SUBROUTINE PCDBSV( N, BWL, BWU, NRHS, A, JA, DESCA, B, IB, DESCB, \$ WORK, LWORK, INFO ) * * * * -- ScaLAPACK routine (version 1.7) -- * University of Tennessee, Knoxville, Oak Ridge National Laboratory, * and University of California, Berkeley. * November 15, 1997 * * .. Scalar Arguments .. INTEGER BWL, BWU, IB, INFO, JA, LWORK, N, NRHS * .. * .. Array Arguments .. INTEGER DESCA( * ), DESCB( * ) COMPLEX A( * ), B( * ), WORK( * ) * .. * * * Purpose * ======= * * PCDBSV solves a system of linear equations * * A(1:N, JA:JA+N-1) * X = B(IB:IB+N-1, 1:NRHS) * * where A(1:N, JA:JA+N-1) is an N-by-N complex * banded diagonally dominant-like distributed * matrix with bandwidth BWL, BWU. * * Gaussian elimination without pivoting * is used to factor a reordering * of the matrix into L U. * * See PCDBTRF and PCDBTRS for details. * * ===================================================================== * * Arguments * ========= * * * N (global input) INTEGER * The number of rows and columns to be operated on, i.e. the * order of the distributed submatrix A(1:N, JA:JA+N-1). N >= 0. * * BWL (global input) INTEGER * Number of subdiagonals. 0 <= BWL <= N-1 * * BWU (global input) INTEGER * Number of superdiagonals. 0 <= BWU <= N-1 * * NRHS (global input) INTEGER * The number of right hand sides, i.e., the number of columns * of the distributed submatrix B(IB:IB+N-1, 1:NRHS). * NRHS >= 0. * * A (local input/local output) COMPLEX pointer into * local memory to an array with first dimension * LLD_A >=(bwl+bwu+1) (stored in DESCA). * On entry, this array contains the local pieces of the * This local portion is stored in the packed banded format * used in LAPACK. Please see the Notes below and the * ScaLAPACK manual for more detail on the format of * distributed matrices. * On exit, this array contains information containing details * of the factorization. * Note that permutations are performed on the matrix, so that * the factors returned are different from those returned * by LAPACK. * * JA (global input) INTEGER * The index in the global array A that points to the start of * the matrix to be operated on (which may be either all of A * or a submatrix of A). * * DESCA (global and local input) INTEGER array of dimension DLEN. * if 1D type (DTYPE_A=501), DLEN >= 7; * if 2D type (DTYPE_A=1), DLEN >= 9 . * The array descriptor for the distributed matrix A. * Contains information of mapping of A to memory. Please * see NOTES below for full description and options. * * B (local input/local output) COMPLEX pointer into * local memory to an array of local lead dimension lld_b>=NB. * On entry, this array contains the * the local pieces of the right hand sides * B(IB:IB+N-1, 1:NRHS). * On exit, this contains the local piece of the solutions * distributed matrix X. * * IB (global input) INTEGER * The row index in the global array B that points to the first * row of the matrix to be operated on (which may be either * all of B or a submatrix of B). * * DESCB (global and local input) INTEGER array of dimension DLEN. * if 1D type (DTYPE_B=502), DLEN >=7; * if 2D type (DTYPE_B=1), DLEN >= 9. * The array descriptor for the distributed matrix B. * Contains information of mapping of B to memory. Please * see NOTES below for full description and options. * * WORK (local workspace/local output) * COMPLEX temporary workspace. This space may * be overwritten in between calls to routines. WORK must be * the size given in LWORK. * On exit, WORK( 1 ) contains the minimal LWORK. * * LWORK (local input or global input) INTEGER * Size of user-input workspace WORK. * If LWORK is too small, the minimal acceptable size will be * returned in WORK(1) and an error code is returned. LWORK>= * NB*(bwl+bwu)+6*max(bwl,bwu)*max(bwl,bwu) * +max((max(bwl,bwu)*NRHS), max(bwl,bwu)*max(bwl,bwu)) * * 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. * > 0: If INFO = K<=NPROCS, the submatrix stored on processor * INFO and factored locally was not * diagonally dominant-like, and * the factorization was not completed. * If INFO = K>NPROCS, the submatrix stored on processor * INFO-NPROCS representing interactions with other * processors was not * stably factorable wo/interchanges, * and the factorization was not completed. * * ===================================================================== * * * Restrictions * ============ * * The following are restrictions on the input