[petsc-dev] Vec set a block of values

[Matthew Knepley]

On Sat, Apr 21, 2018 at 8:47 AM, Junchao Zhang <jczhang at mcs.anl.gov> wrote:

> On Sat, Apr 21, 2018 at 6:21 AM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Fri, Apr 20, 2018 at 5:02 PM, Junchao Zhang <jczhang at mcs.anl.gov>
>> wrote:
>>
>>> 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.
>>>
>>
>> Now this makes no sense. You want to "pad" a Vec? What does this mean?
>>
>> 1) You want to extend its length, which none of these can do. You have to
>> VecCreate another vector.
>>
>> 2) You want some elements to be zero. Just VecSet(v, 0) before setting
>> any elements.
>>
>> If its not those, want is the operation?
>>
> I don't know why no one reply my post to petsc-users. It has background of
> this question.  The word "pad" is also from comments of this example.
>
>
> Subject: How to adapt vectors to matrices loaded from a viewer
> To: PETSc users list <petsc-users at mcs.anl.gov>
>
>  In a PETSc example (ex10.c
> <http://www.mcs.anl.gov/petsc/petsc-current/src/ksp/ksp/examples/tutorials/ex10.c.html>)
> one MatLoad A and then VecLoad b from a viewer. Since one can change mat
> block size through option -matload_block_size, the code tries to pad b if A
> and b's sizes mismatch, using the following test
>
> 186:   MatGetSize(A,&M,NULL);
>
> 187:   VecGetSize(b,&m);
>
> 188:   if (M != m) {   /* Create a new vector b by padding the old one */
>
>   I think the code is wrong. One has to compare local column size of A and
> local size of b, and if there is a mismatch on ANY processor, then one has
> to create a new b.
>   My questions are: Are users supposed to take this complexity? Does PETSc
> provide a neat way to do that, for example, MatVecLoad(A,b,viewer
>
>
That code is nuts and should be deleted. If you know what values the rhs
should have analytically, then just create it.
If you need to load it, then load it. The idea that we can "pad" it is
nonsensical. That should be deleted from the example.

   Matt


>
>>    Matt
>>
>>
>>> --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/
>>>> >> >
>>>> >>
>>>> >>
>>>>
>>>
>>>
>>
>>
>> --
>> 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/ <http://www.caam.rice.edu/~mk51/>
>>
>
>


-- 
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/ <http://www.caam.rice.edu/~mk51/>
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