Let’s consider the effect of this design on concurrency. Whereas the manyto-one model allows the developer to create as many user threads as she wishes,
it does not result in true concurrency, because the kernel can schedule only one thread at a time.
The one-to-one model allows greater concurrency, but the developer has to be careful not to create too many threads within an application
(and in some instances may be limited in the number of threads she can create).