TBB warning on 0 workers when running ExternalWork on a single thread

[makortel]

In comments of #31721 (#31721 (comment), #31721 (comment)) it is reported that in a single-threaded job that uses an ExternalWork TBB gives a warning

TBB Warning: The number of workers is currently limited to 0. The request for 1 workers is ignored. Further requests for more workers will be silently ignored until the limit changes.

We should understand the exact meaning of this warning.

[makortel]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@smuzaffar,@makortel you have been requested to review this Pull request/Issue and eventually sign? Thanks

[cmsbuild]

A new Issue was created by @makortel Matti Kortelainen.

@Dr15Jones, @dpiparo, @silviodonato, @smuzaffar, @makortel, @qliphy can you please review it and eventually sign/assign? Thanks.

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[makortel]

Could the change from tbb::task_scheduler_init to tbb::global_control + explicit tbb::task_arena (#31313) have caused this?

[makortel]

If the message really means that tbb::task_arena::enqueue() no longer creates an additional thread, this is actually the behavior we'd want. In that case it would be nice to be able to suppress the warning.

[makortel]

The warning is emitted here
https://github.com/oneapi-src/oneTBB/blob/d86ed7fbaaa33f1ab6f9ac2037776a1e6c16851a/src/tbb/market.cpp#L103-L106
and uxlfoundation/oneTBB#115 makes me believe that the explicit use of tbb::task_arena plays a role in the warning.