[petsc-dev] Vec set a block of values

[Smith, Barry F.]

  I would just use VecSetValues() since almost all values are local it will be scalable and a little extra time in setting values is not a big deal for this test code.

   Barry


> On Apr 20, 2018, at 4:09 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> 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/
>>>>>> 
>>>>> 
>>>>> 
>>>