Atomic condition variable

[ronag]

I've noticed that using Atomics.notify + Atomics.waitAsync is extremely slow. Just doing a normal Atomics.load + setTimeout polling loop is a lot faster.

Since Atomics is implemented in V8 I'm not sure there is much we can do about it. Other than maybe adding notes in our docs.

Refs: https://github.com/pinojs/thread-stream/blob/main/lib/wait.js
Refs: https://bugs.chromium.org/p/v8/issues/detail?id=13483

[ronag]

@jasnell @addaleax Might be relevant for piscina?

[ronag]

From a quick look in the V8 source code it looks notify is implemented verbatim with the spec without any optimisations, i.e. there is no fast path such as just checking the stored value, and it also uses a global mutex, every time.

[anonrig]

Can you provide a micro-benchmark?

[ronag]

Not really... what would I compare it with?

[anonrig]

I assume it is: Atomics.notify + Atomics.waitAsync vs Atomics.load + setTimeout

[ronag]

Tricky to benchmark due to async stuff. We would need to measure cpu utilization instead of ops per second.