Stage 3 Draft / October 18, 2024

Atomics.pause

1 Atomics.pause ( [ N ] )

This method performs the following steps when called:

  1. If N is neither undefined nor an integral Number, throw a TypeError exception.
  2. If the execution environment of the ECMAScript implementation supports signaling to the operating system or CPU that the current executing code is in a spin-wait loop, such as executing a pause CPU instruction, send that signal. When N is not undefined, it determines the number of times that signal is sent. The number of times the signal is sent for an integral Number N is less than or equal to the number times it is sent for N + 1 if both N and N + 1 have the same sign.
  3. Return undefined.
 Note 1

This method is designed for programs implementing spin-wait loops, such as spinlock fast paths inside of mutexes, to provide a hint to the CPU that it is spinning while waiting on a value. It has no observable behaviour other than timing.

Implementations are expected to implement a pause or yield instruction if the best practices of the underlying architecture recommends such instructions in spin loops. For example, the Intel Optimization Manual recommends the pause instruction.

 Note 2

The N parameter controls how long an implementation pauses. Larger values result in longer waits. Implementations are encouraged to have an internal upper bound on the maximum amount of time paused on the order of tens to hundreds of nanoseconds.

The user, if passing the N parameter, can use it to implement backoff strategies. For backoff strategies where subsequent waits become longer, linear backoff can be implemented by passing linearly increasing values for non-negative N; exponential backoff can be implemented by exponentially increasing values for non-negative N, and so on. For backoff strategies where subsequent waits become shorter, linear backoff can be implemented by passing linearly decreasing values for negative N; exponential backoff where subsequent waits become longer can be implemented by exponentially decreasing values for negative N, and so on.

 Note 3

Due to the overhead of function calls, it is reasonable that an inlined call to this method in an optimizing compiler waits a different amount of time than a non-inlined call.

A Copyright & Software License

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