4.2 psb_gedot — Dot Product

This function computes dot product between two vectors x and y.
If x and y are real vectors it computes dot-product as:

Else if x and y are complex vectors then it computes dot-product as:

psb_gedot(x, y, desc_a, info [,global])

Type:

Synchronous.

On Entry

x

the local portion of global dense matrix x.
Scope: local
Type: required
Intent: in.
Specified as: a rank one or two array or an object of type psb_T_vect_type containing numbers of type specified in Table 2. The rank of x must be the same of y.

y

the local portion of global dense matrix y.
Scope: local
Type: required
Intent: in.
Specified as: a rank one or two array or an object of type psb_T_vect_type containing numbers of type specified in Table 2. The rank of y must be the same of x.

desc_a

contains data structures for communications.
Scope: local
Type: required
Intent: in.
Specified as: an object of type psb_desc_type.

global

Specifies whether the computation should include the global reduction across all processes.
Scope: global
Type: optional.
Intent: in.
Specified as: a logical scalar. Default: global=.true.

On Return

Function value

is the dot product of vectors x and y.
Scope: global unless the optional variable global=.false. has been specified
Specified as: a number of the data type indicated in Table 2.

info

Error code.
Scope: local
Type: required
Intent: out.
An integer value; 0 means no error has been detected.

Notes

1. The computation of a global result requires a global communication, which entails a significant overhead. It may be necessary and/or advisable to compute multiple dot products at the same time; in this case, it is possible to improve the runtime efficiency by using the following scheme:
       vres(1) = psb_gedot(x1,y1,desc_a,info,global=.false.) 
       vres(2) = psb_gedot(x2,y2,desc_a,info,global=.false.) 
       vres(3) = psb_gedot(x3,y3,desc_a,info,global=.false.) 
       call psb_sum(ctxt,vres(1:3))

   In this way the global communication, which for small sizes is a latency-bound operation, is invoked only once.