PILAENVX(3) ScaLAPACK routine of NEC Numeric Library Collection PILAENVX(3)
NAME
PILAENVX - is called from the ScaLAPACK routines to choose problem-
dependent parameters for the local environment
SYNOPSIS
INTEGER FUNCTION PILAENVX(
ICTXT, ISPEC, NAME, OPTS, N1, N2, N3, N4 )
CHARACTER*( * ) NAME, OPTS
INTEGER ICTXT, ISPEC, N1, N2, N3, N4
PURPOSE
PILAENVX is called from the ScaLAPACK routines to choose problem-depen-
dent parameters for the local environment. See ISPEC for a description
of the parameters.
This version provides a set of parameters which should give good, but
not optimal, performance on many of the currently available computers.
Users are encouraged to modify this subroutine to set the tuning param-
eters for their particular machine using the option and problem size
information in the arguments.
This routine will not function correctly if it is converted to all
lower case. Converting it to all upper case is allowed.
ARGUMENTS
ICTXT (local input) INTEGER
On entry, ICTXT specifies the BLACS context handle, indica-
ting the global context of the operation. The context itself
is global, but the value of ICTXT is local.
ISPEC (global input) INTEGER
Specifies the parameter to be returned as the value of
PILAENVX.
= 1: the optimal blocksize; if this value is 1, an unblocked
algorithm will give the best performance (unlikely).
= 2: the minimum block size for which the block routine
should be used; if the usable block size is less than
this value, an unblocked routine should be used.
= 3: the crossover point (in a block routine, for N less
than this value, an unblocked routine should be used)
= 4: the number of shifts, used in the nonsymmetric
eigenvalue routines (DEPRECATED)
= 5: the minimum column dimension for blocking to be used;
rectangular blocks must have dimension at least k by m,
where k is given by PILAENVX(2,...) and m by
PILAENVX(5,...)
= 6: the crossover point for the SVD (when reducing an m by n
matrix to bidiagonal form, if max(m,n)/min(m,n) exceeds
this value, a QR factorization is used first to reduce
the matrix to a triangular form.)
= 7: the number of processors
= 8: the crossover point for the multishift QR method
for nonsymmetric eigenvalue problems (DEPRECATED)
= 9: maximum size of the subproblems at the bottom of the
computation tree in the divide-and-conquer algorithm
(used by xGELSD and xGESDD)
=10: ieee NaN arithmetic can be trusted not to trap
=11: infinity arithmetic can be trusted not to trap
12 <= ISPEC <= 16:
PxHSEQR or one of its subroutines,
see PIPARMQ for detailed explanation
17 <= ISPEC <= 22:
Parameters for PBxTRORD/PxHSEQR (not all), as follows:
=17: maximum number of concurrent computational windows;
=18: number of eigenvalues/bulges in each window;
=19: computational window size;
=20: minimal percentage of flops required for
performing matrix-matrix multiplications instead
of pipelined orthogonal transformations;
=21: width of block column slabs for row-wise
application of pipelined orthogonal
transformations in their factorized form;
=22: the maximum number of eigenvalues moved together
over a process border;
=23: the number of processors involved in AED;
=99: Maximum iteration chunksize in OpenMP parallelization
NAME (global input) CHARACTER*(*)
The name of the calling subroutine, in either upper case or
lower case.
OPTS (global input) CHARACTER*(*)
The character options to the subroutine NAME, concatenated into
a single character string. For example, UPLO = 'U', TRANS =
'T', and DIAG = 'N' for a triangular routine would be specified
as OPTS = 'UTN'.
N1 (global input) INTEGER
N2 (global input) INTEGER
N3 (global input) INTEGER
N4 (global input) INTEGER
Problem dimensions for the subroutine NAME; these may not all
be required.
(PILAENVX) (global output) INTEGER
>= 0: the value of the parameter specified by ISPEC
< 0: if PILAENVX = -k, the k-th argument had an illegal value.
Further Details
===============
The following conventions have been used when calling ILAENV from the
LAPACK routines:
1) OPTS is a concatenation of all of the character options to
subroutine NAME, in the same order that they appear in the
argument list for NAME, even if they are not used in determining
the value of the parameter specified by ISPEC.
2) The problem dimensions N1, N2, N3, N4 are specified in the order
that they appear in the argument list for NAME. N1 is used
first, N2 second, and so on, and unused problem dimensions are
passed a value of -1.
3) The parameter value returned by ILAENV is checked for validity in
the calling subroutine. For example, ILAENV is used to retrieve
the optimal blocksize for STRTRI as follows:
NB = ILAENV( 1, 'STRTRI', UPLO // DIAG, N, -1, -1, -1 )
IF( NB.LE.1 ) NB = MAX( 1, N )
The same conventions will hold for this ScaLAPACK-style variant.
ScaLAPACK routine 31 October 2017 PILAENVX(3)