Comment by bontaq
18 hours ago
How's that O(N^2)? How's it O(N) with caching? Does a 3 turn conversation cost 3 times as much with no caching, or 9 times as much?
18 hours ago
How's that O(N^2)? How's it O(N) with caching? Does a 3 turn conversation cost 3 times as much with no caching, or 9 times as much?
I’m not sure that it’s O(N) with caching but this illustrates the N^2 part:
https://blog.exe.dev/expensively-quadratic
If there was an exponential cost, I would expect to see some sort of pricing based on that. I would also expect to see it taking exponentially longer to process a prompt. I don't believe LLMs work like that. The "scary quadratic" referenced in what you linked seems to be pointing out that cache reads increase as your conversation continues?
If I'm running a database keeping track of a conversation, and each time it writes the entire history of the conversation instead of appending a message, are we calling that O(N^2) now?
> I would also expect to see it taking exponentially longer to process a prompt. I don't believe LLMs work like that.
Try this out using a local LLM. You'll see that as the conversation grows, your prompts take longer to execute. It's not exponential but it's significant. This is in fact how all autoregressive LLMs work.
Yes, that is indeed O(N^2). Which, by the way, is not exponential.
Also by the way, caching does not make LLM inference linear. It's still quadratic, but the constant in front of the quadratic term becomes a lot smaller.
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What we would call O(n^2) in your rewriting message history would be the case where you have an empty database and you need to populate it with a certain message history. The individual operations would take 1, 2, 3, .. n steps, so (1/2)*n^2 in total, so O(n^2).
This is the operation that is basically done for each message in an LLM chat in the logical level: the complete context/history is sent in to be processed. If you wish to process only the additions, you must preserve the processed state on server-side (in KV cache). KV caches can be very large, e.g. tens of gigabytes.