PDRSCL - multiplie an N-element real distributed vector sub( X ) by the
real scalar 1/a
SUBROUTINE PDRSCL( N, SA, SX, IX, JX, DESCX, INCX )
INTEGER IX, INCX, JX, N
DOUBLE PRECISION SA
INTEGER DESCX( * )
DOUBLE PRECISION SX( * )
PDRSCL multiplies an N-element real distributed vector sub( X ) by the
real scalar 1/a. This is done without overflow or underflow as long as
the final result sub( X )/a does not overflow or underflow.
where sub( X ) denotes X(IX:IX+N-1,JX:JX), if INCX = 1,
X(IX:IX,JX:JX+N-1), if INCX = M_X.
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
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
NOTATION STORED IN EXPLANATION
--------------- -------------- --------------------------------------
DT_A (global) descA[ DT_ ] The descriptor type. In this case,
DT_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
N_A (global) descA[ N_ ] The number of columns in the global
MB_A (global) descA[ MB_ ] The blocking factor used to distribu-
te the rows of the array.
NB_A (global) descA[ NB_ ] The blocking factor used to distribu-
te 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
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
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
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
Because vectors may be seen as particular matrices, a distributed
vector is considered to be a distributed matrix.
N (global input) pointer to INTEGER
The number of components of the distributed vector sub( X ). N
SA (global input) DOUBLE PRECISION
The scalar a which is used to divide each component of sub( X
). SA must be >= 0, or the subroutine will divide by zero.
SX (local input/local output) DOUBLE PRECISION array
containing the local pieces of a distributed matrix of
dimension of at least ( (JX-1)*M_X + IX + ( N - 1 )*abs( INCX )
) This array contains the entries of the distributed vector
sub( X ).
IX (global input) pointer to INTEGER
The global row index of the submatrix of the distributed matrix
X to operate on.
JX (global input) pointer to INTEGER
The global column index of the submatrix of the distributed
matrix X to operate on.
DESCX (global and local input) INTEGER array of dimension 8.
The array descriptor of the distributed matrix X.
INCX (global input) pointer to INTEGER
The global increment for the elements of X. Only two values of
INCX are supported in this version, namely 1 and M_X.