psb_halo -- Halo Data Communication

These subroutines gathers the values of the halo elements, and (optionally) scale the result:

$$x \leftarrow \alpha x$$

where:

$x$

is a global dense submatrix.

Table 13: Data types

$\alpha$, $x$	Subroutine
Integer	psb_halo
Short Precision Real	psb_halo
Long Precision Real	psb_halo
Short Precision Complex	psb_halo
Long Precision Complex	psb_halo

call psb_halo(x, desc_a, info)
call psb_halo(x, desc_a, info, alpha, work, data)

Type:

Synchronous.

On Entry

x

global dense matrix $x$.
Scope: local
Type: required
Intent: inout.
Specified as: a rank one or two array with the TARGET attribute containing numbers of type specified in Table 13.

desc_a

contains data structures for communications.
Scope: local
Type: required
Intent: in.
Specified as: a structured data of type descdatapsb_desc_type.

alpha

the scalar $\alpha$.
Scope: global
Type: optional
Intent: in.
Default: $alpha = 1$
Specified as: a number of the data type indicated in Table 13.

work

the work array.
Scope: local
Type: optional
Intent: inout.
Specified as: a rank one array of the same type of $x$ with the POINTER attribute.

data

index list selector.
Scope: global
Type: optional
Specified as: an integer. Values:psb_comm_halo_,psb_comm_mov_, psb_comm_ext_, default: psb_comm_halo_. Chooses the index list on which to base the data exchange.

On Return

x

global dense result matrix $x$.
Scope: local
Type: required
Intent: inout.
Returned as: a rank one or two array containing numbers of type specified in Table 13.

info

the local portion of result submatrix $y$.
Scope: local
Type: required
Intent: out.
An integer value that contains an error code.

Figure 6: Sample discretization mesh.

Usage Example Consider the discretization mesh depicted in fig. 6, partitioned among two processes as shown by the dashed line; the data distribution is such that each process will own 32 entries in the index space, with a halo made of 8 entries placed at local indices 33 through 40. If process 0 assigns an initial value of 1 to its entries in the $x$ vector, and process 1 assigns a value of 2, then after a call to psb_halo the contents of the local vectors will be the following:

Process 0				Process 1
I	GLOB(I)	X(I)		I	GLOB(I)	X(I)
1	1	1.0		1	33	2.0
2	2	1.0		2	34	2.0
3	3	1.0		3	35	2.0
4	4	1.0		4	36	2.0
5	5	1.0		5	37	2.0
6	6	1.0		6	38	2.0
7	7	1.0		7	39	2.0
8	8	1.0		8	40	2.0
9	9	1.0		9	41	2.0
10	10	1.0		10	42	2.0
11	11	1.0		11	43	2.0
12	12	1.0		12	44	2.0
13	13	1.0		13	45	2.0
14	14	1.0		14	46	2.0
15	15	1.0		15	47	2.0
16	16	1.0		16	48	2.0
17	17	1.0		17	49	2.0
18	18	1.0		18	50	2.0
19	19	1.0		19	51	2.0
20	20	1.0		20	52	2.0
21	21	1.0		21	53	2.0
22	22	1.0		22	54	2.0
23	23	1.0		23	55	2.0
24	24	1.0		24	56	2.0
25	25	1.0		25	57	2.0
26	26	1.0		26	58	2.0
27	27	1.0		27	59	2.0
28	28	1.0		28	60	2.0
29	29	1.0		29	61	2.0
30	30	1.0		30	62	2.0
31	31	1.0		31	63	2.0
32	32	1.0		32	64	2.0
33	33	2.0		33	25	1.0
34	34	2.0		34	26	1.0
35	35	2.0		35	27	1.0
36	36	2.0		36	28	1.0
37	37	2.0		37	29	1.0
38	38	2.0		38	30	1.0
39	39	2.0		39	31	1.0
40	40	2.0		40	32	1.0