Hacker News Reader

Comment by read_if_gay_ | Hacker News Reader

running 70b on 24gb has been possible for months (so basically forever) it just got like 2x speedup now. the issue that it's a lobotomized ~2bit quant remains.

View on HN

Quantisation tradeoff is an interesting question. My experience so far is that larger models with lower bit quantisation seem to perform better than smaller models with higher bit quantisation. So, e.g., 30B-4bit appears to perform better than 13B-8bit. But, then, an 8-bit quantisation seems not really distinguishable from the non-quantised model, so what benefit might you gain from not quantising the model.

Of course, speed, accuracy, power consumption, etc are all considerations, sometimes at odds with each other.

It's diminishing returns in terms of perplexity below 4 bits, and this is true even with newer quantisation methods like AWQ and OmniQuant.

I think OmniQuant is notable because it shifts the bend of the curve to 3-bit. While < 3-bit still ramps up, it's notable in that it's usable and doesn't go asymptotic: https://github.com/OpenGVLab/OmniQuant/blob/main/imgs/weight...

What EXL2 seems to bring to the table is that you can target an arbitrary quantize bit-weight (eg, if you're a bit short on VRAM, you don't need to go from 4->3 or 3->2, but can specify say 3.75bpw). You have some control w/ other schemes by setting group size, or with k-quants, but EXL2 is definitely allows you to be finer grained. I haven't gotten a chance to sit down with EXL2 yet, but if no one else does it, it's on my todo-list to be able to do 1:1 perplexity and standard benchmark evals on all the various new quantization methods, just as a matter of curiosity.

Can you do tuning/training on quantized models? Or are they inference only?

This paper[1] attempts to do LoRA fine tuning on quantised LLMs. The LoRa weights themselves are full precision, but they're much smaller. Although, I haven't had a chance to try it yet.

[1]: https://arxiv.org/abs/2306.08162

Yup, but the thing is that can you get model working how you want and can you run it production. My experience is that gpt4 yes, gpt3.5 yes but needs some work Llama2-70b 8-bit or more yes but needs some work after that it really depends usecase.

Something like that 4^30 >>> 8^13?

That's absolutely false, unless you mean running llamacpp with half the layers on the CPU and half the layers on the GPU. And that was like 2t/s.

> running llamacpp with half the layers on the CPU and half the layers on the GPU

How do you do that? Could you please provide a link if you have it handy.

When running you use "-ngl" to specify how many layers are uploaded to the GPU

Just set the --gpu-layers|-ngl flag. Run main with --help to see all the options.

Reply on news.ycombinator.com