PCGERQ2(l)

📦 version 1.5, in ubuntu jammy

💖 PCGERQ2 — bestie edition 💖

okay so. we're in the LAPACK cinematic universe, version 1.5, and today's main character is a routine called PCGERQ2. she's giving parallel linear algebra. she's giving RQ factorization. she is, frankly, iconic. pull up a chair 🫶

✨ NAME ✨

PCGERQ2 — computes an RQ factorization of a complex distributed M-by-N matrix sub( A ) = A(IA:IA+M-1,JA:JA+N-1) = R * Q. no cap. that's the whole job description 💅

🎀 SYNOPSIS 🎀

SUBROUTINE PCGERQ2( M, N, A, IA, JA, DESCA, TAU, WORK, LWORK, INFO )

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

    INTEGER         DESCA( * )

    COMPLEX         A( * ), TAU( * ), WORK( * )

🌸 PURPOSE 🌸

So PCGERQ2 computes an RQ factorization of a complex distributed M-by-N matrix:

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

periodt. ✨

📝 Notes

bestie, before we go any further — every global data object out here is described by an associated description vector. that vector stores all the info you need to establish the mapping between an object element and its corresponding process and memory location. it's the group chat of the array, honestly.

let A be a generic term for any 2D block cyclicly distributed array. such a global array has an associated description vector DESCA. and in the following comments, the character _ should be read as "of the global array". iykyk.

  • 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. lowkey chaotic of her but okay.
  • 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 that 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

👑 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. no negative row energy allowed bestie 🚫
  • 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) COMPLEX 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: if M <= N, the upper triangle of A( IA:IA+M-1, JA+N-M:JA+N-1 ) contains the M by M upper triangular matrix R; if M >= N, the elements on and above the (M-N)-th subdiagonal contain the M by N upper trapezoidal matrix R. the remaining elements, with the array TAU, represent the unitary matrix Q as a product of elementary reflectors (see Further Details). she's doing a lil glow-up on exit, we love a transformation arc 💫
  • 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) COMPLEX, array, dimension LOCr(IA+M-1). this array contains the scalar factors of the elementary reflectors. TAU is tied to the distributed matrix A. they're a package deal, fr fr 🫶
  • WORK — (local workspace/local output) COMPLEX 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 >= Nq0 + MAX( 1, Mp0 ), 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. it's the "just tell me how much room i need, no drama" mode 💅

  • INFO — (local output) INTEGER.
    • = 0: successful exit. slay. ✨
    • < 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.

🦋 FURTHER DETAILS 🦋

The matrix Q is represented as a product of elementary reflectors

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

Each H(i) has the form

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

where tau is a complex scalar, and v is a complex vector with v(n-k+i+1:n) = 0 and v(n-k+i) = 1; conjg(v(1:n-k+i-1)) is stored on exit in A(ia+m-k+i-1,ja:ja+n-k+i-2), and tau in TAU(ia+m-k+i-1). that's the whole lore drop, bestie. write it down 📝

heads up: INFO being negative is not a vibe. it's the routine telling you one of your arguments is giving illegal value. check your scalars and your array entries before you come for her 🫣

and that's the tea, my love. this page is from LAPACK version 1.5, dated 12 May 1997, and it documents PCGERQ2(l) — a true original, a legend, the blueprint 💜 go forth and factor responsibly. mwah. 💋

✨ the end ✨

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