users <
> > > petsc-users at mcs.anl.gov> wrote:
> > > > Hi all Petscer,
> > > > 
> > > >         I have ask helps for some questions. Thanks for the
> > > > replies
> > > > 
> > > > from developer. I have known more about the petsc. I have also
> > > > sought
> > > > 
> > > > helps in the mail lists, and I find that there are many
> > > > subjects
> > > > 
> > > > focused on such a problem. Some subjects are listed as follow
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
https://lists.mcs.anl.gov/pipermail/petsc-users/2019-January/037425.html
> > > > 
> > > > ,
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
https://lists.mcs.anl.gov/mailman/htdig/petsc-users/2015-January/024068.html
> > > > 
> > > > ,
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
https://lists.mcs.anl.gov/mailman/htdig/petsc-users/2008-November/003633.html
> > > > 
> > > > ,
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
https://lists.mcs.anl.gov/pipermail/petsc-users/2019-January/037436.html
> > > > 
> > > > 
> > > > 
> > > > ......
> > > > 
> > > > 
> > > > 
> > > > The problem can be summarized as
> > > > 
> > > > 1. Getting the value of vector from other processor
> > > > 
> > > > 2. How to set the value of ghost cell, the value of the ghost
> > > > cell
> > > > 
> > > > might be as same as a very cell in other processor. 
> > > > 
> > > > 
> > > > 
> > > > I think the above problem is very popular. I suffer such a
> > > > problem when
> > > > 
> > > > I want to treat the periodic boundary for the FVM method.
> > > 
> > > Ah, I think I am now seeing the problem. In DMPlex, we do not
> > > implementperiodicity of the mesh by putting in extra
> > > communication. We make aperiodic mesh directly. For example, in
> > > 1D what you describe would mesha line segment, and then
> > > communicate the values from one end to the other.Instead, in Plex
> > > we just mesh a circle.
> > >   Thanks,
> > >     Matt 
> > > >  After the dm
> > > > 
> > > > object is distributed, the donor cell of bound might be in
> > > > other
> > > > 
> > > > processor. Since the donor cell must be matched as a pair
> > > > correctly,
> > > > 
> > > > using the routines DMGlobaltoLocal and DMLocaltoGlobal can not
> > > > get the
> > > > 
> > > > expected results. 
> > > > 
> > > > 
> > > > 
> > > > In fact, such a problem is not very difficult for a mpi
> > > > program. But we
> > > > 
> > > > are coding with the petsc, we always think that whether we can
> > > > 
> > > > implement more easily.
> > > > 
> > > > 
> > > > 
> > > > Whether our developer can create a demo for us to following. I
> > > > think
> > > > 
> > > > such a demo is very useful for user.
> > > > 
> > > > 
> > > > 
> > > > Thanks
> > > > 
> > > > 
> > > > 
> > > > leejeal
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > 
> > > 
> 
> 
Thanks
leejearl
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