NAME
PSGETRS - solve a system of distributed linear equations op( sub( A )
) * X = sub( B ) with a general N-by-N distributed matrix sub( A )
using the LU factorization computed by PSGETRF
SYNOPSIS
SUBROUTINE PSGETRS( TRANS, N, NRHS, A, IA, JA, DESCA, IPIV, B, IB, JB,
DESCB, INFO )
CHARACTER TRANS
INTEGER IA, IB, INFO, JA, JB, N, NRHS
INTEGER DESCA( * ), DESCB( * ), IPIV( * )
REAL A( * ), B( * )
PURPOSE
PSGETRS solves a system of distributed linear equations sub( A )
denotes A(IA:IA+N-1,JA:JA+N-1), op( A ) = A or A**T and sub( B )
denotes B(IB:IB+N-1,JB:JB+NRHS-1).
Notes
=====
Each global data object is described by an associated description
vector. This vector stores the information required to establish the
mapping between an object element and its corresponding process and
memory location.
Let A be a generic term for any 2D block cyclicly distributed array.
Such a global array has an associated description vector DESCA. In the
following comments, the character _ should be read as "of the global
array".
NOTATION STORED IN EXPLANATION
--------------- -------------- --------------------------------------
DTYPE_A(global) DESCA( DTYPE_ )The descriptor type. In this case,
DTYPE_A = 1.
CTXT_A (global) DESCA( CTXT_ ) The BLACS context handle, indicating
the BLACS process grid A is distribu-
ted over. The context itself is glo-
bal, but the handle (the integer
value) may vary.
M_A (global) DESCA( M_ ) The number of rows in the global
array A.
N_A (global) DESCA( N_ ) The number of columns in the global
array A.
MB_A (global) DESCA( MB_ ) The blocking factor used to distribute
the rows of the array.
NB_A (global) DESCA( NB_ ) The blocking factor used to distribute
the columns of the array.
RSRC_A (global) DESCA( RSRC_ ) The process row over which the first
row of the array A is distributed.
CSRC_A (global) DESCA( CSRC_ ) The process column over which the
first column of the array A is
distributed.
LLD_A (local) DESCA( LLD_ ) The leading dimension of the local
array. LLD_A >= MAX(1,LOCr(M_A)).
Let K be the number of rows or columns of a distributed matrix, and
assume that its process grid has dimension p x q.
LOCr( K ) denotes the number of elements of K that a process would
receive if K were distributed over the p processes of its process
column.
Similarly, LOCc( K ) denotes the number of elements of K that a process
would receive if K were distributed over the q processes of its process
row.
The values of LOCr() and LOCc() may be determined via a call to the
ScaLAPACK tool function, NUMROC:
LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ). An upper
bound for these quantities may be computed by:
LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A
This routine requires square block data decomposition ( MB_A=NB_A ).
ARGUMENTS
TRANS (global input) CHARACTER
Specifies the form of the system of equations:
= ’N’: sub( A ) * X = sub( B ) (No transpose)
= ’T’: sub( A )**T * X = sub( B ) (Transpose)
= ’C’: sub( A )**T * X = sub( B ) (Transpose)
N (global input) INTEGER
The number of rows and columns to be operated on, i.e. the
order of the distributed submatrix sub( A ). N >= 0.
NRHS (global input) INTEGER
The number of right hand sides, i.e., the number of columns of
the distributed submatrix sub( B ). NRHS >= 0.
A (local input) REAL pointer into the local
memory to an array of dimension (LLD_A, LOCc(JA+N-1)). On
entry, this array contains the local pieces of the factors L
and U from the factorization sub( A ) = P*L*U; the unit
diagonal elements of L are not stored.
IA (global input) INTEGER
The row index in the global array A indicating the first row of
sub( A ).
JA (global input) INTEGER
The column index in the global array A indicating the first
column of sub( A ).
DESCA (global and local input) INTEGER array of dimension DLEN_.
The array descriptor for the distributed matrix A.
IPIV (local input) INTEGER array, dimension ( LOCr(M_A)+MB_A )
This array contains the pivoting information. IPIV(i) -> The
global row local row i was swapped with. This array is tied to
the distributed matrix A.
B (local input/local output) REAL pointer into the
local memory to an array of dimension (LLD_B,LOCc(JB+NRHS-1)).
On entry, the right hand sides sub( B ). On exit, sub( B ) is
overwritten by the solution distributed matrix X.
IB (global input) INTEGER
The row index in the global array B indicating the first row of
sub( B ).
JB (global input) INTEGER
The column index in the global array B indicating the first
column of sub( B ).
DESCB (global and local input) INTEGER array of dimension DLEN_.
The array descriptor for the distributed matrix B.
INFO (global output) INTEGER
= 0: successful exit
< 0: If the i-th argument is an array and the j-entry had an
illegal value, then INFO = -(i*100+j), if the i-th argument is
a scalar and had an illegal value, then INFO = -i.