parameters. Some of these * are temporary and will be removed in future releases, while others * may reflect fundamental technical limitations. * * Non-cyclic restriction: VERY IMPORTANT! * P*NB>= mod(JA-1,NB)+N. * The mapping for matrices must be blocked, reflecting the nature * of the divide and conquer algorithm as a task-parallel algorithm. * This formula in words is: no processor may have more than one * chunk of the matrix. * * Blocksize cannot be too small: * If the matrix spans more than one processor, the following * restriction on NB, the size of each block on each processor, * must hold: * NB >= 2*MAX(BWL,BWU) * The bulk of parallel computation is done on the matrix of size * O(NB) on each processor. If this is too small, divide and conquer * is a poor choice of algorithm. * * Submatrix reference: * JA = IB * Alignment restriction that prevents unnecessary communication. * * * ===================================================================== * * * Notes * ===== * * If the factorization routine and the solve routine are to be called * separately (to solve various sets of righthand sides using the same * coefficient matrix), the auxiliary space AF *must not be altered* * between calls to the factorization routine and the solve routine. * * The best algorithm for solving banded and tridiagonal linear systems * depends on a variety of parameters, especially the bandwidth. * Currently, only algorithms designed for the case N/P >> bw are * implemented. These go by many names, including Divide and Conquer, * Partitioning, domain decomposition-type, etc. * * Algorithm description: Divide and Conquer * * The Divide and Conqer algorithm assumes the matrix is narrowly * banded compared with the number of equations. In this situation, * it is best to distribute the input matrix A one-dimensionally, * with columns atomic and rows divided amongst the processes. * The basic algorithm divides the banded matrix up into * P pieces with one stored on each processor, * and then proceeds in 2 phases for the factorization or 3 for the * solution of a linear system. * 1) Local Phase: * The individual pieces are factored independently and in * parallel. These factors are applied to the matrix creating * fillin, which is stored in a non-inspectable way in auxiliary * space AF. Mathematically, this is equivalent to reordering * the matrix A as P A P^T and then factoring the principal * leading submatrix of size equal to the sum of the sizes of * the matrices factored on each processor. The factors of * these submatrices overwrite the corresponding parts of A * in memory. * 2) Reduced System Phase: * A small (max(bwl,bwu)* (P-1)) system is formed representing * interaction of the larger blocks, and is stored (as are its * factors) in the space AF. A parallel Block Cyclic Reduction * algorithm is used. For a linear system, a parallel front solve * followed by an analagous backsolve, both using the structure * of the factored matrix, are performed. * 3) Backsubsitution Phase: * For a linear system, a local backsubstitution is performed on * each processor in parallel. * * * Descriptors * =========== * * Descriptors now have *types* and differ from ScaLAPACK 1.0. * * Note: banded codes can use either the old two dimensional * or new one-dimensional descriptors, though the processor grid in * both cases *must be one-dimensional*. We describe both types below. * * 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 * * * One-dimensional descriptors: * * One-dimensional descriptors are a new addition to ScaLAPACK since * version 1.0. They simplify and shorten the descriptor for 1D * arrays. * * Since ScaLAPACK supports two-dimensional arrays as the fundamental * object, we allow 1D arrays to be distributed either over the * first dimension of the array (as if the grid were P-by-1) or the * 2nd dimension (as if the grid were 1-by-P). This choice is * indicated by the descriptor type (501 or 502) * as described below. * * IMPORTANT NOTE: the actual BLACS grid represented by the * CTXT entry in the descriptor may be *either* P-by-1 or 1-by-P * irrespective of which one-dimensional descriptor type * (501 or 502) is input. * This routine will interpret the grid properly either way. * ScaLAPACK routines *do not support intercontext operations* so that * the grid passed to a single ScaLAPACK routine *must be the same* * for all array descriptors