ilaenv

Developer Reference for Intel® oneAPI Math Kernel Library for C

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ID 766684

Date 6/24/2024

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ilaenv

Environmental enquiry function that returns values for tuning algorithmic performance.

Syntax

MKL_INT ilaenv (const MKL_INT *ispec, const char *name, const char *opts, const MKL_INT *n1, const MKL_INT *n2, const MKL_INT *n3, const MKL_INT *n4);

Include Files

mkl.h

Description

The 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.

Product and Performance Information
Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex. Notice revision #20201201

Product and Performance Information

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.

Notice revision #20201201

Input Parameters

ispec

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

The name of the calling subroutine, in either upper case or lower case.

opts

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'.

NOTE:

Use only uppercase characters for the opts string.

n1, n2, n3, n4

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

Output Parameters

value

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

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

Return Values

ilaenv returns value.

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:

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.
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.
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', strcat (uplo, diag), n, -1, -1, -1> );
  if( nb <= 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; 
}

Parent topic: LAPACK Utility Functions and Routines

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Developer Reference for Intel® oneAPI Math Kernel Library for C

ilaenv

Syntax

Include Files

Description

Input Parameters

Output Parameters

Return Values

Application Notes

See Also