# ilaenv

Environmental enquiry function that returns values for tuning algorithmic performance.

## Syntax

value = ilaenv( ispec, name, opts, n1, n2, n3, n4 )

## Description

Enquiry function ilaenv is called from the LAPACK routines to choose problem-dependent parameters for the local environment. See ispec below for a description of the parameters.

This version provides a set of parameters that 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.

## Input Parameters

ispec

  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 and QZ methods 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 ?gelsd and ?gesdd)

  =10: ieee NaN arithmetic can be trusted not to trap

  =11: infinity arithmetic can be trusted not to trap

  12 ≤ ispec ≤ 16: ?hseqr or one of its subroutines, see iparmq for detailed explanation.

name

  CHARACTER*(*). The name of the calling subroutine, in either upper case or lower case.

opts

  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, n2, n3, n4

  INTEGER. Problem dimensions for the subroutine name; these may not all be required.

## Output Parameters

value

  INTEGER.

  If `value ≥ 0`: the value of the parameter specified by ispec;

  If `value = -k < 0`: the k-th argument had an illegal value.

## Application Notes

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 )`

Below is an example of ilaenv usage in C language:

#include <stdio.h>

#include "mkl.h"

int main(void)

{

int size = 1000;

int ispec = 1;

int dummy = -1;

int blockSize1 = ilaenv(&ispec, "dsytrd", "U", &size, &dummy, &dummy, &dummy);

int blockSize2 = ilaenv(&ispec, "dormtr", "LUN", &size, &size, &dummy, &dummy);

printf("DSYTRD blocksize = %d\n", blockSize1);

printf("DORMTR blocksize = %d\n", blockSize2);

return 0;

}

## See Also

?hseqr Computes all eigenvalues and (optionally) the Schur factorization of a matrix reduced to Hessenberg form.
iparmq Environmental enquiry function which returns values for tuning algorithmic performance.
Parent topic: Utility Functions and Routines