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])
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|
| dot, x, y | Function |
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|
| Short Precision Real | psb_gedot |
| Long Precision Real | psb_gedot |
| Short Precision Complex | psb_gedot |
| Long Precision Complex | psb_gedot |
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| |
Table 2: Data types
-
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
- 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.