R CRAN Check fail when using parallel functions

Question

I want to submit a package to CRAN that uses parallel computation using parallel::makeCluster(parallel::detectCores()).

When I build the package everything works fine, but when I check the package (devtools::check(document = FALSE)) it returns the error:

Running examples in ‘TESTER-Ex.R’ failed
The error most likely occurred in:

> base::assign(".ptime", proc.time(), pos = "CheckExEnv")
> ### Name: hello_world
> ### Title: Prints hello world
> ### Aliases: hello_world
> 
> ### ** Examples
> 
> hello_world()
Error in .check_ncores(length(names)) : 8 simultaneous processes spawned
Calls: hello_world -> <Anonymous> -> makePSOCKcluster -> .check_ncores
Execution halted

I recreated the error in an MWE-package (TESTER), which only has one function hello_world, I have uploaded the whole TESTER-package to GitHub but the it should be reproducible from the following function.

#' Prints hello world
#'
#' @return nothing
#' @export
#'
#' @examples
#' hello_world()
hello_world <- function() {

  # initiate cluster
  cl <- parallel::makeCluster(parallel::detectCores())

  # stop cluster
  parallel::stopCluster(cl)

  cat("Hello World\n")
  return(invisible(NULL))
}

I have looked through Writing R Extensions but couldn't find anything related to this issue, neither could I find a question on SO.

Any ideas what causes this error and how to resolve it?

Stockstill

Answer 1

CRAN limits the number of cores available to packages to 2, for performance reasons. There was a thread in the mailing list, I believe, but I can't find it right now.

I use something like the following in my tests:

chk <- Sys.getenv("_R_CHECK_LIMIT_CORES_", "")

if (nzchar(chk) && chk == "TRUE") {
    # use 2 cores in CRAN/Travis/AppVeyor
    num_workers <- 2L
} else {
    # use all cores in devtools::test()
    num_workers <- parallel::detectCores()
}

Collaboration

I believe you mean this document: cran.r-project.org/web/packages/policies.html – Stockstill

2 cores... maybe someone should tell CRAN we are in 2020. – Foothold

Answer 2

I think the issue is that the library parallel is not loaded

in your NAMESPACE file you should load the package

import(parallel)

Bub

It seems that parallel is part of basic R, as in, we don't need to import it directly. Alexis answer was the key and resolves the issue – Stockstill