Busywait

Busy-wait, also known as busy waiting or polling, is a synchronization technique in which a thread or process waits for a condition to become true by repeatedly checking it in a loop, without yielding the processor. The loop typically runs while a condition remains unmet, consuming CPU time.

In practice, busy-waiting appears in spinlocks, where a thread loops on a lock flag until it becomes available, and in hardware polling loops that wait for a device status bit to change. It is also used in some low-level or embedded contexts where the cost of blocking or context switching is undesirable and the wait is expected to be very short.

The main advantage of busy-waiting is low latency: when the condition becomes true, the waiting thread can proceed immediately without the overhead of scheduling or waking another task. It has minimal setup and can be simpler to implement in certain low-level environments. However, it wastes CPU cycles, increases power consumption, and scales poorly on multi-core systems where other threads could be productive while one spins. It is generally unsuitable for long waits or workloads with many competing tasks, where blocking synchronization primitives or event-driven designs are preferable.

Mitigations and alternatives include inserting a brief pause or yielding the processor inside the loop, using back-off strategies to reduce contention, or replacing the busy-wait with blocking primitives such as mutexes, semaphores, or condition variables. In hardware contexts, using interrupts instead of polling can also eliminate unnecessary spinning. Busy-wait is most appropriate for very short waits or tightly controlled environments where latency is critical.