Basic Facts about GPUs
damek.github.io
damek.github.io
For example, the arithmetic intensity break-even point (ridge-point) is very different once you leave the NVIDIA-land. If we take AMD Instinct MI300, it has up to 160 TFLOPS FP32 paired with ~6 TB/s of HBM3/3E bandwidth gives a ridge-point near 27 FLOPs/byte which is about double that of the A100’s 13 FLOPs/byte. The larger on-package HBM (128 – 256 GB) GPU memory also shifts the practical trade-offs between tiling depth and occupancy. Although this is very expensive and does not have CUDA (which can be good and bad at the same time).
There are so many potential bottlenecks (normally just memory access patterns, but without tools to verify you have to design and run manual experiments).
Now, the last of it I bothered with was 9 months ago; enough is enough.
No wonder nobody on this site trusts AMD.
Xbox, Playstation, and Steam Deck seem to be doing pretty nicely with AMD.
AMD make excellent graphics hardware, and the graphics tools are also fantastic. AMD's pricing and market positioning can be questionable but the hardware is great. They're not as strong with machine learning tasks, and they're in a follower position for tensor acceleration, but for graphics they are very solid.
1. Both AMD and NVIDIA have "tensorcore" ISA instructions (ie real silicon/data-path, not emulation) which have zero use case in graphics
2. Ain't no one playing video games on MI300/H100 etc and the ISA/architecture reflects that
> but for graphics they are very solid.
Hmmm I wonder if AMD's overfit-to-graphics architectural design choices are a source of friction as they now transition to serving the ML compute market... Hmmm I wonder if they're actively undoing some of these choices...
Nvidia 15 years ago was overfit to graphics. Nvidia just made smarter choices, sold more hardware and re-invested their winnings into software and improving their hardware. Now they're just as good at GPGPU with a stronger software stack.
AMD has struggled to be anything other than a follower in the market and has suffered quite a lot as a result. Even in graphics. Mesh shaders in DX12 was the result of NVIDIA dictating a new execution model that was very favorable to their new hardware while AMD had already had a similar (but not perfectly compatible) system since the Vega called primitive shaders.
When I think of matrix multiplication in graphics I primarily think of transforms between spaces: moving vertices from object space to camera space, transforming from camera space to screen space, ... This is a big part of the math done in regular rendering and needs to be done for every visible vertex in the scene - typically in the millions in modern games.
I suppose the difference here is that DLSS is a case where you primarily do large numbers of consecutive matrix multiplications with little other logic, since it's more ANN code than graphics code.
> graphics is just dead and AMD and NVIDIA should throw everything else they do in the bin to chase the LLM bag
No graphics means that games of the future will be like:
"You have been eaten by a ClautGemPilot."
I think it's very naive to assume that GPU's will continue to dominate the AI landscape.
Buy v. rent calculus is only viable if there's no asymmetry between the two. Oftentimes, what you can rent you cannot buy, and vice-versa, what you can buy—you could never rent. Even if you _could_ buy an actual TPU, you wouldn't be able to run it anyway, as it's all built around sophisticated networking and switching topologies[1]. The same goes for GPU deployments of comparable scale: what made you think that you could buy and run GPU's at scale?
It's a fantasy.
First of all I don't understand how that's an answer. Second of all it's laughably wrong - I can name 5 firms (outside of FAANG) off the top of my head with >1k Blackwell devices and they're making very good money (have you ever heard of quantfi....). Third of all, how is TPU going to conquer absolutely anything when (as you admit) you couldn't run one even if you could buy one?
i hope you realize how silly you sound when
1. NVDA's market cap is 70% more than GOOG's
2. there is literally not a single other viable competitor to GPGPU amongst the 30 or so "accelerator" companies that all swear their thing will definitely be the one, even with many of them approaching 10 years in the market by now (cerebras, samba nova, groq, dmatrix, blah blah blah).
So, according to that, GPUs aren't really going anywhere unless there's a new player in a town who will compete with the Nvidia and sell at lower prices.
...
> the recent Versal chips allow for AI compute-in-network capabilities that puts NVIDIA Bluefield's supposed programmability to shame
I'm always just like... who are you people. Like what is the profile of a person that just goes around proclaiming wild things as if they're completely established. And I see this kind of comment on hn very frequently. Like you either work for Tenstorrent or you're an influencer or a zdnet presenter or just ... because none of this even remotely true.
Reminds me of
"My father would womanize; he would drink. He would make outrageous claims like he invented the question mark. Sometimes, he would accuse chestnuts of being lazy."
> I think it's very naive to assume that GPU's will continue to dominate the AI landscape
I'm just curious - how much of your portfolio is AMD and how much is NVDA and how much is GOOG?
