Answer:
1) Answer is in explanation.
2) waiting on application, waiting for hardware to communicate, waiting for a file to become available, waiting for user input...
3) No, busy waiting cannot be avoided altogether.
Explanation:
1) The term "busy waiting" means a process simply spins (does nothing but continue to test it's entry conditions), while it is waiting to enter it's critical session. This continues to use (waste) CPU cycles which is inefficient. The CPU is not engaged in any real productive activity during this period, and the process does not progress towards completion.
2) Other kinds of waiting in an operating system are:
* Waiting on application
* Waiting for hardware to communicate
* Waiting for a file to become available
* Waiting for user input.
3) Busy waiting cannot be avoided altogether; some events cannot trigger a wake-up, for example, on Unix, a process cannot "sleep untill a file is modified" because the operating system does not provide any mechanism to automatically wake up the process when the even occurs. Some amount of repeated polling (checking) is required.
Generally, busy waiting is mostly avoidable on uniprocessor machines, but mostly unavoidable on multi processor machines. Therefore, busy waiting, cannot be avoided altogether.
