Force single-threaded RcppParallel processing in forked parallelization

[HenrikBengtsson]

RcppParallel::setThreadOptions(numThreads = 1L)

https://rcppcore.github.io/RcppParallel/

[HenrikBengtsson]

Forcing single-threaded processing in nested parallel workers is the most conservative approach. We could relax this a bit by looking at the number of threads per core the CPU supports, cf. futureverse/parallelly#47

[HenrikBengtsson]

Can also just set environment variable RCPP_PARALLEL_NUM_THREADS; that avoids introducing a new package dependency. From RcppParallel 4.4.0 (2018-03-02) NEWS:

• Allow setting the number of threads to use via RCPP_PARALLEL_NUM_THREADS environment variable.

RcppParallel::setThreadOptions() sets this internally;

> RcppParallel::setThreadOptions
function (numThreads = "auto", stackSize = "auto") 
{
    if (identical(numThreads, "auto")) 
        numThreads <- -1L
    else if (!is.numeric(numThreads)) 
        stop("numThreads must be an integer")
    else numThreads <- as.integer(numThreads)
    if (identical(stackSize, "auto")) 
        stackSize <- 0L
    else if (!is.numeric(stackSize)) 
        stop("stackSize must be an integer")
    else stackSize <- as.integer(stackSize)
    if (!is.null(dllInfo) && isUsingTbb()) 
        setTbbThreadOptions(numThreads, stackSize)
    if (numThreads == -1L) 
        Sys.unsetenv("RCPP_PARALLEL_NUM_THREADS")
    else Sys.setenv(RCPP_PARALLEL_NUM_THREADS = numThreads)
}
<environment: namespace:RcppParallel>

[HenrikBengtsson]

Starting by doing it in forked parallel processing when option future.fork.multithreading.enable=FALSE, cf. how with already do it for OpenMP.