By default, the __init__ function of IocpProactor in windows_events.py calls CreateIoCompletionPort with a 4th argument of 0xffffffff, yet MSDN doesn't document this as a valid argument.    https://docs.microsoft.com/en-us/windows/desktop/fileio/createiocompletionport

It looks like the 4th arg (NumberOfConcurrentThreads) is meant to be either a positive integer or 0.  0 is a special value meaning "If this parameter is zero, the system allows as many concurrently running threads as there are processors in the system."

Why does asyncio use 0xffffffff instead as the default value?