Q. of multi-componet system and data from input file

Barry Smith bsmith at mcs.anl.gov
Mon May 12 19:07:33 CDT 2008

On May 12, 2008, at 1:59 PM, tsjb00 wrote:

> Hi, there! I am a beginner of PETSc and I have some questions about  
> using PETSc to solve for a multi-componet system. The code is  
> supposed to be applicable to different systems, where number of  
> components, properties of components ,etc. would be input for the  
> program.
> Say I define DA with dof=number of components = nc, number of grid  
> in x,y,z = nx,ny,nz respectively. When I use DA related functions,  
> it seems that by default the data objects (vectors, arrays, etc.)  
> would be of nx*ny*nz*nc. However, some physical variables are  
> independent of specific components, which means I need to handle  
> data objects of nx*ny*nz*integral. My questions are:
> Does PETSc include tools or examples to deal with such problems?
> If not, how can I make sure the 'nx*ny*nz*any integral' data objects  
> are distributed over the nodes in a way defined by DA? I am using  
> PETSc_Decide for partitioning right now. I would prefer that at  
> least the number of processors be flexible.

    I do not understand your question but here is a stab at it. For  
each different nc you need in your code you simply create a different  
DA. For example if you have
two fields you want together and also have three fields together you  
would create one DA for the 2 dof and one for the 3 dof. Adding a  
couple more DA's won't take
much memory or time.

    You can use DAVecGetArray() to access the values for a fixed dof,  
if sometimes you want nc to be different for different runs within the  
loops you can use DAVecGetArrayDOF(). You access values via x[k][j][i] 
[l] where l goes from 0 to dof-1 for the dof that you used to create  
the DA.


> I need to read in a property f(x,y,z) from a data file and then  
> distribute the data across different processors. Any suggestions on  
> this would be appreciated. My concern is that if I use MPI_Send/ 
> Receive, the data to be transferred might correspond to  
> discontinuous indices due to the partitioning.
> Many thanks in advance!
> BJ
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