RFR: 8366441: AArch64: Support WFET in OnSpinWait [v3]

[Andrew Haley]

On Tue, 10 Feb 2026 22:19:18 GMT, Ruben <duke at openjdk.org> wrote:

> The current WFET sequence does not allow a delay less than 80ns based on the above data: `onSpinWait delay` is 80ns / 142ns on Arm Cortex-X925 / Cortex-A725, when the delay of 1ns is requested. It might be possible to achieve a lower minimal delay by adjusting the sequence, and the minimal delay of the current sequence might be different on other hardware.
> 
> The default delay of 100ns was chosen based on the above measurements - as providing improvement for both ProducerConsumer and SharedCounter microbenchmarks.

100ns doesn't sound way out to me. The 40ns figure is for a "raw" memory transaction, such as a single memory write. Inevitably a real thread handshake will take a few such transactions.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/27030#issuecomment-3883362898