[petsc-dev] Vec set a block of values

[Junchao Zhang]

VecScatter is too heavy (in both coding and runtime) for this simple task.
I just want to pad a vector loaded from a PetscViewer to match an MPIBAIJ
matrix. Thus the majority is memcpy, with few neighborhood off-processor
puts.

--Junchao Zhang

On Fri, Apr 20, 2018 at 3:57 PM, Jed Brown <jed at jedbrown.org> wrote:

> Junchao, If you need to access off-process values and put them into a
> new vector, you should use VecScatter.
>
> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
>
> >   Setting large contiguous blocks of values is not a common use case. In
> finite elements the values are not contiguous.
> >
> >> On Apr 20, 2018, at 3:45 PM, Zhang, Junchao <jczhang at mcs.anl.gov>
> wrote:
> >>
> >> I agree the extra overhead can be small, but users are forced to write
> a loop where one single line gives the best.
> >>
> >> --Junchao Zhang
> >>
> >> On Fri, Apr 20, 2018 at 3:36 PM, Smith, Barry F. <bsmith at mcs.anl.gov>
> wrote:
> >>
> >>    When setting values into matrices and vectors we consider the
> "extra" overhead of needing to pass in the indices for all the values
> (instead of being able to set an arbitrary block of values without using
> indices for each one) to be a minimal overhead that we can live with.
> >>
> >>    Barry
> >>
> >>
> >> > On Apr 20, 2018, at 3:33 PM, Junchao Zhang <jczhang at mcs.anl.gov>
> wrote:
> >> >
> >> >
> >> > On Fri, Apr 20, 2018 at 3:18 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
> >> > On Fri, Apr 20, 2018 at 4:10 PM, Junchao Zhang <jczhang at mcs.anl.gov>
> wrote:
> >> > To pad a vector, i.e., copy a vector to a new one, I have to call
> VecSetValue(newb,1,&idx,...) for each element. But to be efficient, what I
> really needs is to set a block of values in one call. It looks PETSc does
> not have a routine for that(?). I looked at VecSetValuesBlocked, but it
> looks it is not for that purpose.
> >> > Should we have something like VecSetValuesBlock(Vec v,PetscInt
> i,PetscInt cnt,PetscScalar *value, InsertMode mode) to set cnt values
> starting at index i?
> >> >
> >> > Use VecGetArray().
> >> > Did you mean VecGetArray b and newb, do a memcpy from b to new and
> then restore them? If yes, it does not work since some of the values I want
> to set might be remote.
> >> > E.g, I have 4 processors. b's size is 181 and is distributed as 46,
> 45,45,45, newb is distributed as 48,45,45,45 to match a matrix of block
> size 3.
> >> >
> >> >
> >> >   Matt
> >> >
> >> > --Junchao Zhang
> >> >
> >> >
> >> >
> >> > --
> >> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> >> > -- Norbert Wiener
> >> >
> >> > https://www.cse.buffalo.edu/~knepley/
> >> >
> >>
> >>
>
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