[petsc-users] Question about a parallel implementation of PCFIELDSPLIT

[Barry Smith]

   Generally fieldsplit is used on problems that have a natural "split" of the variables into two or more subsets. For example u0,v0,u1,v1,u2,v2,u3,v4 This is often indicated in the vectors and matrices with the "blocksize" argument, 2 in this case. DM also often provides this information. 

   When laying out a vector/matrix with a blocksize one must ensure that an equal number of of the subsets appears on each MPI process. So, for example, if the above vector is distributed over 3 MPI processes one could use   u0,v0,u1,v1       u2,v2      u3,v3  but one cannot use u0,v0,u1    v1,u2,v2   u3,v3.  Another way to think about it is that one must split up the vector as indexed by block among the processes. For most multicomponent problems this type of decomposition is very natural in the logic of the code.

  Barry
 

> On Jan 19, 2024, at 3:19 AM, Pantelis Moschopoulos <pmoschopoulos at outlook.com> wrote:
> 
> Dear all,
> 
> When I am using PCFIELDSPLIT and pc type "schur" in serial mode everything works fine. When I turn now to parallel, I observe that the number of ranks that I can use must divide the number of N without any remainder, where N is the number of unknowns. Otherwise, an error of the following form emerges: "Local columns of A10 3473 do not equal local rows of A00 3471".
> 
> Can I do something to overcome this?
> 
> Thanks,
> Pantelis

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