ingveFeb 8

Why E cores make Apple silicon fast

https://eclecticlight.co/2026/02/08/last-week-on-my-mac-why-e-cores-make-apple-silicon-fast/

Last Week on My Mac: Why E cores make Apple silicon fast

Apple silicon architecture is designed to get background processes out of the way of our apps running in the foreground, by using the E cores.

The Eclectic Light Company
Show threaddrob518
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.
Feb 8 at 1:56pmWeb
Show threadmasklinnFeb 8
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.
Show threaddrob518Feb 8
How is the QoS communicated to the scheduler? Is there a mark on the binary or does the code do it at thread startup?
Show threadsaagarjhaFeb 8
It’s done in code.