# [petsc-users] What does PCASMSetOverlap do?

Zhuo Chen chenzhuotj at gmail.com
Wed Apr 13 08:30:42 CDT 2022

```Dear Matthew and Mark,

Thank you very much for the reply! Much appreciated!

The question was about a 1D problem. I think I should say core 1 has row
1:32 instead of 1:32, 1:32 as it might be confusing.

So the overlap is extended to both directions for the middle processor but
only toward the increasing direction for the first processor and the
decreasing direction for the last processor. In 1D, this makes sense as the
domain is contiguous. However, in 2D with domain decomposition with spacial
overlaps, this overlapping would not work as one subdomain can have several
neighbor domains. Mark mentioned generalized ASM, is that the correct
direction that I should look for?

Best regards.

On Wed, Apr 13, 2022 at 9:14 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Wed, Apr 13, 2022 at 9:11 AM Mark Adams <mfadams at lbl.gov> wrote:
>
>>
>>
>> On Wed, Apr 13, 2022 at 8:56 AM Matthew Knepley <knepley at gmail.com>
>> wrote:
>>
>>> On Wed, Apr 13, 2022 at 6:42 AM Mark Adams <mfadams at lbl.gov> wrote:
>>>
>>>> No, without overlap you have, let say:
>>>> core 1:   1:32, 1:32
>>>> core 2:   33:64,  33:64
>>>>
>>>> Overlap will increase the size of each domain so you get:
>>>> core 1:   1:33, 1:33
>>>> core 2:   32:65,  32:65
>>>>
>>>
>>> I do not think this is correct. Here is the algorithm. Imagine the
>>> matrix is a large graph. When you divide rows, you
>>> can think of that as dividing the vertices into sets. If overlap = 1, it
>>> that are just 1 edge away from my set.
>>>
>>
>> I think that is what was said. You increase each subdomain by one row of
>> vertices.
>> So in 1D, vertex 32 and 33 are in both subdomains and you have an overlap
>> region of size 2.
>> They want an overlap region of size 1, vertex 33.
>>
>
> This is true, but I did not think they specified a 1D mesh.
>
>   Matt
>
>
>>
>>>   Thanks,
>>>
>>>      Matt
>>>
>>>
>>>> What you want is reasonable but requires PETSc to pick a separator set,
>>>> which is not well defined.
>>>> You need to build that yourself with gasm (I think) if you want this.
>>>>
>>>> Mark
>>>>
>>>> On Wed, Apr 13, 2022 at 3:17 AM Zhuo Chen <chenzhuotj at gmail.com> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I hope that everything is going well with everybody.
>>>>>
>>>>> I have a question about the PCASMSetOverlap. If I have a 128x128
>>>>> matrix and I use 4 cores with overlap=1. Does it mean that from core 1 to
>>>>> core 4, the block ranges are (starting from 1):
>>>>>
>>>>> core 1:   1:33, 1:33
>>>>> core 2:   33:65,  33:65
>>>>> core 3:   65:97,  65:97
>>>>> core 4:   95:128, 95:128
>>>>>
>>>>> Or is it something else? I cannot tell from the manual.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Zhuo Chen
>>>>> Department of Astronomy
>>>>> Tsinghua University
>>>>> Beijing, China 100084
>>>>> *https://czlovemath123.github.io/ <https://czlovemath123.github.io/>*
>>>>>
>>>>
>>>
>>> --
>>> What most experimenters take for granted before they begin their
>>> experiments is infinitely more interesting than any results to which their
>>> -- Norbert Wiener
>>>
>>> https://www.cse.buffalo.edu/~knepley/
>>> <http://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
> -- Norbert Wiener
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>

--
Zhuo Chen
Department of Astronomy
Tsinghua University
Beijing, China 100084
*https://czlovemath123.github.io/ <https://czlovemath123.github.io/>*
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