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macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

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Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#51
post #33

This implies you'd run more than one Mac Studio in a cluster, and I have a few concerns regarding Mac clustering (as someone who's managed a number of tiny clusters, with various hardware): 1. The power button is in an awkward location, meaning rackmounting them (either 10" or 19" rack) is a bit cumbersome (at best) 2. Thunderbolt is great for peripherals, but as a semi-permanent interconnect, I have worries over the…

It’s been terrible for years/forever. Even Xserves didn’t really meet the needs of a professional data centre. And it’s got worse as a server OS because it’s not a core focus. Don’t understand why anyone tries to bother - apart from this MLX use case or as a ProRes render farm.

iOS build runner. Good luck developing cross-platform apps without a Mac!

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#52

Earlier quoted context omitted.

I'm not sure where else you can get a half TB of 800GB/s memory for < $10k. (Though that's the M3 Ultra, don't know about the M5). Is there something competitive in the nvidia ecosystem?

I wasn't aware that M3 Ultra offered a half terabyte of unified memory, but an RTX5090 has double that bandwidth and that's before we even get into B200 (~8TB/s).

You could get x1 M3 Ultra w/ 512gb of unified ram for the price of x2 RTX 5090 totaling 64gb of vram not including the cost of a rig capable of utilizing x2 RTX 5090.

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#54
This doesn’t remotely surprise me, and I can guess Apple’s AI endgame:

* They already cleared the first hurdle to adoption by shoving inference accelerators into their chip designs by default. It’s why Apple is so far ahead of their peers in local device AI compute, and will be for some time.

* I suspect this introduction isn’t just for large clusters, but also a testing ground of sorts to see where the bottlenecks lie for distributed inference in practice.

* Depending on the telemetry they get back from OSes using this feature, my suspicion is they’ll deploy some form of distributed local AI inference system that leverages their devices tied to a given iCloud account or on the LAN to perform inference against larger models, but without bogging down any individual device (or at least the primary device in use)

For the endgame, I’m picturing a dynamically sharded model across local devices that shifts how much of the model is loaded on any given device depending on utilization, essentially creating local-only inferencing for privacy and security of their end users. Throw the same engines into, say, HomePods or AppleTVs, or even a local AI box, and voila, you’re golden.

EDIT: If you're thinking, "but big models need the higher latency of Thunderbolt" or "you can't do that over Wi-Fi for such huge models", you're thinking too narrowly. Think about the devices Apple consumers own, their interconnectedness, and the underutilized but standardized hardware within them with predictable OSes. Suddenly you're not jamming existing models onto substandard hardware or networks, but rethinking how to run models effectively over consumer distributed compute. Different set of problems.

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#55

dang I wish I could share md tables. Here’s a text edition: For $50k the inference hardware market forces a trade-off between capacity and throughput: * Apple M3 Ultra Cluster ($50k): Maximizes capacity (3TB). It is the only option in this price class capable of running 3T+ parameter models (e.g., Kimi k2), albeit at low speeds (~15 t/s). * NVIDIA RTX 6000 Workstation ($50k): Maximizes throughput (>80 t/s). It is sup…

You can keep scaling down! I spent $2k on an old dual-socket xeon workstation with 768GB of RAM - I can run Deepseek-R1 at ~1-2 tokens/sec.

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#56
post #50

Earlier quoted context omitted.

For a bit more context, those posts are using pipeline parallelism. For N machines put the first L/N layers on machine 1, next L/N layers on machine 2, etc. With pipeline parallelism you don't get a speedup over one machine - it just buys you the ability to use larger models than you can fit on a single machine. The release in Tahoe 26.2 will enable us to do fast tensor parallelism in MLX. Each layer of the model is…

But that's only for prefilling right? Or is it beneficial for decoding too (I guess you can do KV lookup on shards, not sure how much speed-up that will be though).

No you use tensor parallelism in both cases.

The way it typically works in an attention block is: smaller portions of the Q, K and V linear layers are assigned to each node and are processed independently. Attention, rope norm etc is run on the node-specific output of that. Then, when the output linear layer is applied an "all reduce" is computed which combines the output of all the nodes.

EDIT: just realized it wasn't clear -- this means that each node ends up holding a portion of the KV cache specific to its KV tensor shards. This can change based on the specific style of attention (e.g., in GQA where there are fewer KV heads than ranks you end up having to do some replication etc)

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#57

Earlier quoted context omitted.

machine capex currently dominates power

Sounds like an ecosystem ripe for horizontally scaling cheaper hardware.

If I understand correctly, a big problem is that the calculation isn't embarrasingly parallel: the various chunks are not independent, so you need to do a lot of IO to get the results from step N from your neighbours to calculate step N+1.

Using more smaller nodes means your cross-node IO is going to explode. You might save money on your compute hardware, but I wouldn't be surprised if you'd end up with an even greater cost increase on the network hardware side.

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#58
post #47

Earlier quoted context omitted.

For #2, OWC puts a screw hole above their dock's thunderbolt ports so that you can attach a stabilizer around the cord https://www.owc.com/solutions/thunderbolt-dock It's a poor imitation of old ports that had screws on the cables, but should help reduce inadvertent port stress. The screw only works with limited devices (ie not the Mac Studio end of the cord) but it can also be adhesive mounted. https://eshop.macsale…

That screw hole is just the regular locking USB-C variant, is it not? See for example: https://www.startech.com/en-jp/cables/usb31cctlkv50cm

Looks like it! Thanks for pointing this out, I had no idea it was a standard.

Apparently since 2016 https://www.usb.org/sites/default/files/documents/usb_type-c...

So for any permanent Thunderbolt GPU setups, they should really be using this type of cable

Re: macOS 26.2 enables fast AI clusters with RDMA over Thunderbolt

#59

This doesn’t remotely surprise me, and I can guess Apple’s AI endgame: * They already cleared the first hurdle to adoption by shoving inference accelerators into their chip designs by default. It’s why Apple is so far ahead of their peers in local device AI compute, and will be for some time. * I suspect this introduction isn’t just for large clusters, but also a testing ground of sorts to see where the bottlenecks l…

I think you are spot on, and this fits perfectly within my mental model of HomeKit; tasks are distributed to various devices within the network based on capabilities and authentication, and given a very fast bus Apple can scale the heck out of this.
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