PSGELQF(l)

💖 PSGELQF 💖

okay bestie, welcome to PSGELQF, a LAPACK routine, version 1.5, and she is booked and busy computing LQ factorizations of real distributed matrices. pull up a chair, grab a strawberry 🍓, and let's get into it.

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🌸 NAME 🌸

PSGELQF — computes an LQ factorization of a real distributed M-by-N matrix

sub( A ) = A(IA:IA+M-1,JA:JA+N-1) = L * Q

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✨ SYNOPSIS ✨

SUBROUTINE PSGELQF( M, N, A, IA, JA, DESCA, TAU, WORK, LWORK, INFO )

    INTEGER         IA, INFO, JA, LWORK, M, N

    INTEGER         DESCA( * )

    REAL            A( * ), TAU( * ), WORK( * )

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🦋 PURPOSE 🦋

PSGELQF computes an LQ factorization of a real distributed M-by-N matrix

sub( A ) = A(IA:IA+M-1,JA:JA+N-1) = L * Q.

💅 Notes

Every global data object out here has an associated description vector riding with it. That vector is literally the whole blueprint — it stores the info you need to map an element of the object to the process and the memory location it actually lives in.

So let A be a generic term for any 2D block cyclicly distributed array, bestie. A global array like that has an associated description vector DESCA. In all the commentary below, read the character _ as "of the global array". iykyk 🫶

Here's the whole notation situation:

• DTYPE_A (global) — stored in DESCA( DTYPE_ ) — the descriptor type. In this case, DTYPE_A = 1.
• CTXT_A (global) — stored in DESCA( CTXT_ ) — the BLACS context handle, indicating the BLACS process grid A is distributed over. The context itself is global, but the handle (the integer value) may vary.
• M_A (global) — stored in DESCA( M_ ) — the number of rows in the global array A.
• N_A (global) — stored in DESCA( N_ ) — the number of columns in the global array A.
• MB_A (global) — stored in DESCA( MB_ ) — the blocking factor used to distribute the rows of the array.
• NB_A (global) — stored in DESCA( NB_ ) — the blocking factor used to distribute the columns of the array.
• RSRC_A (global) — stored in DESCA( RSRC_ ) — the process row over which the first row of the array A is distributed.
• CSRC_A (global) — stored in DESCA( CSRC_ ) — the process column over which the first column of the array A is distributed.
• LLD_A (local) — stored in DESCA( LLD_ ) — the leading dimension of the local array. LLD_A >= MAX(1,LOCr(M_A)).

Now let K be the number of rows or columns of a distributed matrix, and assume its process grid has dimension p x q.

LOCr( K ) denotes the number of elements of K that a process would receive if K were distributed over the p processes of its process column.

Similarly, LOCc( K ) denotes the number of elements of K that a process would receive if K were distributed over the q processes of its process row.

The values of LOCr() and LOCc() may be determined via a call to the ScaLAPACK tool function, NUMROC:

LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ).

An upper bound for these quantities may be computed by:

LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A

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🎀 ARGUMENTS 🎀

• M — (global input) INTEGER — the number of rows to be operated on, i.e. the number of rows of the distributed submatrix sub( A ). M >= 0.
• N — (global input) INTEGER — the number of columns to be operated on, i.e. the number of columns of the distributed submatrix sub( A ). N >= 0.
• A — (local input/local output) REAL pointer into the local memory to an array of dimension (LLD_A, LOCc(JA+N-1)). On entry, the local pieces of the M-by-N distributed matrix sub( A ) which is to be factored. On exit, the elements on and below the diagonal of sub( A ) contain the M by min(M,N) lower trapezoidal matrix L (L is lower triangular if M <= N); the elements above the diagonal, with the array TAU, represent the orthogonal matrix Q as a product of elementary reflectors (see Further Details).
• IA — (global input) INTEGER — the row index in the global array A indicating the first row of sub( A ).
• JA — (global input) INTEGER — the column index in the global array A indicating the first column of sub( A ).
• DESCA — (global and local input) INTEGER array of dimension DLEN_. The array descriptor for the distributed matrix A.
• TAU — (local output) REAL, array, dimension LOCr(IA+MIN(M,N)-1). This array contains the scalar factors of the elementary reflectors. TAU is tied to the distributed matrix A. 🔗
• WORK — (local workspace/local output) REAL array, dimension (LWORK). On exit, WORK(1) returns the minimal and optimal LWORK.
• LWORK — (local or global input) INTEGER — the dimension of the array WORK. LWORK is local input and must be at least

LWORK >= MB_A * ( Mp0 + Nq0 + MB_A ),

where

IROFF = MOD( IA-1, MB_A ), ICOFF = MOD( JA-1, NB_A ), IAROW =
INDXG2P( IA, MB_A, MYROW, RSRC_A, NPROW ), IACOL = INDXG2P( JA,
NB_A, MYCOL, CSRC_A, NPCOL ), Mp0   = NUMROC( M+IROFF, MB_A,
MYROW, IAROW, NPROW ), Nq0   = NUMROC( N+ICOFF, NB_A, MYCOL,
IACOL, NPCOL ),

and NUMROC, INDXG2P are ScaLAPACK tool functions; MYROW, MYCOL, NPROW and NPCOL can be determined by calling the subroutine BLACS_GRIDINFO.

If LWORK = -1, then LWORK is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. Each of these values is returned in the first entry of the corresponding work array, and no error message is issued by PXERBLA.

• INFO — (global output) INTEGER
  • = 0: successful exit 🎉
  • < 0: if the i-th argument is an array and the j-entry had an illegal value, then INFO = -(i*100+j); if the i-th argument is a scalar and had an illegal value, then INFO = -i.

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🔥 FURTHER DETAILS 🔥

The matrix Q is represented as a product of elementary reflectors

Q = H(ia+k-1) H(ia+k-2) . . . H(ia), where k = min(m,n).

Each H(i) has the form

H(i) = I - tau * v * v'

where tau is a real scalar, and v is a real vector with v(1:i-1)=0 and v(i) = 1; v(i+1:n) is stored on exit in A(ia+i-1,ja+i:ja+n-1), and tau in TAU(ia+i-1).

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and that's the whole page, bestie, no notes, we ate 💅✨ this one's from LAPACK version 1.5, dated 12 May 1997 — vintage, iconic, still slaying. mwah 🎀🦋