Called from the ScaLAPACK routines to choose problem-dependent parameters for the local environment.
pilaenvxis called from the ScaLAPACK routines to choose problem-dependent parameters for the local environment. See
ispecfor 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.
- (local input)INTEGER.On entry,ictxtspecifies the BLACS context handle, indicating the global context of the operation. The context itself is global, but the value ofictxtis local.
- (global input)INTEGER.Specifies the parameter to be returned as the value ofpilaenvx.= 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 leastkbym, wherekis given byandpilaenvx(2,...)mby.pilaenvx(5,...)= 6: the crossover point for the SVD (when reducing anmbynmatrix 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?gelsdand?gesdd).=10: IEEE NaN arithmetic can be trusted not to trap.=11: infinity arithmetic can be trusted not to trap.12 <=ispec<= 16:p?hseqror one of its subroutines, seepiparmqfor detailed explanation.17 <=ispec<= 22:Parameters forpb?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.
- (global input)CHARACTER*(*).The name of the calling subroutine, in either upper case or lower case.
- (global input)CHARACTER*(*).The character options to the subroutine name, concatenated into a single character string. For example,,uplo= 'U', andtrans= 'T'for a triangular routine would be specified asdiag= 'N'.opts= 'UTN'
- n1,n2,n3, andn4
- (global input)INTEGER.Problem dimensions for the subroutine name; these may not all be required.
- (global output)INTEGER.>= 0: the value of the parameter specified byispec.< 0: ifpilaenvx= -k, thek-th argument had an illegal value.
The following conventions have been used when calling
ilaenvfrom the LAPACK routines:
- optsis 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 byispec.
- The problem dimensionsn1,n2,n3, andn4are specified in the order that they appear in the argument list forname.n1is used first,n2second, and so on, and unused problem dimensions are passed a value of -1.
- The parameter value returned byilaenvis checked for validity in the calling subroutine. For example,ilaenvis used to retrieve the optimal block size forstrtrias 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.