Time to build a GPU OS? Here is the first step

Neat! As someone working in this space and feeling like I've been taking crazy pills from how these "duh, CPU solved this 30 years ago" things keep slipping it's great to see more people bridging the gap! Unfortunately CUDA/HIP (and the entire stack beneath them) virtual memory management ops are very expensive host APIs (remapping a big block of pages can be O(n^2) with page count and fully synchronize host/device (forced wait idle), take kernel locks, etc) so it hasn't been viable in all cases. If your workloads are submit/wait with host in the loop the VM tricks are ok but if you are trying to never block the GPU (pipeline depth > 0) you really want to avoid anything that does a page table modification (until we get GPUs that can pipeline those). vkQueueBindSparse is one of the few async APIs I've seen, and CUDA has cuMemMapArrayAsync but I haven't yet used it (because arrays are annoying and without being able to inspect the driver I'm sure it's probably doing the wrong thing).

I've had good luck with indirection tables used during lookup inside of the kernels consuming/producing the kvcache data - it's essentially user-mode remapping like they do here: you can publish a buffer offset table and threads are uniform, have coalesced reads to the table, and cache the offsets no problem. You have the same memory locality issues as VM (contiguous virtual but potentially random physical) but are not limited to device page sizes and since you can update while work is in-flight you can be much more aggressive about reuse and offload (enqueue DMA to cold storage to evict from VRAM, enqueue DMA to copy from cold memory into reused VRAM, enqueue offset table update, enqueue work using them, repeat - all without host synchronization). You can also defrag in-flight if you do want to try to restore the physical locality. It's nothing crazy and fairly normal in CPU land (or even classic virtual texturing), but in ML GPU land I could write a big paper on it and call it SuperDuperFancyAttention4 and publish press releases...

Actual title: "Solve the GPU Cost Crisis with kvcached: A library to enable virtualized, elastic KV cache for LLM serving on shared GPUs"

In my imagination, I thought that the large GPU clusters were dynamically allocating whole machines to different tasks depending on load.

So, hypothetically, if ChatGPT's peak load and their minimum load were a 3× ratio, they'd reallocate 2/3 of their servers to training when it's not peak time.

Doing the same thing inside an individual GPU seems irrelevant to anyone operating at scale when they can approximate the same behavior with entire servers or even entire racks.

AI-written paper posted by JiaRong Xing

Username is Jrxing

"GPU OS" turns out to be just more LLM spam

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