[petsc-users] Error in vec/vec/examples/tutorials/ex19.c

[Smith, Barry F.]

> On Nov 30, 2018, at 5:35 PM, Sajid Ali <sajidsyed2021 at u.northwestern.edu> wrote:
> 
> I thought I fixed it by using VecDuplicate instead of setting up x2 & copying but I got more confused as to why the VecWrite had different dimensions in each case. 
> 
> Does blocking take a Nx1 vector and convert it to N/BLOCK_SIZE x BLOCK_SIZE vector (that's how it looked it in the output hdf5 files) ?

   Sort of (and yes in the hdf5 file it is stored this way) in memory the block size just requires that each process have a number of blocks (that is part of a block cannot be on one process and the other part on the next one. Thus in splitting a vector of size 2 to two processes it gets different results for different block sizes.

> 
> Since I didn't find TimeStep to be a feature of HDF5, am I right in assuming that it's a PETSc feature that creates an extra dimension to store multiple vectors (with the new dimension being indexed by PetscViewerHDF5SetTimeStep) ?

    Yes

> 
> On Fri, Nov 30, 2018 at 5:17 PM Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
> 
> 
> > On Nov 30, 2018, at 1:30 PM, Sajid Ali via petsc-users <petsc-users at mcs.anl.gov> wrote:
> > 
> > Hi, 
> > 
> > I tried running ex19.c and I get the following error (I added a small snippet to print the local size on rank0 as well) :
> > 
> > [sajid at xrm temp]$ mpirun -np 2 ./ex19 
> > local_sizes : 3 4 
> > [1]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> > [1]PETSC ERROR: Arguments are incompatible
> > [1]PETSC ERROR: Incompatible vector local lengths parameter # 1 local size 3 != parameter # 2 local size 2
> > [1]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> > [1]PETSC ERROR: Petsc Release Version 3.10.2, unknown 
> > [1]PETSC ERROR: ./ex19 on a  named xrm by sajid Fri Nov 30 13:28:36 2018
> > [1]PETSC ERROR: [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> > [0]PETSC ERROR: Arguments are incompatible
> > [0]PETSC ERROR: Incompatible vector local lengths parameter # 1 local size 3 != parameter # 2 local size 4
> > [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> > [0]PETSC ERROR: Petsc Release Version 3.10.2, unknown 
> > [0]PETSC ERROR: ./ex19 on a  named xrm by sajid Fri Nov 30 13:28:36 2018
> > 
> > Does this mean that PETSC_DECIDE is inconsistent between two calls in the same file ? 
> 
>    No. The reason for the error is a bug in the code. The x1 vector has a block size of 1 hence when it is divided among two processes it gets 3 entries on each process. The x2 vector has a block size of 2 so when it is divided among two processes it gets 4 entries on the first process and 2 on the second. The call to VecCopy() does not work because the two vectors have different layouts.
> 
>    I have attached a fixed version of the code (that does not copy from x1 to x2). 
> 
>      Barry
> 
> 
> 
> > 
> > Thank You, 
> > Sajid Ali
> > Applied Physics
> > Northwestern University
> 
> 
> 
> -- 
> Sajid Ali
> Applied Physics
> Northwestern University