[Rd] How can a package be aware of whether it's on CRAN

[Henrik Bengtsson]

> Is there any reliable way to let packages to know if they are on CRAN, so they can set omp cores to 2 by default?

Instead of testing for "on CRAN" or not, you can test for 'R CMD
check' running or not. 'R CMD check' sets environment variable
_R_CHECK_LIMIT_CORES_=TRUE. You can use that to limit your code to run
at most two (2) parallel threads or processes.

The parallelly::availableCores() function is agile to this and many
other settings, i.e. it'll return 2 when running via 'R CMD check'. As
the author, I obviously suggest using that function to query what
amount of CPU resources your R process is allowed to use. For more
info, see <https://parallelly.futureverse.org/index.html#availablecores-vs-paralleldetectcores>.

/Henrik

PS. I'm in the camp of *not* having R packages parallelize by default.
At least not until R and its community have figured out how to avoid
ending up with nested parallelization (e.g. via dependencies) by
mistake.  We would also need a standard for end-users (and the sysadms
on the machines they're running) to control the default number of CPU
cores the R session may use.  Right now we only have a few scattered
settings for separate purposes, e.g. option 'mc.cores'/env var
'MC_CORES', and option 'Ncpus', which is not enough for establishing a
de facto standard.

On Tue, Nov 23, 2021 at 11:11 AM Dipterix Wang <dipterix.wang using gmail.com> wrote:
>
> Dear R wizards,
>
> I recently received an email from Prof. Ripley. He pointed out that my package seriously violates the CRAN policy: "using 8 threads is a serious violation of the CRAN policy”. By default the number of cores my package uses is determined from system CPU cores. After carefully reading all the CRAN policies, now I understand that CRAN does not allow a package to use more than 2 CPU cores when checking a package. I can easily change my code to let my tests comply to that constraint.
>
> However, this warning worries me because my package uses OpenMP. I got “caught" partially because I printed the number of cores used in the package startup message, and one of my test exceeded the time limit (which leads to manual inspection). However, what if I develop a package that imports on those openmp-dependent packages? (For example, data.table, fst…) These packages use more than 2 cores by default. If not carefully treated, it’ll be very easy to exceed that limit, and it’s very hard for CRAN to detect it.
>
> Is there any reliable way to let packages to know if they are on CRAN, so they can set omp cores to 2 by default?
>
> Best,
> - Dipterix
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