Re: [petsc-dev] Vec set a block of values
On Sat, Apr 21, 2018 at 9:07 AM, Junchao Zhang <[email protected]> wrote:
On Sat, Apr 21, 2018 at 7:51 AM, Matthew Knepley <[email protected]> wrote:
On Sat, Apr 21, 2018 at 8:47 AM, Junchao Zhang <[email protected]> wrote:
On Sat, Apr 21, 2018 at 6:21 AM, Matthew Knepley <[email protected]> wrote:
On Fri, Apr 20, 2018 at 5:02 PM, Junchao Zhang <[email protected]> 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 <[email protected]>
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.
Maybe the code should say "load a vector, and then copy to a longer one and replace the old one ". It looks like we pad the original vector. So the question is still there: How to write a neat code to copy a vector to a longer one?
1) It makes no sense to copy it. What would the values even mean? Just create a vector of the right size. 2) If you do have a notion of what the values mean, USE VecScatter. That is is exactly what it is for. Matt
Matt
Matt
--Junchao Zhang
On Fri, Apr 20, 2018 at 3:57 PM, Jed Brown <[email protected]> wrote:
Junchao, If you need to access off-process values and put them into a new vector, you should use VecScatter.
"Smith, Barry F." <[email protected]> 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 <[email protected]> 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. < [email protected]> 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 <[email protected]> wrote:
On Fri, Apr 20, 2018 at 3:18 PM, Matthew Knepley < [email protected]> wrote: On Fri, Apr 20, 2018 at 4:10 PM, Junchao Zhang < [email protected]> 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
-- 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
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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. -- Norbert Wiener
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Matthew Knepley