[petsc-dev] Vec set a block of values

[Jed Brown]

Junchao Zhang <jczhang at mcs.anl.gov> writes:

> 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.

At what address do those puts go, how do you avoid race conditions from
multiple processors having overlapping neighborhoods, and how does the
recipient know that the put has completed?  Just use VecScatter.  It
could be optimized to recognize contiguous runs above a certain size and
convert to memcpy.

> --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/
>> >> >
>> >>
>> >>
>>