In concurrent programming and operating systems, what is meant by busy waiting?

Introduction / Context:
Busy waiting is an important term in operating systems and concurrent programming that describes a particular way of waiting for a condition to change. It has implications for CPU utilisation and system responsiveness. This question asks you to identify what busy waiting means in the context of processes or threads competing for resources.

Given Data / Assumptions:

• Multiple processes or threads may need to wait for a lock, flag or event.
• The operating system can either block them or allow them to run.
• Busy waiting is one possible design choice for the waiting behaviour.
• No numerical calculations are needed.

Concept / Approach:
Busy waiting occurs when a process or thread repeatedly tests a condition in a tight loop while remaining in the running state. It does not voluntarily relinquish the processor, so it consumes CPU cycles even though it is not making useful progress. This can lead to poor CPU utilisation if many threads spin simultaneously. Blocking or sleeping instead of busy waiting allows the scheduler to run other processes while the waiting thread is paused until the condition changes.

Step-by-Step Solution:
Step 1: Recall typical examples where a thread checks a shared variable in a while loop until it changes. Step 2: Note that in busy waiting, the loop body does not call blocking functions; it only checks and repeats. Step 3: Understand that this keeps the thread runnable and consuming CPU cycles. Step 4: Compare this behaviour with the options given in the question. Step 5: Select the option that explicitly describes continuous checking of a condition in a loop without relinquishing the CPU.

Verification / Alternative check:
Many synchronisation examples mention that using a simple spin lock on a single processor can be wasteful because it causes busy waiting. Descriptions emphasise that busy waiting consumes CPU time but performs no useful work while the condition remains false. This matches the behaviour described in the correct option and distinguishes busy waiting from simply being idle or blocked.

Why Other Options Are Wrong:
Option B describes a process that is doing heavy computation, which is useful work rather than waiting. Option C mentions waiting for user input while the CPU is idle; this situation is usually handled by blocking the process rather than busy waiting. Option D talks about repeatedly booting an operating system, which is unrelated to how a single process waits inside a running system.

Common Pitfalls:
Learners sometimes think that any loop is busy waiting, but loops that perform useful computation are not considered busy waiting. Another pitfall is to assume that busy waiting is always bad; in some short waits or on multiprocessor systems, busy waiting can be acceptable or even beneficial, but the definition still requires spinning while checking a condition without yielding the CPU.

Final Answer:
Busy waiting means that a process continuously checks a condition in a loop without relinquishing the CPU, instead of blocking and allowing others to run.