Question on Index Sets and VecScatters

[Matthew Knepley]

On Dec 18, 2007 1:43 PM, Randall Mackie <randy at geosystem.us> wrote:
> 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?

SELF.

> 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.

Comms on IndexSets do not actually matter.

  Thanks,

    Matt

> 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 at geosystem.us> 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
>
>



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What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which
their experiments lead.
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