passed to that routine. * * NOTE: In all cases where 1D descriptors are used, 2D descriptors * may also be used, since a one-dimensional array is a special case * of a two-dimensional array with one dimension of size unity. * The two-dimensional array used in this case *must* be of the * proper orientation: * If the appropriate one-dimensional descriptor is DTYPEA=501 * (1 by P type), then the two dimensional descriptor must * have a CTXT value that refers to a 1 by P BLACS grid; * If the appropriate one-dimensional descriptor is DTYPEA=502 * (P by 1 type), then the two dimensional descriptor must * have a CTXT value that refers to a P by 1 BLACS grid. * * * Summary of allowed descriptors, types, and BLACS grids: * DTYPE 501 502 1 1 * BLACS grid 1xP or Px1 1xP or Px1 1xP Px1 * ----------------------------------------------------- * A OK NO OK NO * B NO OK NO OK * * Note that a consequence of this chart is that it is not possible * for *both* DTYPE_A and DTYPE_B to be 2D_type(1), as these lead * to opposite requirements for the orientation of the BLACS grid, * and as noted before, the *same* BLACS context must be used in * all descriptors in a single ScaLAPACK subroutine call. * * Let A be a generic term for any 1D 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( 1 ) The descriptor type. For 1D grids, * TYPE_A = 501: 1-by-P grid. * TYPE_A = 502: P-by-1 grid. * CTXT_A (global) DESCA( 2 ) 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. * N_A (global) DESCA( 3 ) The size of the array dimension being * distributed. * NB_A (global) DESCA( 4 ) The blocking factor used to distribute * the distributed dimension of the array. * SRC_A (global) DESCA( 5 ) The process row or column over which the * first row or column of the array * is distributed. * LLD_A (local) DESCA( 6 ) The leading dimension of the local array * storing the local blocks of the distri- * buted array A. Minimum value of LLD_A * depends on TYPE_A. * TYPE_A = 501: LLD_A >= * size of undistributed dimension, 1. * TYPE_A = 502: LLD_A >=NB_A, 1. * Reserved DESCA( 7 ) Reserved for future use. * * * * ===================================================================== * * Code Developer: Andrew J. Cleary, University of Tennessee. * Current address: Lawrence Livermore National Labs. * This version released: August, 2001. * * ===================================================================== * * .. * .. Parameters .. REAL ONE, ZERO PARAMETER ( ONE = 1.0E+0 ) PARAMETER ( ZERO = 0.0E+0 ) COMPLEX CONE, CZERO PARAMETER ( CONE = ( 1.0E+0, 0.0E+0 ) ) PARAMETER ( CZERO = ( 0.0E+0, 0.0E+0 ) ) INTEGER INT_ONE PARAMETER ( INT_ONE = 1 ) INTEGER DESCMULT, BIGNUM PARAMETER (DESCMULT = 100, BIGNUM = DESCMULT * DESCMULT) INTEGER BLOCK_CYCLIC_2D, CSRC_, CTXT_, DLEN_, DTYPE_, \$ LLD_, MB_, M_, NB_, N_, RSRC_ PARAMETER ( BLOCK_CYCLIC_2D = 1, DLEN_ = 9, DTYPE_ = 1, \$ CTXT_ = 2, M_ = 3, N_ = 4, MB_ = 5, NB_ = 6, \$ RSRC_ = 7, CSRC_ = 8, LLD_ = 9 ) * .. * .. Local Scalars .. INTEGER ICTXT, MYCOL, MYROW, NB, NPCOL, NPROW, \$ WS_FACTOR * .. * .. External Subroutines .. EXTERNAL PCDBTRF, PCDBTRS, PXERBLA * .. * .. Executable Statements .. * * Note: to avoid duplication, most error checking is not performed * in this routine and is left to routines * PCDBTRF and PCDBTRS. * * Begin main code * INFO = 0 * * Get block size to calculate workspace requirements * IF( DESCA( DTYPE_ ) .EQ. BLOCK_CYCLIC_2D ) THEN NB = DESCA( NB_ ) ICTXT = DESCA( CTXT_ ) ELSEIF( DESCA( DTYPE_ ) .EQ. 501 ) THEN NB = DESCA( 4 ) ICTXT = DESCA( 2 ) ELSE INFO = -( 6*100 + DTYPE_ ) CALL PXERBLA( ICTXT, \$ 'PCDBSV', \$ -INFO ) RETURN ENDIF * CALL BLACS_GRIDINFO( ICTXT, NPROW, NPCOL, MYROW, MYCOL ) * * * Size needed for AF in factorization * WS_FACTOR = NB*(BWL+BWU)+6*MAX(BWL,BWU)*MAX(BWL,BWU) * * Factor the matrix * CALL PCDBTRF( N, BWL, BWU, A, JA, DESCA, WORK, \$ MIN( LWORK, WS_FACTOR ), WORK( 1+WS_FACTOR ), \$ LWORK-WS_FACTOR, INFO ) * * Check info for error conditions * IF( INFO.NE.0 ) THEN IF( INFO .LT. 0 ) THEN CALL PXERBLA( ICTXT, 'PCDBSV', -INFO ) ENDIF RETURN END IF * * Solve the system using the factorization * CALL PCDBTRS( 'N', N, BWL, BWU, NRHS, A, JA, DESCA, B, IB, DESCB, \$ WORK, MIN( LWORK, WS_FACTOR ), WORK( 1+WS_FACTOR), \$ LWORK-WS_FACTOR, INFO ) * * Check info for error conditions * IF( INFO.NE.0 ) THEN CALL PXERBLA( ICTXT, 'PCDBSV', -INFO ) RETURN END IF * RETURN * * End of PCDBSV * END