JAM_SEMAPHORE documentation?

Hi Builders,
Is there documentation on JAM_SEMAPHORE ?  I see examples; but google did not reveal any documentation.
Why?
I'm trying to find way to help the build team speed up our nightly builds?

I retained a semaphore on the link when I created clang-vxworks.jam
https://github.com/boostorg/build/blob/bc7659b9afe79f6ea8e70e8ea4a6974286d67d0d/src/tools/clang-vxworks.jam#L117
but I question if it is really required anymore?

I'd also like to serialize test execution
For VxWorks we telnet and execute the test on a remote target board using a "launcher"
https://github.com/Wind-River/vxworks7-layer-for-boost/blob/master/usr_src/vxworks_boost_test_run.exp

For the simulated and smaller arm targets, we need to ensure only one telnet session is active, or the test target runs out of memory.
(We are cross-compiling on big servers, so the build can be many-threaded, but the target is an embedded target and may be quite resource constrained )
https://github.com/boostorg/build/blob/518bc4a1a1a1980134121070593fabea026dce16/src/tools/testing.jam#L189

How can I add a semaphore on "run" and "run-fail" rules that only gets applied  to target-os=vxworks ?

Thanks,

Brian Kuhl
Wind River

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