# [petsc-users] What does PCASMSetOverlap do?

Matthew Knepley knepley at gmail.com
Wed Apr 13 08:14:40 CDT 2022

```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