table of contents
PDGERFS(l) | LAPACK routine (version 1.5) | PDGERFS(l) |
NAME¶
PDGERFS - improve the computed solution to a system of linear equations and provides error bounds and backward error estimates for the solutionsSYNOPSIS¶
- SUBROUTINE PDGERFS(
- TRANS, N, NRHS, A, IA, JA, DESCA, AF, IAF, JAF, DESCAF, IPIV, B, IB, JB, DESCB, X, IX, JX, DESCX, FERR, BERR, WORK, LWORK, IWORK, LIWORK, INFO )
PURPOSE¶
PDGERFS improves the computed solution to a system of linear equations and provides error bounds and backward error estimates for the solutions. NotesDTYPE_A = 1.
the BLACS process grid A is distribu-
ted over. The context itself is glo-
bal, but the handle (the integer
value) may vary.
array A.
array A.
the rows of the array.
the columns of the array.
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.
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( 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
ARGUMENTS¶
- TRANS (global input) CHARACTER*1
- Specifies the form of the system of equations. = 'N': sub( A ) * sub( X )
= sub( B ) (No transpose)
- N (global input) INTEGER
- The order of the matrix sub( A ). N >= 0.
- NRHS (global input) INTEGER
- The number of right hand sides, i.e., the number of columns of the matrices sub( B ) and sub( X ). NRHS >= 0.
- A (local input) DOUBLE PRECISION pointer into the local
- memory to an array of local dimension (LLD_A,LOCc(JA+N-1)). This array contains the local pieces of the distributed matrix sub( A ).
- 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.
- AF (local input) DOUBLE PRECISION pointer into the local
- memory to an array of local dimension (LLD_AF,LOCc(JA+N-1)). This array contains the local pieces of the distributed factors of the matrix sub( A ) = P * L * U as computed by PDGETRF.
- IAF (global input) INTEGER
- The row index in the global array AF indicating the first row of sub( AF ).
- JAF (global input) INTEGER
- The column index in the global array AF indicating the first column of sub( AF ).
- DESCAF (global and local input) INTEGER array of dimension DLEN_.
- The array descriptor for the distributed matrix AF.
- IPIV (local input) INTEGER array of dimension LOCr(M_AF)+MB_AF.
- This array contains the pivoting information as computed by PDGETRF. IPIV(i) -> The global row local row i was swapped with. This array is tied to the distributed matrix A.
- B (local input) DOUBLE PRECISION pointer into the local
- memory to an array of local dimension (LLD_B,LOCc(JB+NRHS-1)). This array contains the local pieces of the distributed matrix of right hand sides sub( B ).
- 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.
- X (local input and output) DOUBLE PRECISION pointer into the
- local memory to an array of local dimension (LLD_X,LOCc(JX+NRHS-1)). On entry, this array contains the local pieces of the distributed matrix solution sub( X ). On exit, the improved solution vectors.
- IX (global input) INTEGER
- The row index in the global array X indicating the first row of sub( X ).
- JX (global input) INTEGER
- The column index in the global array X indicating the first column of sub( X ).
- DESCX (global and local input) INTEGER array of dimension DLEN_.
- The array descriptor for the distributed matrix X.
- FERR (local output) DOUBLE PRECISION array of local dimension
- LOCc(JB+NRHS-1). The estimated forward error bound for each solution vector of sub( X ). If XTRUE is the true solution corresponding to sub( X ), FERR is an estimated upper bound for the magnitude of the largest element in (sub( X ) - XTRUE) divided by the magnitude of the largest element in sub( X ). The estimate is as reliable as the estimate for RCOND, and is almost always a slight overestimate of the true error. This array is tied to the distributed matrix X.
- BERR (local output) DOUBLE PRECISION array of local dimension
- LOCc(JB+NRHS-1). The componentwise relative backward error of each solution vector (i.e., the smallest re- lative change in any entry of sub( A ) or sub( B ) that makes sub( X ) an exact solution). This array is tied to the distributed matrix X.
- WORK (local workspace/local output) DOUBLE PRECISION array,
- dimension (LWORK) On exit, WORK(1) returns the minimal and optimal LWORK.
- LWORK (local or global input) INTEGER
- The dimension of the array WORK. LWORK is local input and must be at least LWORK >= 3*LOCr( N + MOD(IA-1,MB_A) ) If LWORK = -1, then LWORK is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. Each of these values is returned in the first entry of the corresponding work array, and no error message is issued by PXERBLA.
- IWORK (local workspace/local output) INTEGER array,
- dimension (LIWORK) On exit, IWORK(1) returns the minimal and optimal LIWORK.
- LIWORK (local or global input) INTEGER
- The dimension of the array IWORK. LIWORK is local input and must be at least LIWORK >= LOCr( N + MOD(IB-1,MB_B) ). If LIWORK = -1, then LIWORK is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. Each of these values is returned in the first entry of the corresponding work array, and no error message is issued by PXERBLA.
- INFO (global output) INTEGER
- = 0: successful exit
PARAMETERS¶
ITMAX is the maximum number of steps of iterative refinement. Notes ===== This routine temporarily returns when N <= 1. The distributed submatrices op( A ) and op( AF ) (respectively sub( X ) and sub( B ) ) should be distributed the same way on the same processes. These conditions ensure that sub( A ) and sub( AF ) (resp. sub( X ) and sub( B ) ) are "perfectly" aligned. Moreover, this routine requires the distributed submatrices sub( A ), sub( AF ), sub( X ), and sub( B ) to be aligned on a block boundary, i.e., if f(x,y) = MOD( x-1, y ): f( IA, DESCA( MB_ ) ) = f( JA, DESCA( NB_ ) ) = 0, f( IAF, DESCAF( MB_ ) ) = f( JAF, DESCAF( NB_ ) ) = 0, f( IB, DESCB( MB_ ) ) = f( JB, DESCB( NB_ ) ) = 0, and f( IX, DESCX( MB_ ) ) = f( JX, DESCX( NB_ ) ) = 0.12 May 1997 | LAPACK version 1.5 |