pilaenvx

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 11/07/2023

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pilaenvx

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

Syntax

result = pilaenvx (ictxt, ispec, name, opts, n1, n2, n3, n4 )

Include Files

mkl.fi

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) INTEGER. 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) INTEGER.

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) CHARACTER*(*).

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

opts

(global input) 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, and n4

(global input) INTEGER. Problem dimensions for the subroutine name; these may not all be required.

Output Parameters

result

(global output) INTEGER.

>= 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:

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, 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.
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.LE.1 ) NB = MAX( 1, N )

The same conventions hold for this ScaLAPACK-style variant.

Parent topic: ScaLAPACK Auxiliary Routines

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Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

pilaenvx

Syntax

Include Files

Description

Input Parameters

Output Parameters

Application Notes