I'm always just like... who are you people: financiers, or hackers? :-) I don't work for TT, but I am a founder in the vertical AI space. Firstly, every major player is making AI accelerators of their own now, and guess what, most state-of-the-art designs have very little in common with a GPGPU design of yester-year. We have thoroughly evaluated various options, including buying/renting NVIDIA hardware; unfortunately, it didn't make any sense—neither in terms of cost, nor capability. Buying (and waiting _months_ for) NVIDIA rack-fuls is the quickest way to bankrupt your business with CAPEX. Renting the same hardware is merely moving the disease to OPEX, and in post-ZIRP era this is equally devastating.
No matter how much HBM memory you get for whatever individual device, no matter the packaging—it's never going to be enough. The weights alone are quickly dwarfed by K/V cache pages anyway. This is doubly true, if you're executing highly-concurrent agents that share a lot of the context, or doing dataset-scale inference transformations. The only thing that matters, truly, is the ability to scale-out, meaning fabrics, RDMA over fabrics. Even the leading-edge GPU systems aren't really good at it, because none of the interconnect is actually programmable.
The current generation of TT cards (7nm) has four 800G NIC's per card, and the actual Blackhole chips[1] support up to 12x400G. You can approach TT, they will license you the IP, and you get to integrate it at whatever scale you please (good luck even getting in a room with Arm people!) and because TT's whole stack is open source, you get to "punch in" whatever topology you want[2]. In other words, at least with TT you would get a chance to scale-out without bankrupting your business.
The compute hierarchy is fresh and in line with the latest research, their toolchain is as as hackable as it gets, and stands multiple heads above anything that AMD or Intel had ever released. Most importantly, because TT is currently under-valued, it presents an outstanding opportunity for businesses like ours in navigating around the established cost-centers. For example, TT still offers "Galaxy" deployments which used to contain 32 previous-generation (Wormhole) devices in a 6U air-cooled chassis. It's not a stretch that a similar setup, composed of 32 liquid-cooled Blackholes (2 TB GDDR6, 100 Tbps interconnect) would fit in a 4U chassis. AFAIK, There's no GPU deployment in the world at that density. Similarly to TPU design, it's also infinitely scalable by means of 3+D twisted torus topologies.
What's currently missing in the TT ecosystem: (1) the "superchip" package including state of the art CPU cores, like TT-Ascalon, that they would also happily license to you, and perhaps more importantly, (2) compute-in-network capability, so that the stupidly-massive TT interconnect bandwidth could be exploited/informed by applications.
Firstly, the Grendel superchip is expected to hit the market by the end of next year.
Secondly, because the interconnect is not some proprietary bullshit from Mellanox, you get to introduce the programmable-logic NIC's into the topology, and maybe even avoid IP encapsulation altogether! There are many reasons to do so, and indeed, Versal FPGA's have lots to offer in terms of hard IP in addition to PL. K/V cache management with offloading to NVMe-oF clusters, prefix-matching, reshaping, quantization, compression, and all the other terribly-parallel tasks which are basically intractable for anything other than FPGA's.
Today, if we wanted to do a large-scale training run, we would simply go for the most cost-effective option available at scale, which is renting TPU v6 from Google. This is a temporary measure, if anything, because compute-in-network in AI deployments is still a novelty, and nobody can really do it at sufficiently-large scale yet. Thankfully, Xilinx is getting there[3]. AWS offers f1 instances, it does offer NVMe-accelerated ones, as well as AI acclerators, but there's a good reason they're unable to offer all three at the same time.
[1] https://riscv.epcc.ed.ac.uk/assets/files/hpcasia25/Tenstorre...
[2] https://github.com/tenstorrent/tt-metal/blob/main/tech_repor...
[3] https://www.amd.com/en/products/accelerators/alveo/v80.html
MI250 to A100 would be which would be very similar.
Again, I emphasize "modern"
An NVIDIA GPU from circa 2003 is completely different and has baked in circuitry specific to the rendering pipelines used for videogames at that time.
So most of this post is not quite general to all "GPUs" which a much broader category of devices that don't necessarily encompass the type of general purpose computation we use modern Nvidia GPUs for.
It might be worth trying to increase the contrast a bit.
The content is really great though!
I'm 99% sure that author had designed this website on an Apple Mac with so called "font smoothing" enabled, which makes all regular fonts artificially "semi-bold". So to make a normal looking font, Mac designers use this thinner font weight and then Apple helpfully makes it kinda "normal".
As mentioned below, 19.5 TFLOPS is the FP32 compute roofline, which doesn't support Tensor Cores. If you want to use those you need to use FP16 and you can get substantially improved performance.
You wouldn't believe how many PhDs I've met who have no idea what a roofline is.
