Does anyone have any insight into the MacOS scheduler and the algorithm it uses to place threads on E vs. P cores? Is it as simple as noting whether a thread was last suspended blocking on I/O or for a time slice timeout and mapping I/O blockers to E cores and time slice blockers to P cores? Or does the programmer indicate a static mapping at thread creation? I write code on a Mac all the time, but I use Clojure and all the low level OS decisions are opaque to me.

Check out the scheduler documentation that Apple has in the xnu repo. https://github.com/apple-oss-distributions/xnu/blob/main/doc...

https://developer.apple.com/videos/play/tech-talks/110147/

The baseline is static: low QoS tasks are dispatched to the E cores, while high QoS tasks are dispatched to P cores. IIRC high QoS cores can migrate to the E cores if all P cores are loaded, but my understanding is that the lowest QoS tasks (background) never get promoted to P cores.

The article mentions P or E is generally decided by if it's a "background" process (whatever than means). Possible some (undocumented) designation in code or directive to the compiler of the binary decides this at compile time.