alt.hn

2/8/2026 at 8:38:55 PM

Apple XNU: Clutch Scheduler

 https://github.com/apple-oss-distributions/xnu/blob/main/doc/scheduler/sched_clutch_edge.md

by tosh

2/8/2026 at 10:05:16 PM

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/

by trueno

2/9/2026 at 4:22:54 AM

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

by saagarjha

2/9/2026 at 3:58:26 PM

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

by KerrAvon

2/9/2026 at 4:04:41 PM

Please could you elaborate?

by denotational

2/10/2026 at 12:41:26 PM

I mean that goes without saying on anything I read on the damn internet. With that said, I'm not seeing a comparative informational source anywhere that even takes the time of day. Feel free to bombard me with new resources.

by trueno

2/8/2026 at 10:20:32 PM

> The XNU kernel runs on a variety of platforms

This is fascinating, would love to know where it’s used! (Besides macOS)

by cadamsdotcom

2/8/2026 at 10:26:45 PM

I believe it means Apple's other hardware platforms (phones, tablets, smart TVs, VR headsets, smartwatches)

by csb6

2/9/2026 at 12:35:42 AM

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).

by LoganDark

2/9/2026 at 4:08:36 AM

And touchbars too, strangely enough.

by nxobject

2/9/2026 at 4:11:26 AM

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.

by LoganDark

2/9/2026 at 12:44:22 PM

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

by asimovDev

2/9/2026 at 4:27:37 AM

All of Apple's platforms.

by internet2000

2/8/2026 at 10:22:25 PM

Perhaps they mean ISAs

by electronsoup

2/9/2026 at 12:14:44 AM

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.

by LoganDark

2/9/2026 at 12:28:14 AM

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

by fragmede

2/9/2026 at 10:58:39 AM

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.

by simonh

2/9/2026 at 2:14:51 PM

NeXTSTEP also ran on SPARC iirc

by mghackerlady

2/9/2026 at 5:40:27 PM

OpenSTEP actually.

by pjmlp

2/9/2026 at 8:54:31 PM

OpenSTEP had SPARC support, yes, but NeXTSTEPs last release had support for m68k, x86, and SPARC. 3.3 had support for PA-RISC

by mghackerlady

2/8/2026 at 11:44:27 PM

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

by xphos

2/9/2026 at 5:00:08 AM

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

by skissane

2/9/2026 at 9:32:43 AM

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 :)

by jabwd

2/9/2026 at 4:38:36 PM

For Private Cloud Compute: “a new operating system: a hardened subset of the foundations of iOS and macOS tailored to support Large Language Model (LLM) inference workloads while presenting an extremely narrow attack surface.” https://security.apple.com/blog/private-cloud-compute/

by glhaynes

2/10/2026 at 3:52:13 AM

I wonder if there is any chance we might see another Xserve?

If they’ve got Apple Silicon servers in their own data centres…

by skissane

2/9/2026 at 9:37:28 AM

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

by therein

2/9/2026 at 12:11:32 PM

Any sources or more information on that?

by Longhanks

2/9/2026 at 3:22:32 PM

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).

by twalla

2/9/2026 at 5:39:49 PM

NeXT added a Java variant to WebObjects and it was for several years the main server side infrastructure, after being acquired by Apple.

Nowadays you can usually still find Java and JVM languages like Clojure (Apple Maps), on Apple's job ads.

How much of it is still Java based, no idea.

I imagine XCode Cloud has nothing to with it for example.

by pjmlp

2/9/2026 at 8:46:03 PM

Unfortunately I am the source in this case. It is from having worked on them personally. :)

by therein

2/9/2026 at 12:39:40 AM

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

by wiml

2/9/2026 at 4:43:50 AM

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

by userbinator

2/9/2026 at 6:01:14 AM

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.

by dagmx

2/9/2026 at 1:55:02 PM

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

by xphos

2/9/2026 at 1:06:36 AM

Is mc68k or PPC still in there anywhere?

by kjs3

2/9/2026 at 2:54:02 AM

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.

by monocasa

2/9/2026 at 2:14:12 PM

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

by mghackerlady

2/10/2026 at 12:45:57 AM

I seriously doubt it.

Apple's ARM implementation is in a really good place right now. It would take something extremely compelling to get them to consider any other architecture for an application processor, especially considering that it'd mean giving up some degree of control.

Power is probably not where Apple would choose to go unless something really unusual happened. It's essentially just IBM's pet architecture at this point.

by duskwuff

2/11/2026 at 3:09:22 PM

When I mean minimal arm base, I basically mean making sure XNU at least compiles on PPC64. I don't doubt they'd likely never have a use for it, but they maintained intel support behind closed doors when macos forked from NeXTstep and nobody thought they'd need that

by mghackerlady

2/9/2026 at 7:59:33 PM

I would honestly be shocked if they were.

They've been making quite a few changes to the virtual memory code over the past decade, and keeping those vestigial arch's around is a pretty big maintenance burden. It'd probably be less work to just add the arch as if it were new when it's needed at this point since the kernel itself is pretty portable.

by monocasa

2/9/2026 at 5:50:48 AM

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. …"

by poige

2/9/2026 at 3:37:38 PM

Edge is also described in the linked document.

by dcrazy

2/8/2026 at 10:46:09 PM

Does this contribute to macOS's suitability for DAW applications or is that more the baked in low-latency audio drivers?

by almoni

2/8/2026 at 11:11:19 PM

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.

by bigyabai

2/9/2026 at 4:04:12 PM

CoreAudio was developed alongside xnu / IOKit for Mac OS X, so it’s kind of all of it. Apple had the opportunity to start fresh with a built-in super low-latency audio subsystem at the turn of the century, and they took it.

by KerrAvon

2/9/2026 at 5:05:44 AM

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

by lukeh

2/9/2026 at 12:07:56 AM

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.

by dcrazy

2/9/2026 at 3:14:43 PM

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

by travisgriggs

2/9/2026 at 11:18:08 AM

[dead]

by MarginalGainz