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\name{workers}
% Environment variables
\alias{BIOCPARALLEL_WORKER_NUMBER}
\alias{BIOCPARALLEL_WORKER_MAX}
\alias{R_PARALLELLY_AVAILABLECORES_FALLBACK}
\title{Environment control of worker number}
\description{
Environment variables, global options, and aspects of the computing
environment controlling default and maximum worker number.
}
\details{
By default, BiocParallel \code{Param} objects use almost all
(\code{parallel::detectCores() - 2}) available cores as
workers. Several variables can determine alternative default number of
workers. Elements earlier in the description below override elements
later in the description.
\describe{
\item{\code{_R_CHECK_LIMIT_CORES_}:}{Environment variable defined in
base R, described in the 'R Internals' manual
(\code{RShowDoc("R-ints")}). If defined and not equal to
\code{"false"} or \code{"FALSE"}, default to 2 workers.}
\item{\code{IS_BIOC_BUILD_MACHINE}:}{Environment variable used by
the Bioconductor build system; when defined, default to 4
workers.}
\item{\code{getOption("mc.cores")}:}{Global R option (initialized
from the environment variable \code{MC_CORES}) with non-negative
integer number of workers, also recognized by the base R
'parallel' package.}
\item{\code{BIOCPARALLEL_WORKER_MAX:}}{Environment variable,
non-negative integer number of workers. Use this to set both the
default and maximum worker number to a single value.}
\item{\code{BIOCPARALLEL_WORKER_NUMBER}:}{Environment variable,
non-negative integer number of workers. Use this to set a default
worker number without specifying \code{BIOCPARALLEL_WORKER_MAX},
or to set a default number of workers less than the maximum
number.}
\item{\code{R_PARALLELLY_AVAILABLECORES_FALLBACK}:}{Environment
variable, non-negative integer number of workers, also recognized by
the 'parallelly' family of packages.}
}
A subset of environment variables and other aspects of the computing
environment also \emph{enforce} limits on worker number. Usually, a
request for more than the maximum number of workers results in a
warning message and creation of a 'Param' object with the maximum
rather than requested number of workers.
\describe{
\item{\code{_R_CHECK_LIMIT_CORES_}:}{Environment variable defined in
base R. \code{"warn"} limits the number of workers to 2, with a
warning; \code{"false"}, or \code{"FALSE"} does not limit worker
number; any other value generates an error.}
\item{\code{IS_BIOC_BUILD_MACHINE}:}{Environment variable used by
the Bioconductor build system. When set, limit the number of
workers to 4.}
\item{\code{BIOCPARALLEL_WORKER_MAX:}}{Environment variable,
non-negative integer.}
\item{Number of available connections:}{R has an internal limit
(126) on the number of connections open at any time. 'SnowParam()'
and 'MulticoreParam()' use 1 connection per worker, and so are
limited by the number of available connections.}
}
}
\examples{
## set up example
original_worker_max <- Sys.getenv("BIOCPARALLEL_WORKER_MAX", NA_integer_)
original_worker_n <- Sys.getenv("BIOCPARALLEL_WORKER_NUMBER", NA_integer_)
Sys.setenv(BIOCPARALLEL_WORKER_MAX = 4)
Sys.setenv(BIOCPARALLEL_WORKER_NUMBER = 2)
bpnworkers(SnowParam()) # 2
bpnworkers(SnowParam(4)) # OK
bpnworkers(SnowParam(5)) # warning; set to 4
## clean up
Sys.unsetenv("BIOCPARALLEL_WORKER_MAX")
if (!is.na(original_worker_max))
Sys.setenv(BIOCPARALLEL_WORKER_MAX = original_worker_max)
Sys.unsetenv("BIOCPARALLEL_WORKER_NUMBER")
if (!is.na(original_worker_n))
Sys.setenv(BIOCPARALLEL_WORKER_NUMBER = original_worker_n)
}
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