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Re: Question on Index Sets and VecScatters

Matt,

Just a quick follow up question. The local vectors created in (1)
are SEQ vectors on PETSC_COMM_SELF. To create the index sets, it
seems like I should just use ISCreateStride, using 0, m for start
and length of the index set. My question is should the communicator
be PETSC_COMM_SELF or PETSC_COMM_WORLD?

Similarly, the index set for the global vector should also be
created with ISCreateStride, using k_0 and m for start and lengths.
Same question about the communicator.

Thanks, Randy


Matthew Knepley wrote: 
You could do it like that, but it seems pretty wasteful, especially in parallel
where you might be sending a considerable amount of data. Why not do
something like this:

  1) Average all slabs into a local vector, indexed by the given k value,
      meaning you have a map {k_0, k_1, ..., k_m} --> {0,1, ...,m}.

  2) Now construct a scatter that maps each local vector into a parallel
       vector of all the ks. The IndexSet for the from (local) vector will be
       {0, 1, ..., m} and the IndexSet for the to (global) vector will be
       {k_0, k_1, ... , k_m} on each process.

  3) When you scatter use ADD_VALUES. Then you will have the sum, and
      just scale the vector by the slab size.

Does this makes sense to you?

  Thanks,

    Matt

On Dec 13, 2007 5:32 PM, Randall Mackie <randy@xxxxxxxxxxxx> wrote: 
I have a situation where I've put a model vector m(i,j,k) into a parallel
PETSc vector for use in my modeling code. However, I'm now adding a bit of code
where I want to do some calculations based on the 1D average of the model.
In other words, for each k, I want to average m(i,j), and so produce a new
model vector m_avg(k).

So, to do this, it would seem that I need to create a VecScatter that will,
for each layer, scatter all the m(i,j) into a 2D vector, then I can take
the average. It would seem that I need to create an Index Set to do this,
but I'm a bit confused as to how to go about it actually, since I've never
used Index Sets.

Can someone outline the basic steps given my description above?

Thanks, Randy

--
Randall Mackie
GSY-USA, Inc.
PMB# 643
2261 Market St.,
San Francisco, CA 94114-1600
Tel (415) 469-8649
Fax (415) 469-5044

California Registered Geophysicist
License No. GP 1034






--
Randall Mackie
GSY-USA, Inc.
PMB# 643
2261 Market St.,
San Francisco, CA 94114-1600
Tel (415) 469-8649
Fax (415) 469-5044

California Registered Geophysicist
License No. GP 1034