From mskb01 at gmail.com  Sun May 24 23:56:13 2026
From: mskb01 at gmail.com (Kumar MS)
Date: Sun, 24 May 2026 17:56:13 -0400
Subject: [Rcpp-devel] Nesting RcppParallel Worker objects and
 oversubscription
Message-ID: <CAFbaAiC5CFSrBGTQvXTOqLD=FtL_aeN+GCv0Ze8HT3f2wJ=9Hw@mail.gmail.com>

Hello,

I have an Rcpp function that nests RcppParallel workers. ( I am doing
cross-validation. The outer worker parallelizes over a grid of
hyperparameter values, and the inner worker parallelizes over folds ).

Before calling this function, I do a
`RcppParallel::setThreadOptions(6)` to set a desired number of threads
of six. However, I observe that whenever the nested worker gets
called, the total number of threads goes much higher (~12 or higher)
with CPU usage shooting to ~1200% on an M4 Mac with the code compiled
and linked against Homebrew's intel tbb.

I have also tried `tbb::global_control
ctll(tbb::global_control::max_allowed_parallelism, 6);` I see the same
behavior.

It appears nesting ignores the global setting?

Thank you,
Kumar

From kevinushey at gmail.com  Mon May 25 00:12:09 2026
From: kevinushey at gmail.com (Kevin Ushey)
Date: Sun, 24 May 2026 15:12:09 -0700
Subject: [Rcpp-devel] Nesting RcppParallel Worker objects and
 oversubscription
In-Reply-To: <CAFbaAiC5CFSrBGTQvXTOqLD=FtL_aeN+GCv0Ze8HT3f2wJ=9Hw@mail.gmail.com>
References: <CAFbaAiC5CFSrBGTQvXTOqLD=FtL_aeN+GCv0Ze8HT3f2wJ=9Hw@mail.gmail.com>
Message-ID: <CAJXgQP1VhTSXs+OimGAjc2k+bEd5x6JFimJtfPYq4=e5YuGynw@mail.gmail.com>

The short answer here is -- don't do that.

The behavior you're seeing is expected ? TBB is not designed for nested
parallelFor calls with externally managed thread counts. Both
setThreadOptions and global_control::max_allowed_parallelism constrain the
thread pool size, but not oversubscription across nested task arenas, and
TBB may spin up additional threads in nested contexts specifically to avoid
deadlock. The cleanest fix is to flatten your parallelism into a single
parallelFor over the full grid ? folds index space, decomposing the
combined index inside the worker ? this gives TBB a complete view of the
work, avoids the nesting issue entirely, and typically improves load
balancing as a bonus.

Best,
Kevin

On Sun, May 24, 2026, 2:56?PM Kumar MS <mskb01 at gmail.com> wrote:

> Hello,
>
> I have an Rcpp function that nests RcppParallel workers. ( I am doing
> cross-validation. The outer worker parallelizes over a grid of
> hyperparameter values, and the inner worker parallelizes over folds ).
>
> Before calling this function, I do a
> `RcppParallel::setThreadOptions(6)` to set a desired number of threads
> of six. However, I observe that whenever the nested worker gets
> called, the total number of threads goes much higher (~12 or higher)
> with CPU usage shooting to ~1200% on an M4 Mac with the code compiled
> and linked against Homebrew's intel tbb.
>
> I have also tried `tbb::global_control
> ctll(tbb::global_control::max_allowed_parallelism, 6);` I see the same
> behavior.
>
> It appears nesting ignores the global setting?
>
> Thank you,
> Kumar
> _______________________________________________
> Rcpp-devel mailing list
> Rcpp-devel at lists.r-forge.r-project.org
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>
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