spinwaits

Spinwaits are a synchronization technique in which a thread waiting for a condition to become true repeatedly tests the condition in a tight loop, rather than sleeping or blocking. They are closely related to busy-waiting and are often used in low-level code that requires very low latency or where the expected wait time is short.

In practice, a spinwait loops while the condition is not satisfied, optionally performing a pause or backoff in each iteration to reduce resource contention. On modern CPUs, instructions such as a processor pause or a hint to the scheduler help the core avoid pipeline stalls and reduce power usage while spinning. Some implementations include short sleeps or exponential backoff, especially when waits may be longer than a few dozen cycles.

Spinwaits are commonly used to wait for lightweight events such as a flag becoming visible, a lock becoming available, or a memory location changing under lock-free data structures. They are typically preferred when blocking would be more expensive than spinning and when the wait time is expected to be brief. However, spinning consumes CPU time, can waste cycles on multi-core systems, and can cause priority inversion or starvation if misused.

In practice, spinwaits are often complemented with blocking synchronization as a fallback: if the wait exceeds a small threshold, the thread yields the processor or blocks on a traditional primitive such as a condition variable or futex. Developers must consider the platform’s scheduling behavior, cache coherence, and memory ordering guarantees when implementing spinwaits.