table of contents
ILAENV(1) | LAPACK auxiliary routine (version 3.2) | ILAENV(1) |
NAME¶
ILAENV - is called from the LAPACK routines to choose problem-dependent parameters for the local environment
SYNOPSIS¶
- INTEGER FUNCTION
- ILAENV( ISPEC, NAME, OPTS, N1, N2, N3, N4 )
CHARACTER*( * ) NAME, OPTS INTEGER ISPEC, N1, N2, N3, N4
PURPOSE¶
ILAENV is called from the LAPACK routines to choose
problem-dependent parameters for the local environment. See ISPEC for a
description of the parameters.
ILAENV returns an INTEGER
if ILAENV >= 0: ILAENV returns the value of the parameter specified by
ISPEC if ILAENV < 0: if ILAENV = -k, the k-th argument had an illegal
value. 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 parameters 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¶
- ISPEC (input) INTEGER
- Specifies the parameter to be returned as the value of ILAENV. = 1: the
optimal blocksize; if this value is 1, an unblocked algorithm will give
the best performance. = 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 ILAENV(2,...) and m by ILAENV(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: xHSEQR or one of its subroutines, see IPARMQ for detailed explanation - NAME (input) CHARACTER*(*)
- The name of the calling subroutine, in either upper case or lower case.
- OPTS (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 (input) INTEGER
- N2 (input) INTEGER N3 (input) INTEGER N4 (input) INTEGER Problem dimensions for the subroutine NAME; these may not all be required.
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 )
November 2008 | LAPACK auxiliary routine (version 3.2) |