pbdMPI (version 0.5-2)

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",
  rng = FALSE,
  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 a "seq" three parameters `c(from, to, by)` of the base `seq()` function (replaced deprecated "iopair" for 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

Is the primary load and location balance specification. The choices are: "balance" the chunks so they differ by no more than 1 item (used most frequently and default); "cycle" is the same as "balance" in terms of load but differs on location in that chunks are not contiguous, rather are assigned in a cycled way to ranks (note that "balance" and "cycle" are the same if `form` is "number"); "equal" maximizes the number of same size chunks resulting in one or more smaller or even zero size chunks carrying the leftover (required by pbdDMAT 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).

rng

If TRUE, set up different L'Ecuyere random number generator streams. Switch to stream i with comm.set.stream(i), where i is a global index. If form = "vector" random streams are set up for each index in the vector and only those needed by each rank are kept. If form = "number", each rank will use a different stream, set by default (so comm.set.stream does not need to be used). Additional ... parameter seed, passed to comm.set.seed, can be set for reproducibility.

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 that determines MPI rank numbers.

...

If rng = TRUE, then a seed parameter should be provided for comm.set.seed.

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.

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 (as in this example).
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)
}