Developer Reference for Intel® oneAPI Math Kernel Library - C

Developer Reference

Contents

pilaenvx

Called from the ScaLAPACK routines to choose problem-dependent parameters for the local environment.

Syntax

MKL_INT pilaenvx
(
const
MKL_INT*
ictxt
,
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*
Include Files
  • mkl.h
Description
pilaenvx
 is called from the ScaLAPACK routines to choose problem-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, though not optimal, performance on many of the currently available computers. You are encouraged to modify this subroutine to set the tuning parameters for your particular machine using the option and problem size information in the arguments.
Input Parameters
ictxt
(local input)On entry,
ictxt
 specifies the BLACS context handle, indicating the global context of the operation. The context itself is global, but the value of
ictxt
 is local.
ispec
(global input)
Specifies the parameter to be returned as the value of
pilaenvx
.
= 1: the optimal blocksize; if this value is 1, an unblocked algorithm will give the best performance (unlikely).
= 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
pilaenvx
(2,...)
 and
m
 by
pilaenvx
(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 method 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:
p?hseqr
 or one of its subroutines, see
piparmq
 for detailed explanation.
17 <=
ispec
 <= 22:
Parameters for
pb?trord
/
p?hseqr
 (not all), as follows:
=17: maximum number of concurrent computational windows;
=18: number of eigenvalues/bulges in each window;
=19: computational window size;
=20: minimal percentage of FLOPS required for performing matrix-matrix multiplications instead of pipelined orthogonal transformations;
=21: width of block column slabs for row-wise application of pipelined orthogonal transformations in their factorized form;
=22: the maximum number of eigenvalues moved together over a process border;
=23: the number of processors involved in Aggressive Early Deflation (AED);
=99: Maximum iteration chunksize in OpenMP parallelization.
name
(global input)
The name of the calling subroutine, in either upper case or lower case.
opts
(global input) 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
, and
n4
(global input) Problem dimensions for the subroutine name; these may not all be required.
Output Parameters
result
(global output)
>= 0: the value of the parameter specified by
ispec
.
< 0: if
pilaenvx
 = -
k
, 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
    , and
    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 block size for
    strtri
     as follows:
 NB = ilaenv( 1, 'STRTRI', UPLO // DIAG, N, -1, -1, -1 );
if( NB<=1 ) {
   NB = MAX( 1, N );
}
The same conventions hold for this ScaLAPACK-style variant.

Product and Performance Information

1

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