I did a microbenchmark recently and found that on node 24, awaiting a sync function is about 90 times slower than just calling it. If the function is trivial, which can often be the case.

If you go back a few versions, that number goes up to around 105x. I don’t recall now if I tested back to 14. There was an optimization to async handling in 16 that I recall breaking a few tests that depended on nextTick() behavior that stopped happening, such that the setup and execution steps started firing in the wrong order, due to a mock returning a number instead of a Promise.

I wonder if I still have that code somewhere…

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rrr_oh_man 3 hours ago | parent | next [-]

> I did a microbenchmark recently and found that on node 24, awaiting a sync function is about 90 times slower than just calling it. If the function is trivial, which can often be the case.

I dabble in JS and… what?! Any idea why?

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hinkley 3 hours ago | parent [-]

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.

If 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.

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conartist6 2 hours ago | parent [-]

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?

	▲	hinkley 2 hours ago \| parent [-]
		Essentially 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. I 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.

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conartist6 5 hours ago | parent | prev [-]

Here is my test harness and results: https://github.com/conartist6/async-perf