I really appreciate this type of articles. I feel like a lot of knowledge in LLM training and inference is locked inside the heads of practitioners. Similar to compiler engineers before.
To work in LLM training/inference you’re expected to know this stuff but to know this stuff you need to be working in the space.
Gentle reminder that while most money is spent on LLM inference, the vast majority of useful AI use is in fact not LLMs. Also, more and more work is poured into making small models. One thing I like about the whole export controls saga is that people are finding creative ways to squeeze performance out of these devices as witnessed in this post. But, if you then look at solutions like vLLM, vLLM will just fill whatever VRAM is available, no matter the context size, or the model size. So then you have two things to worry about:
First, where do you know exactly what the optimal VRAM assignment per model, per context size is, which seems to be currently based purely on experience and second how do you make sure that only that amount is available to your infra/containers, which is being handled by DRA and stuff like https://project-hami.io
While only tangentially related to the blog post here. The title is picked in such a way that I couldn't help, but put the shameless plug here. When he wrote popping the bubble, I thought we're talking about devices and reducing NVIDIA dependency, but this seems very focused on Cuda.
Disclaimer: I work with Dynamia.ai, the founders of which created HAMi.
> you find that the GPU often sits idle, not for lack of work, but because the CPU hasn't told it what to do next yet. This phenomenon is called a GPU bubble.
This is true, but I've never heard anyone refer to this as a GPU bubble before.
I think most people hear "GPU bubble" and think of a financial bubble of some kind.
Very odd, but perhaps more familiar to graphics programmers? I will say I'd probably call it a stall, which is exactly what the Vulkan docs call it moments later, so :shrug:
it's not stalled, as that would imply that it waits for something, which is not necessarily the case with bubbles. most often it shows lack of proper pipelining or wrong pipeline dependencies (pipeline A waits for pipeline B, pipeline C waits for pipeline B, while pipeline B waits for an event X, now you've just made all three pipelines stalled on event X - not good).
When an engine stalls, the implication is that the chain reaction that drives it is failing - I don’t think that is the case with a GPU as it will quite happily sit there drawing watts til you give it things. In systems nomenclature the inverse term for bubble is utilisation. This or that link is or node is using x% of its capacity. Indeed, if you monitor your GPU with nvidia-smi you will see that very term in the instrumentation.
I really appreciate this type of articles. I feel like a lot of knowledge in LLM training and inference is locked inside the heads of practitioners. Similar to compiler engineers before.
To work in LLM training/inference you’re expected to know this stuff but to know this stuff you need to be working in the space.
Thank you for the kind words. We will write and share more of these.
Gentle reminder that while most money is spent on LLM inference, the vast majority of useful AI use is in fact not LLMs. Also, more and more work is poured into making small models. One thing I like about the whole export controls saga is that people are finding creative ways to squeeze performance out of these devices as witnessed in this post. But, if you then look at solutions like vLLM, vLLM will just fill whatever VRAM is available, no matter the context size, or the model size. So then you have two things to worry about:
First, where do you know exactly what the optimal VRAM assignment per model, per context size is, which seems to be currently based purely on experience and second how do you make sure that only that amount is available to your infra/containers, which is being handled by DRA and stuff like https://project-hami.io
While only tangentially related to the blog post here. The title is picked in such a way that I couldn't help, but put the shameless plug here. When he wrote popping the bubble, I thought we're talking about devices and reducing NVIDIA dependency, but this seems very focused on Cuda.
Disclaimer: I work with Dynamia.ai, the founders of which created HAMi.
Different bubble than the one I was hoping for.
This appears to be different than the recent "Speculative Pipeline Decoding" paper: https://arxiv.org/abs/2605.30852
> you find that the GPU often sits idle, not for lack of work, but because the CPU hasn't told it what to do next yet. This phenomenon is called a GPU bubble.
This is true, but I've never heard anyone refer to this as a GPU bubble before.
I think most people hear "GPU bubble" and think of a financial bubble of some kind.
It appears to be a real term? https://docs.vulkan.org/tutorial/latest/Synchronization/Asyn...
Very odd, but perhaps more familiar to graphics programmers? I will say I'd probably call it a stall, which is exactly what the Vulkan docs call it moments later, so :shrug:
"bubble" used to be used a lot more when talking about very deep pipelines, eg Pentium 4 depth.
Pretty sure that would be "[GPU performance] bottlenecked [by the CPU]" in most common terms.
while the title is misreading, when reading GPU profiling data, we do call these bubbles - where the GPU _could_ do something, but it's idle.
any time your GPU is idle = you are losing $$$ = your TCO is going up. you don't want that.
I saw it in literature on cpu pipelines in quotes, never without.
I've never seen it in quotes, but yeah it is a very common term in pipelined CPUs.
It's very common to call it a GPU bubble in gamedev, though not strictly for CPU induced bubbles.
The term I would use would be “underutilised”
"stall" is the best term I can think of as in "pipeline stall".
Better term, anyone?
it's not stalled, as that would imply that it waits for something, which is not necessarily the case with bubbles. most often it shows lack of proper pipelining or wrong pipeline dependencies (pipeline A waits for pipeline B, pipeline C waits for pipeline B, while pipeline B waits for an event X, now you've just made all three pipelines stalled on event X - not good).
When an engine stalls, the implication is that the chain reaction that drives it is failing - I don’t think that is the case with a GPU as it will quite happily sit there drawing watts til you give it things. In systems nomenclature the inverse term for bubble is utilisation. This or that link is or node is using x% of its capacity. Indeed, if you monitor your GPU with nvidia-smi you will see that very term in the instrumentation.
Yes, the title seems off - I also thought I am going to be reading about the AI/pricing bubble.
I love the brand name, Moondream