Operating Systems Question Bank with Answers 13

What is busy waiting with respect to a critical section problem? Can busy waiting be avoided?

Question:

What is busy waiting with respect to a critical section problem? Can busy waiting be avoided?

Answer:

Waiting is the act of suspending the current thread of execution until some future event which might be the availability of a contested resource, the passage of time, or the release of a lock.

Busy waiting means a process is waiting for an event to occur and it does so by executing instructions. Systems implement busy waiting by simply spinning in a tight loop, constantly checking if the event in question has occurred. Busy waiting wastes CPU cycles since nothing useful is done during looping.

Alternative to busy waiting is sleeping. There needs to be built a list of threads who wish to wait, called a wait queue. The kernel is asked to wake up a process from the list whenever the event in question happens. For example, the kernel might be asked to wake up a thread when a specific mutex becomes available. You then yield to the kernel, allowing it to schedule something else instead of you.

The benefit of sleeping over busy looping is that the kernel can run something useful instead for the duration of the wait. The downside is the overhead: Managing the list, putting the thread to sleep, and context switching into a new process.

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