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 ei‐
genvalue routines (DEPRECATED) = 5: the minimum column dimen‐
sion 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 non‐
symmetric 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 )
LAPACK auxiliary routine (versioNovember 2008 ILAENV(1)
