# [petsc-users] What does PCASMSetOverlap do?

Wed Apr 13 08:11:14 CDT 2022

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.

>
>   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