Developer Reference for Intel® oneAPI Math Kernel Library - C

Developer Reference

Contents

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
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'
.
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:
  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', 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; 
}

Product and Performance Information

1

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