PSLAHQR(l) ) PSLAHQR(l)NAME
PSLAHQR - i an auxiliary routine used to find the Schur decomposition
and or eigenvalues of a matrix already in Hessenberg form from cols ILO
to IHI
SYNOPSIS
SUBROUTINE PSLAHQR( WANTT, WANTZ, N, ILO, IHI, A, DESCA, WR, WI, ILOZ,
IHIZ, Z, DESCZ, WORK, LWORK, IWORK, ILWORK, INFO )
LOGICAL WANTT, WANTZ
INTEGER IHI, IHIZ, ILO, ILOZ, ILWORK, INFO, LWORK, N
INTEGER DESCA( * ), DESCZ( * ), IWORK( * )
REAL A( * ), WI( * ), WORK( * ), WR( * ), Z( * )
PURPOSE
PSLAHQR is an auxiliary routine used to find the Schur decomposition
and or eigenvalues of a matrix already in Hessenberg form from cols ILO
to IHI. Notes
=====
Each global data object is described by an associated description vec‐
tor. This vector stores the information required to establish the map‐
ping 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 col‐
umn.
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
ARGUMENTS
WANTT (global input) LOGICAL
= .TRUE. : the full Schur form T is required;
= .FALSE.: only eigenvalues are required.
WANTZ (global input) LOGICAL
= .TRUE. : the matrix of Schur vectors Z is required;
= .FALSE.: Schur vectors are not required.
N (global input) INTEGER
The order of the Hessenberg matrix A (and Z if WANTZ). N >= 0.
ILO (global input) INTEGER
IHI (global input) INTEGER It is assumed that A is already
upper quasi-triangular in rows and columns IHI+1:N, and that
A(ILO,ILO-1) = 0 (unless ILO = 1). PSLAHQR works primarily with
the Hessenberg submatrix in rows and columns ILO to IHI, but
applies transformations to all of H if WANTT is .TRUE.. 1 <=
ILO <= max(1,IHI); IHI <= N.
A (global input/output) REAL array, dimension
(DESCA(LLD_),*) On entry, the upper Hessenberg matrix A. On
exit, if WANTT is .TRUE., A is upper quasi-triangular in rows
and columns ILO:IHI, with any 2-by-2 or larger diagonal blocks
not yet in standard form. If WANTT is .FALSE., the contents of
A are unspecified on exit.
DESCA (global and local input) INTEGER array of dimension DLEN_.
The array descriptor for the distributed matrix A.
WR (global replicated output) REAL array,
dimension (N) WI (global replicated output) REAL array,
dimension (N) The real and imaginary parts, respectively, of
the computed eigenvalues ILO to IHI are stored in the corre‐
sponding elements of WR and WI. If two eigenvalues are computed
as a complex conjugate pair, they are stored in consecutive
elements of WR and WI, say the i-th and (i+1)th, with WI(i) > 0
and WI(i+1) < 0. If WANTT is .TRUE., the eigenvalues are stored
in the same order as on the diagonal of the Schur form returned
in A. A may be returned with larger diagonal blocks until the
next release.
ILOZ (global input) INTEGER
IHIZ (global input) INTEGER Specify the rows of Z to which
transformations must be applied if WANTZ is .TRUE.. 1 <= ILOZ
<= ILO; IHI <= IHIZ <= N.
Z (global input/output) REAL array.
If WANTZ is .TRUE., on entry Z must contain the current matrix
Z of transformations accumulated by PDHSEQR, and on exit Z has
been updated; transformations are applied only to the submatrix
Z(ILOZ:IHIZ,ILO:IHI). If WANTZ is .FALSE., Z is not refer‐
enced.
DESCZ (global and local input) INTEGER array of dimension DLEN_.
The array descriptor for the distributed matrix Z.
WORK (local output) REAL array of size LWORK
LWORK (local input) INTEGER
WORK(LWORK) is a local array and LWORK is assumed big enough so
that LWORK >= 3*N + MAX( 2*MAX(DESCZ(LLD_),DESCA(LLD_)) +
2*LOCc(N), 7*Ceil(N/HBL)/LCM(NPROW,NPCOL)) )
IWORK (global and local input) INTEGER array of size ILWORK
ILWORK (local input) INTEGER
This holds the some of the IBLK integer arrays. This is held
as a place holder for the next release.
INFO (global output) INTEGER
< 0: parameter number -INFO incorrect or inconsistent
= 0: successful exit
> 0: PSLAHQR failed to compute all the eigenvalues ILO to IHI
in a total of 30*(IHI-ILO+1) iterations; if INFO = i, elements
i+1:ihi of WR and WI contain those eigenvalues which have been
successfully computed.
Logic: This algorithm is very similar to _LAHQR. Unlike
_LAHQR, instead of sending one double shift through the largest
unreduced submatrix, this algorithm sends multiple double
shifts and spaces them apart so that there can be parallelism
across several processor row/columns. Another critical differ‐
ence is that this algorithm aggregrates multiple transforms
together in order to apply them in a block fashion.
Important Local Variables: IBLK = The maximum number of bulges
that can be computed. Currently fixed. Future releases this
won't be fixed. HBL = The square block size
(HBL=DESCA(MB_)=DESCA(NB_)) ROTN = The number of transforms to
block together NBULGE = The number of bulges that will be
attempted on the current submatrix. IBULGE = The current num‐
ber of bulges started. K1(*),K2(*) = The current bulge loops
from K1(*) to K2(*).
Subroutines: This routine calls: PSLACONSB -> To determine
where to start each iteration PSLAWIL -> Given the shift, get
the transformation SLASORTE -> Pair up eigenvalues so that
reals are paired. PSLACP3 -> Parallel array to local repli‐
cated array copy & back. SLAREF -> Row/column reflector
applier. Core routine here. PSLASMSUB -> Finds negligible
subdiagonal elements.
Current Notes and/or Restrictions: 1.) This code requires the
distributed block size to be square and at least six (6);
unlike simpler codes like LU, this algorithm is extremely sen‐
sitive to block size. Unwise choices of too small a block size
can lead to bad performance. 2.) This code requires A and Z to
be distributed identically and have identical contxts. 3.)
This release currently does not have a routine for resolving
the Schur blocks into regular 2x2 form after this code is com‐
pleted. Because of this, a significant performance impact is
required while the deflation is done by sometimes a single col‐
umn of processors. 4.) This code does not currently block the
initial transforms so that none of the rows or columns for any
bulge are completed until all are started. To offset pipeline
start-up it is recommended that at least 2*LCM(NPROW,NPCOL)
bulges are used (if possible) 5.) The maximum number of bulges
currently supported is fixed at 32. In future versions this
will be limited only by the incoming WORK array. 6.) The
matrix A must be in upper Hessenberg form. If elements below
the subdiagonal are nonzero, the resulting transforms may be
nonsimilar. This is also true with the LAPACK routine. 7.)
For this release, it is assumed RSRC_=CSRC_=0 8.) Currently,
all the eigenvalues are distributed to all the nodes. Future
releases will probably distribute the eigenvalues by the column
partitioning. 9.) The internals of this routine are subject to
change.
Implemented by: G. Henry, November 17, 1996
ScaLAPACK version 1.7 13 August 2001 PSLAHQR(l)
