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/

Please take anything this fella writes with several grains of salt.

He doesn’t read code. Convert it to some log messages and he might explore it.

Read through much of this. Definitely started feeling like “a picture might be worth 1,000+ words”.

Darwin had bunch of schedulers except this one: dualq, multiq, etc

In fact here's the one used in Sonoma: sysctl kern.sched -> edge

which seems to be an extension over "clutch":

https://github.com/apple-oss-distributions/xnu/blob/main/osf...

"… Single-cluster, symmetric (SMP) systems can run with just the Clutch policy, but multi-cluster, asymmetric (AMP) systems must further enable the Edge policy extension to Clutch in order to manage scheduling across the multiple CPU clusters. …"

Edge is also described in the linked document.

> 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).

that lightning AV adapter is crazy, iirc it creates an ethernet connection to airplay the display to the device

And touchbars too, strangely enough.

For Intel platforms, the Touch Bar is driven by the trusted coprocessor (T1/T2), but that itself runs bridgeOS which indeed is Darwin/watchOS-based. With Apple Silicon I don't know if bridgeOS is still used; the SEP runs an L4.

All of Apple's platforms.

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

Does Apple use macOS in servers in its datacentres? Or are they all Linux?

Surely at a minimum they need macOS for CI.

Apple does have one advantage here-they can legally grant themselves permission to run macOS internally on non-Apple hardware, and I don’t believe doing so legally obliges them to extend the same allowance to their customers.

But that might give them a reason to keep x86_64 alive for internal use, since that platform (still) gives you more options for server-class hardware than ARM does

They do run Apple Silicon in data centers, so perhaps another custom version of Darwin + their system frameworks. It is hard to tell without some leaks :)

They use Ubuntu on x86-64 servers, at least for iCloud. Backends for iCloud, Photos and Backups etc. are written in Java.

Any sources or more information on that?

For the Java bit at least, this aligns with job descriptions I’ve seen and recruiter outreach I’ve received (long time ago though, maybe 5 years).

PPC32/64 of course, and for a long time Darwin still contained remnants of its predecessor's support for SPARC, PA-RISC, and m68k.

Which Apple products run arm32 XNU? Their first Apple Silicon CPUs were already arm64.

Well there were still the historical arm32 chips in their iOS devices, but until recently the watches were a cursed arm64_32 (or something like that) which is arm64 with 32 bit pointers iirc.

I should have just soureced this, They had PowerPC not RISCV in there source tree that was the X factor one. The Arm32 bit variant is closed sourced (leaked before) but was supported until IOS 11. XNU is really old almost 30 years! And before XNU there was the MACH kernel and the larger BSD tree it was built on which is an argument that it probably had a initial MIPs release too but I couldn't source the truth on that.

[1] https://en.wikipedia.org/wiki/XNU

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.

I wouldn't be surprised if they keep a minimal Power base maintained behind closed doors. It's how they managed to jump ship to intel so quickly, they never stopped maintaining NeXTSTEPs x86 port

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.

Yeah they're was that whole x86 thing thru did for quite a while.

Twice, on the basis that NEXT used the same kernel and that ran on 68k and Intel when Apple bought them and later ported it for Power PC. When Steve Jobs went back to Apple, for a long time he ran NEXT on a Thinkpad.

NeXTSTEP also ran on SPARC iirc

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.

IOKit was designed to support CoreAudio from the start, cc gvdl.