Has anyone benchmarked kotlin's SharedFlowImpl? I'm seeing thread contention on a SharedFlow with several collectors, and a quick peek shows that that emitting & taking values all uses a single central lock per instance (.. and it's `this` which is an antipattern) github.com/Kotlin/kotli... - ThreadSky

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p-y.wtf • 2 days ago

Has anyone benchmarked kotlin's SharedFlowImpl?

I'm seeing thread contention on a SharedFlow with several collectors, and a quick peek shows that that emitting & taking values all uses a single central lock per instance (.. and it's `this` which is an antipattern)

https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-core/common/src/flow/SharedFlow.kt#L639-L655

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virag.si•2 days ago

Yes we did and found the exact same thing. We had to get rid of most of fast emitting SharedFlows because of that issue - they are incredibly inefficient and cause significant amount of lock contention if used from multiple threads.

StateFlows are better if you can live with those.

virag.si•2 days ago

And even StateFlows have a pretty high CPU cost if you profile them - IIRC we've seen about 60-70% overhead of flow code vs. our collectors/mappers.

virag.si•2 days ago

In the end, the Flows are great for slower emissions (let's say less than once per 10 frames), but don't scale well for things that update their values per-frame.

p-y.wtf•2 days ago

That issue seems to be pointing to the number of collectors:

"unfortunately, a single collect scales linearly with the number of collectors existing, use-case (each collector scales linearly -> the total CPU burnt is quadratic)"

https://github.com/Kotlin/kotlinx.coroutines/issues/4030

virag.si•2 days ago

Yeah, that fits our investigation as well - more collectors, more lock contention on the internal SharedFlow buffer access.

Simple is fast :)

amanda-hinchman.bsky.social•2 days ago

Thread contentions doesn't look great. These are hot flows, the cost is bound to be high!

Folks might want to look into creating cold flow, not sure if it also has the same problems but at least there is a shut off valve.

andrew-ke11y.bsky.social•2 days ago

I'd love to get to a point where I understand problems like these, and have the time and ability to debug them, and identify a fix. Hats off to you!

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