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by raphlinus·4y ago·view on hn ↗
The approach we're exploring for such integration is having the host pass a VkCommandBuffer/MTLCommandBuffer to the guest, which then records its dispatches onto that (the guest also having a VkDevice/MTLDevice handle for resource creation). This has a downside of requiring a single CPU thread at the integration point (and also not exploiting async compute on GPUs that legitimately run more than one compute queue in parallel), but on the other hand it avoids cross command buffer synchronization overhead (which I haven't measured, but if it's like GPU-to-CPU latency, it'd be very much worth avoiding).

Going direct to Metal, instead of through MoltenVK, would seem very appealing to me. Metal is pretty easy, and the idea of having an extremely light runtime that Just Works could be really nice for the user experience.

If you're planning to open-source the runtime, I'd be happy to chat about my experience building piet-gpu-hal. I haven't actually tried running any machine learning workloads on it, but I think it could be adapted for that without too much pain.

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Exactly the kind of things we've been talking about! A fun and challenging tradeoff space and it's always great to connect with others!

Ahh linebender - I hadn't connected the name with your github account - piet-gpu is great, as is your blog! Also, for anyone skimming the comments this talk is fantastic and I share it with anyone new to the GPGPU space: https://www.youtube.com/watch?v=DZRn_jNZjbw

We waffled a bit with the API granularity in the beginning and it's taken building out most of the rest of the project in order to nail it down (the big refactor still pending). The biggest issue is that in simple models we'll end up emitting a single command buffer but anything with control flow (that we can't predicate), data dependencies (sparsity, thresholding, etc), or CPU work in the middle (IO, custom user code, etc) can break that up. We also hit cases where we need to flush work - such as if we run out of usable memory and need to defragment or resize our pools. We want to be able to (but aren't yet) reuse command buffers (CUDA graphs, etc) and that requires being able to both cache them and recreate them on demand (if we resize a pool we have to invalidate all cached command buffers using those resources, as update-after-bind is not universally available and if shapes change there's big ripples). Since most models beyond simple vision ones are ~thousands of dispatches it also lets us better integrate into multithreaded applications like you mention as apps can record commands for themselves in parallel without synchronization. It still would be nice to have certain operations inlined, though, and for that we want to allow custom hooks that we call into to add commands to the command buffers, turning things inside-out to make small amounts of work like image transformations in-between model layers possible (I'm really hoping we can avoid modeling the entire graphics pipeline in the compiler and this would be a way around that :). We haven't yet started on scheduling across queues but that's also very interesting especially in multi-GPU cases (with x4/x8 GPUs being common in datacenters, or NUMA CPU clusters that can be scheduled similarly).

We're fully open source (https://github.com/google/iree) but have been operating quietly while we get the groundwork in place - it's taken some time but now we're finally starting to stumble into success on certain problem categories (like transformers as in the post). Right now it's mostly just organized as a systems/compiler nerd honeypot for people looking for an ML/number crunching framework that (purposefully) doesn't look like any of the existing ones :)

Would love to chat more - even if just to commiserate over GPU APIs and such - everyone is welcome on the discord where a bunch of us nerds have gathered or we could grab virtual coffee (realized just now that this hn acct is ancient - I'm benvanik@google.com :)