[petsc-users] How subdomain talks to neighbors?

[(Rebecca) Xuefei YUAN]

Dear all,

I have a question about how subdomains "talk" to neighbor subdomains.

For example, I have a 6X5 mesh, the solution at each mesh f[j][i] is given

	for (j = jFirst; j <= jLast; j++){
		for (i = iFirst; i <= iLast; i++){
			f[j][i] = i+j*info.mx;
		}
	}
i.e.,
0   1  2  3  4  5
6   7  8  9 10 11
12 13 14 15 16 17
18 19 20 21 22 23
24 25 26 27 28 29

When I read this solution (binary file) as an input of another set of  
code with np=2, the array gives me

rank=0: xs=0,x=3,ys=0,y=5

0   1  2  3  4  5
3   4  5  6  7  8
6   7  8  9 10 11
9  10 11 12 13 14
12 13 14  X  X  X

rank=1: xs=3,x=3,ys=0,y=5

X   X  X 15 16 17
15 16 17 18 19 20
18 19 20 21 22 23
21 22 23 24 25 26
24 25 26 27 28 29

or np=3 gives

rank=0: xs=0,x=2,ys=0,y=5

0   1  2  3  4  5
2   3  4  5  6  7
4   5  6  7  8  9
6   7  8  9  X  X
8   9  X  X  X  X

rank=1: xs=2,x=2,ys=0,y=5

X   X 10 11 12 13
10 11 12 13 14 15
12 13 14 15 16 17
14 15 16 17 18 19
16 17 18 19  X  X

rank=2: xs=4,x=2,ys=0,y=5

X   X  X  X 20 21
X   X 20 21 22 23
20 21 22 23 24 25
22 23 24 25 26 27
24 25 26 27 28 29

, where X means this location is not visited. Still I do not know how  
subdomains talk to each other. Any references on how to understand  
those communication between subdomains?

Thanks a lot!

Rebecca