7.15 psb_nrm2 — Global 2-norm reduction

call psb_nrm2(ctxt, dat [, root, mode, request])

This subroutine implements a 2-norm value reduction operation based on the underlying communication library.

Type:

Synchronous.

On Entry

ctxt

the communication context identifying the virtual parallel machine.
Scope: global.
Type: required.
Intent: in.
Specified as: an integer variable.

dat

The local contribution to the global minimum.
Scope: local.
Type: required.
Intent: inout.
Specified as: a real variable, which may be a scalar, or a rank 1 array.  Kind, rank and size must agree on all processes.

root

Process to hold the final value, or -1 to make it available on all processes.
Scope: global.
Type: optional.
Intent: in.
Specified as: an integer value -1 <= root <= np - 1, default -1.

mode

Whether the call is started in non-blocking mode and completed later, or is executed synchronously.
Scope: global.
Type: optional.
Intent: in.
Specified as: an integer value. The action to be taken is determined by its bit fields, which can be set with bitwise OR. Basic action values are psb_collective_start_, psb_collective_end_. Default: both fields are selected (i.e. require synchronous completion).

request

A request variable to check for operation completion.
Scope: local.
Type: optional.
Intent: inout.
If mode does not specify synchronous completion, then this variable must be present.

On Return

dat

On destination process(es), the result of the 2-norm reduction.
Scope: global.
Type: required.
Intent: inout.
Specified as: a real variable, which may be a scalar, or a rank 1 array.
Kind, rank and size must agree on all processes.

request

A request variable to check for operation completion.
Scope: local.
Type: optional.
Intent: inout.
If mode does not specify synchronous completion, then this variable must be present.

Notes

1. This reduction is appropriate to compute the results of multiple (local) NRM2 operations at the same time.
2. Denoting by dat_i the value of the variable dat on process i, the output res is equivalent to the computation of

   with care taken to avoid unnecessary overflow.

3. The dat argument is both input and output, and its value may be changed even on processes different from the final result destination.
4. The mode argument can be built with the bitwise IOR() operator; in the following example, the argument is forcing immediate completion, hence the request argument needs not be specified:

     call psb_nrm2(ctxt,dat,mode=ior(psb_collective_start_,psb_collective_end_))
   

5. When splitting the operation in two calls, the dat argument must not be accessed between calls:

     call psb_nrm2(ctxt,dat,mode=psb_collective_start_,request=nrm2_request)
     ....... ! Do not access dat
     call psb_nrm2(ctxt,dat,mode=psb_collective_end_,request=nrm2_request)