It's quite an interesting approach for limiting the amount of concurrency in singlethreaded environments. I first saw that in the documentation of the C++ seastar library which is also mostly a singlethreaded eventloop. See the chapter "Limiting parallelism with semaphores" here: http://docs.seastar-project.org/master/md_doc_tutorial.html

Their semaphore approach allows then to start an operation exactly X amounts of time and the next one will wait until another one finishes. Seems like an interesting approach. e.g. to limit the number of parallel connections.