Comment by lhl | Hacker News Reader

Comment by lhl | Hacker News Reader

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Although there are multiple bottlenecks, my understanding (and why at a certain point, throwing more threads doesn't work) is that inference for dense LLMs are largely limited by memory bandwidth. Most desktop computers will have dual channel DDR4/DDR5 memory which will be hard pressed to get >60GB/s. A last-gen Epyc/Threadripper Pro should have 8 channel memory DDR4-3200 support, which should get you a theoretical max of 204.8 GB/s (benchmarking ends up more around 150GB/s in AIDA64).

The latest Genoa has 12 channel DDR5-4800 support (and boosted AVX-512) and I'd imagine should perform quite well, but if you primarily want to run inference on a quantized 65B model, I think you're best bang/buck (for local hardware) would be 2 x RTX 3090s (each of those has 24GB of GDDR6X w/ just shy of 1TB/s of memory bandwidth).

Yeah, it's really so bad on desktops.

With my LLaMA AVX implementation on 32bit floats [0] there no performance gain after 2 threads, so remaining 14 threads available are of no use, there no memory bandwidth to load them with work :)

[0] https://github.com/gotzmann/llama.go

To the extent that you're memory bandwidth limited you should be able to do multiple inferences at once --- latency stays high but getting multiple samplings can be extremely useful for many uses and can cover up somewhat for high latency.

To an extent, but memory bandwidth soon becomes a bottleneck there too. The hidden state and the KV cache are large so it becomes a matter of how fast you can move data in and out of your L2 cache. If you don’t have a unified memory pool it gets even worse.

Thank you, that makes sense. I had no idea that there was such a dramatic difference in memory bandwidth between desktop and server CPUs.

The two-channel DDR5 in desktops can't even do two channels very well -- if you try to put 64GB RAM in (two dual-rank 32GB DIMMs) then you lose around 50% of the bandwidth compared to a single rank DIMM (e.g. from 8GHz to 4GHz speeds, and increased latency).

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