Apple XNU: Clutch Scheduler
github.com101 points by tosh 7 hours ago
101 points by tosh 7 hours ago
interesting. id love an eclecticlight breakdown of this. they're one of the few if only that write anything worth reading on apple hardware, i once found a QOS/scheduler insight through those guys when I couldn't get my c/cpp project pinned to the cores I wanted on m-series. https://eclecticlight.co/m1-macs/
> The XNU kernel runs on a variety of platforms
This is fascinating, would love to know where it’s used! (Besides macOS)
I believe it means Apple's other hardware platforms (phones, tablets, smart TVs, VR headsets, smartwatches)
It's used in iOS as well. iOS runs in some unexpected places, like for example Studio Display. Also, the Apple Lightning Digital AV Adapter runs Darwin (because RTKit didn't exist yet).
Perhaps they mean ISAs
Well x86 at one point, arm both the 32 and 64 bit versions. I think they had RISCV support in their source tree at one point but not really at a commercial level. It does cover a lot different levels of hardware though
PPC32/64 of course, and for a long time Darwin still contained remnants of its predecessor's support for SPARC, PA-RISC, and m68k.
Is mc68k or PPC still in there anywhere?
I'm sure there's vestiges of them somewhere, but the underlying support (the architecture specific parts of the mach portion of the kernel) is gone for those archs.
IIRC, Apple uses 'platform' to refer to an SoC integration. For example, M1, M2 and etc. are separate platforms. M5 in Vision Pro is a separate platform than M5 in MacBook Pro. I believe Apple's XNU does somewhat still support non-Apple Silicon as well though.
Does this contribute to macOS's suitability for DAW applications or is that more the baked in low-latency audio drivers?
Audio actually runs on a dedicated realtime thread. This used to be scheduled differently, but nowadays it might be implemented by the FIXPRI bucket described in this document.
CoreAudio probably deserves most of the credit, there. Similar ASIO-style solutions like JACK, DirectSound and now Pipewire hit the sub-30ms mark without any big scheduler tweaks.