ilaenv(3)

NAME

ILAENV - i called from the LAPACK routines to choose prob

lem-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 prob

lem-dependent 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 subrou

tine 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 convert

ed to all lower case. Converting it to all upper case is al

lowed.

ARGUMENTS

ISPEC (input) INTEGER

Specifies the parameter to be returned as the val

ue 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 rou

tine should be used. = 3: the crossover point (in a block rou

tine, for N less than this value, an unblocked routine should be

used) = 4: the number of shifts, used in the nonsymmetric eigen

value 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 eigenvalue problems. = 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

NAME (input) CHARACTER*(*)

The name of the calling subroutine, in either up

per case or lower case.

OPTS (input) CHARACTER*(*)

The character options to the subroutine NAME, con

catenated 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 IS

PEC < 0: if ILAENV = -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 op

tions 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 dimen

sions 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(3)