Hmm
later found vllm uses paged kv cache with layout that matches how the GPU wants to read fully coalesced without strided jumps. llama.cpp was using a flat layout that’s fine for single prompt but breaks L2 access patterns when batching.
reshaped kv tensors in llama.cpp to interleave ; made it [head, seq, dim] instead of [seq, head, dim], closer to how vllm feeds data into fused attention kernel. 2x speedup right there w.r.t same ops.
GPU was never the bottleneck. it was memory layout not aligning with SM’s expected access stride. vllm just defaults to layouts that make better use of shared memory and reduce global reads. that’s the real reason it scales better per batch.
this took its own time of say 2+days and had to dig under the nice looking GPU graphs to find real bottlenecks, it was widly trial and error tbf,
> anybody got idea on how to do this kinda experiment in hot reload mode without so much hassle??
Even if llamacpp isnt used for batch inference now, this can allow those to finally run llamacpp for batching and on any hardware since vLLM supports only select hardware. Maybe finally we can stop all this gpu api software fragmentation and cuda moat as llamacpp benchmarks have shown Vulkan to be as or more performant than cuda or sycl.
[1] https://miro.medium.com/v2/resize:fit:1400/format:webp/1*lab...
I believe batching is a concept only useful when during the training or fine tuning process.
For local hosting, a more likely scenario where you could use batching is if you had a lot of different data you wanted to process (lots of documents or whatever). You could batch them in sets of x and have it complete in 1/x the time.
A less likely scenario is having enough users that you can make the first user wait a few seconds while you wait to see if a second user submits a request. If you do get a second request, then you can batch them and the second user will get their result back much faster than if they had had to wait for the first user’s request to complete first.
Most people doing local hosting on consumer hardware won’t have the extra VRAM for the KV cache for multiple simultaneous inferences though.
Self attention premise is exactly that it isn't context free so it is also incorrect to say that batched requests do not mathematically affect each other. They do, and that's by design.
The GPU can’t do anything with weights while they are in VRAM. They have to be moved into the GPU itself first.
So it is about memory round-trips, but not between RAM and VRAM. It’s the round trips between the VRAM and the registers in the GPU die. When batch processing, the calculations for all batched requests can be done while the model parameters are in the GPU registers. Compared to if they were done sequentially, you would multiply the number of trips between the VRAM and the GPU by the number of individual inferences.
Also, batched prompts and outputs are indeed mathematically independent from each other.
Moving data to and from VRAM is ~100ns of latency. Moving data from RAM to VRAM through PCIe 5.0 is 1-10us of latency. So, ~1 to ~2 orders of magnitude of difference.
And this is the reason why batching is used - you don't want to pay the price of that latency for each and every CPU-to-GPU request but you want to push as much data as you can through a single round-trip.
That’s the core point though. If you do batches the cache and registers are already primed and ready. The model runs in steps/layers accessing different weights in VRAM along the way. When batching you take advantage of this.
I’m in agreement that RAM to VRAM is important too but I feel the key speed up for inference batching is my above point.
Every weight has to be touched for every forward pass, meaning you have to wait for 16G to transfer from VRAM -> SRAM -> registers. That's not even close to 100ns: on a 4090 with ~1TB/s memory bandwidth that's 16 milliseconds. PCIe latency to launch kernels or move 20 integers or whatever is functionally irrelevant on this scale.
The real reason for batching is it lets you re-use that gigantic VRAM->SRAM transfer across the batch & sequence dimensions. Instead of paying a 16ms memory tax for each token, you pay it once for the whole batched forward pass.
[i1 i2 ]⋅[w1 w3 ; w2 w4 ] = [i1 ⋅w1 +i2 ⋅w3 i1 ⋅w2 +i2 ⋅w4 ]
Cool. Now what happens if we make the input vector a 2x2 matrix with, for some reason, a second set of two input values:
[i1 i2 ; j1 j2 ]⋅[w1 w3 ; w2 w4 ] = [i1 ⋅ w1 +i2 ⋅ w3 i1 ⋅ w2 +i2 ⋅ w4 ; j1 ⋅ w1 +j2 ⋅ w3 j1 ⋅ w2 +j2 ⋅w4 ]
Look at that! The input has 2 rows, each row has an input value for the network and the output matrix has 2 rows, each containing the outputs for the respective inputs. So you can "just" apply your neural network to any number of input values by just putting one to each row. You could do 2, or 1000 this way ... and a number of values would only need to be calculated once.
ah right so the GPU was the bottleneck then
"The computational power of the cores on the GPU was never the issue-- however the code that I wrote resulted in a memory bandwidth bottleneck that starved the GPU cores of data to work on, which is firmly within my responsibilities as a programmer -- to fully understand the bandwidth and latency characteristics of the device(s) i'm running on"