pbdMPI (version 0.4-6)

comm.chunk: comm.chunk

Description

Given a total number of items, N, comm.chunk splits the number into chunks. Tailored especially for situations in SPMD style programming, potentially returning different results to each rank. Optionally, results for all ranks can be returned to all.

Usage

comm.chunk(
  N,
  form = "number",
  type = "balance",
  lo.side = "right",
  all.rank = FALSE,
  p = NULL,
  rank = NULL,
  comm = .pbd_env$SPMD.CT$comm
)

Value

A numeric value from 0:N or a vector giving a subset of 1:N (depending on form) for the rank. If all.rank is TRUE, a vector or a list of vectors, respectively.

Arguments

N

The number of items to split into chunks.

form

Output a chunk as a single "number", as a "vector" of items from 1:N, or as an "iopair" giving offset and length in a file. Forms "ldim" and "bldim" are available only with type "equal" and are intended for setting "ddmatrix" (see package pbdDMAT) slots.

type

Either "balance" the chunks so they differ by no more than 1 item (used most frequently for best balance) or force as many as possible to be "equal" with possibly one or more smaller or even zero size chunks (required by ScaLAPACK's block-cyclic layouts).

lo.side

If exact balance is not possible, put the smaller chunks on the "left" (low ranks) or on the "right" (high ranks).

all.rank

FALSE returns only the chunk for rank r. TRUE returns a vector of length p (when form="number"), and a list of length p (when form="vector") each containing the output for the corresponding rank.

p

The number of chunks (processors). Normally, it is NOT specified and defaults to NULL, which assigns comm.size(comm).

rank

The rank of returned chunk. Normally, it is NOT specified and defaults to NULL, which assigns comm.rank(comm)). Note that ranks are numbered from 0 to p-1, whereas the list elements for all.rank=TRUE are numbered 1 to p.

comm

The communicator across which the split will be taken.

Details

Various chunking options are possible when the number does not split evenly into equal chunks. The output form can be a number, a vector of items, or a few other special forms intended for pbdR components.

If the communicator was divided with comm.split(), the chunking is done within all sub-communicators respective to their size. This is useful for setting up collective operations that take place on disjoint subsets of ranks.

Examples

if (FALSE) {
## Note that the p and rank parameters are provided by comm.size() and
## comm.rank(), respectively, when running SPMD in parallel. Normally, they
## are not specified unless testing in serial mode.
library(pbdIO)

comm.chunk(16, all.rank = TRUE, p = 5)
comm.chunk(16, type = "equal", all.rank = TRUE, p = 5)
comm.chunk(16, type = "equal", lo.side = "left", all.rank = TRUE, p = 5)
comm.chunk(16, p = 5, rank = 0)
comm.chunk(16, p = 5, lo.side = "left", rank = 0)
}