
ILAENV(l) ) ILAENV(l)
NAME
ILAENV  i called from the LAPACK routines to choose problemdependent 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 problemdependent 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, perfor
mance 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 perfor
mance. = 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 non
symmetric eigenvalue routines = 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 and QZ methods for nonsymmetric ei
genvalue problems. = 9: maximum size of the subproblems at the bottom of the com
putation tree in the divideandconquer 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
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.
>= 0: the value of the parameter specified by ISPEC < 0: if ILAENV = k, the kth
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 )
LAPACK version 3.0 15 June 2000 ILAENV(l) 
