Any await runs the logic that attempts to release the main message pump to check for other tasks or incoming IO events. And it looks like that takes around 90 instructions to loop back around to running the next line of the code, when the process is running nothing else.
replyIf you’re doing real work, 90 instructions ain’t much but it’s not free either. If you’ve got an async accumulator (eg, otel, Prometheus) that could be a cost you care about.
How did you come up with 90? Can you shed any might on the difference between the cost of promise resolution and the cost of await? Is there any cost component with how deep in the call stack you are when an await happens?
replyEssentially for loop of 10k iterations comparing `fn()` versus `await fn()` fed into a microbenchmark tool, with some fiddling to detect if elimination was happening or ordering was changing things.
replyI was bumping into PRs trying to eliminate awaits in long loops and thinking surely the overhead can’t be so high to warrant doing this, especially after node ~16. I was wrong.
Clarifying:
reply‘fn()’ is the exact same function call in both cases. Only the benchmark itself was async versus sync.
Because the case under test is what’s the cost of making an async api when 90% of the calls are sync? And the answer is a lot higher than I thought.