Why is a yielding mutex implementation not recommended?

Question

While implementing a mutex there are several architectural choices, like,

1. Spinning mutex (spinlock)

2. Sleeping mutex (a FIFO sleep queue is maintained while WAITING)

3. Yielding mutex (call the scheduler to run another process when WAITING)

Why is the Yielding Mutex least preferred? And how severe the consequences would be in using it?

Answer 1

The sleeping mutex has more fairness. Yielding mutex can cause starvation.

The problem with the yielding model is that a process may be asked to yield over and over, while other processes scoop the mutex (see also barging), or only have to wait a much shorter time.

Depending on how new processes are added to the queue, it could even happen that every time a certain process reaches its turn, it is forced to yield because the mutex is already taken, starving the process.

The FIFO model ensures that waiting processes are served on a first-come/first-served basis. It is harder to implement in the OS, but more fair.

The models can get more complicated. For instance, the OS may also have priorities for processes, and the priorities can change over time. Then the queue system can get even more tricky to implement.