# [petsc-users] Why use MATMPIBAIJ?

Matthew Knepley knepley at gmail.com
Fri Jan 22 07:57:22 CST 2016

```On Fri, Jan 22, 2016 at 7:27 AM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:

> DO you mean the option pc_fieldsplit_block_size? In this thread:
>
> http://petsc-users.mcs.anl.narkive.com/qSHIOFhh/fieldsplit-error
>

No. "Block Size" is confusing on PETSc since it is used to do several
things. Here block size
is being used to split the matrix. You do not need this since you are
matrix block size is used two ways:

1) To indicate that matrix values come in logically dense blocks

2) To change the storage to match this logical arrangement

After everything works, we can just indicate to the submatrix which is
extracted that it has a
certain block size. However, for the Laplacian I expect it not to matter.

> It assumes you have a constant number of fields at each grid point, am I
> right? However, my field split is not constant, like
> [u1_x   u1_y    u1_z    p_1    u2_x    u2_y    u2_z    u3_x    u3_y
>  u3_z    p_3    u4_x    u4_y    u4_z]
>
> Subsequently the fieldsplit is
> [u1_x   u1_y    u1_z    u2_x    u2_y    u2_z    u3_x    u3_y    u3_z
> u4_x    u4_y    u4_z]
> [p_1    p_3]
>
> Then what is the option to set block size 3 for split 0?
>
> Sorry, I search several forum threads but cannot figure out the options as
> you said.
>
>
>
>> You can still do that. It can be done with options once the decomposition
>> is working. Its true that these solvers
>> work better with the block size set. However, if its the P2 Laplacian it
>> does not really matter since its uncoupled.
>>
>> Yes, I agree it's uncoupled with the other field, but the crucial factor
> defining the quality of the block preconditioner is the approximate
> inversion of individual block. I would merely try block Jacobi first,
> because it's quite simple. Nevertheless, fieldsplit implements other nice
> things, like Schur complement, etc.
>

I think concepts are getting confused here. I was talking about the
interaction of components in one block (the P2 block). You
are talking about interaction between blocks.

Thanks,

Matt

> Giang
>
>
>
> On Fri, Jan 22, 2016 at 11:15 AM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Fri, Jan 22, 2016 at 3:40 AM, Hoang Giang Bui <hgbk2008 at gmail.com>
>> wrote:
>>
>>> Hi Matt
>>> I would rather like to set the block size for block P2 too. Why?
>>>
>>> Because in one of my test (for problem involves only [u_x u_y u_z]), the
>>> gmres + Hypre AMG converges in 50 steps with block size 3, whereby it
>>> increases to 140 if block size is 1 (see attached files).
>>>
>>
>> You can still do that. It can be done with options once the decomposition
>> is working. Its true that these solvers
>> work better with the block size set. However, if its the P2 Laplacian it
>> does not really matter since its uncoupled.
>>
>> This gives me the impression that AMG will give better inversion for "P2"
>>> block if I can set its block size to 3. Of course it's still an hypothesis
>>> but worth to try.
>>>
>>> Another question: In one of the Petsc presentation, you said the Hypre
>>> AMG does not scale well, because set up cost amortize the iterations. How
>>> is it quantified? and what is the memory overhead?
>>>
>>
>> I said the Hypre setup cost is not scalable, but it can be amortized over
>> the iterations. You can quantify this
>> just by looking at the PCSetUp time as your increase the number of
>> processes. I don't think they have a good
>> model for the memory usage, and if they do, I do not know what it is.
>> However, generally Hypre takes more
>> memory than the agglomeration MG like ML or GAMG.
>>
>>   Thanks,
>>
>>     Matt
>>
>>
>>>
>>> Giang
>>>
>>> On Mon, Jan 18, 2016 at 5:25 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>
>>>> Hoang Giang Bui <hgbk2008 at gmail.com> writes:
>>>>
>>>> > Why P2/P2 is not for co-located discretization?
>>>>
>>>> Matt typed "P2/P2" when me meant "P2/P1".
>>>>
>>>
>>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> -- Norbert Wiener
>>
>
>

--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their