Shu-yu Guo (Google)

Lars Hansen (Mozilla)

Atomics.waitAsync

for Stage 3

Recap

Atomics.waitAsync(TA,idx,v,t)

If TA[idx] != v, return a promised resolved to “not-equal”.

Else returns a promise that is resolved either when

An agent calls Atomics.notify(TA,idx,c); resolved with “ok”.

Timeout expires; resolved with “timed-out”.

Fairness: Single FIFO Queue

• No more line jumping for Atomics.wait
• Single FIFO waiter queue per SAB location, just like now

Motivating Anomaly

let p1 = Atomics.waitAsync(i32a,0,v);

let p2 = Atomics.waitAsync(i32a,0,v);

Atomics.wait(i32a,0,v);

Resolving p1 and p2 don’t do anything useful if the agent is blocked!

But We Can’t Solve the General Problem

let p1 = Atomics.waitAsync(i32a,0,v);

let p2 = Atomics.waitAsync(i32a,0,v);

Atomics.wait(i32a,1,v);

Wrong trade off to solve the very narrow same-location case

Single FIFO Queue Implications

• Allows the programmer to resolve Promises in agents that are blocked
• Easy to reason about the order of Promise settlements and agent wake-ups
• Trivially fair
• PR #20

Stage 3?