[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,	Thanks again for your helps. You have help me a lot.
Petsc is a great library. 	I need to learn more about it, thanks.
LeejearlOn Thu, 2019-01-17 at 21:41 -0500, Matthew Knepley wrote:
> On Thu, Jan 17, 2019 at 9:39 PM leejearl <leejearl at mail.nwpu.edu.cn>
> wrote:
> > 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?
> 
> Just to be clear, your method is perfectly fine. Mine is not more
> efficient. I just think its easier.You can see periodic meshes
> generated using DMPlexCreateBoxMesh().
>   Thanks,
>     Matt 
> > >   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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