[menaerus]

Interesting approach.

Am I correct to understand that you're basically minimizing the latencies and required compute/mem-bw by avoiding the KV cache? And encoding the (local) context in the input tokens instead?

I ask this because you set the prompt/context size to 0 (--ctx-size 0) and the batch size to 1024 (-b 1024). Former would mean that llama.cpp will only be using the context that is already encoded in the model itself but no local (code) context besides the one provided in the input tokens but perhaps I misunderstood something.

Thanks for your contributions and obviously the large amount of time you take to document your work!

[ggerganov]

The primary tricks for reducing the latency are around context reuse, meaning that the computed KV cache of tokens from previous requests is reused for new requests and thus computation is saved.

To get high-quality completions, you need to provide a large context of your codebase so that the generated suggestion is more inline with your style and implementation logic. However, naively increasing the context will quickly hit a computation limit because each request would need to compute (a.k.a prefill) a lot of tokens.

The KV cache shifts used here is an approach to reuse the cache of old tokens by "shifting" them in new absolute positions in the new context. This way a request that would normally require a context of lets say 10k tokens, could be processed more quickly by computing just lets say 500 tokens and reusing the cache of the other 9.5k tokens, thus cutting the compute ~10 fold.

The --ctx-size 0 CLI arg simply tells the server to allocate memory buffers for the maximum context size supported by the model. For the case of Qwen Coder models, this corresponds to 32k tokens.

The batch sizes are related to how much local context around your cursor to use, along with the global context from the ring buffer. This is described in more detail in the links, but simply put: decreasing the batch size will make the completion faster, but with less quality.

[menaerus]

Ok, so --ctx-size with a value != 0 means that we can override the default model context size. Since for obvious computation cost reasons we cannot use the 32k fresh context per each request, the trick you make is to use the 1k context (batch that includes local and semi-local code) that you enrich with the previous model responses by keeping them in and feeding them from KV cache? To increase the correlation between the current request and previous responses you do the shifting in KV cache?

[ggerganov]

Yes, exactly. You can set --ctx-size to a smaller value if you know that you will not hit the limit of 32k - this will save you VRAM.

To control how much global context to keep in the ring buffer (i.e. the context that is being reused to enrich the local context), you can adjust the "ring_n_chunks" and "rink_chunk_size". With the default settings, this amounts to about 8k tokens of context on our codebases when the ring buffer is full, which is a conservative setting. Increasing these numbers will make the context bigger, will improve the quality but will affect the performance.

There are a few other tricks to reduce the compute for the local context (i.e. the 1k batch of tokens), so that in practice, a smaller amount is processed. This further saves compute during the prefill.

[menaerus]

Since qwen 2.5 turbo with 1M context size is advertised to be able to crunch ~30k LoC, I guess we can say then that the 32k qwen 2.5 model is capable of ~960 LoC and therefore 32k model with an upper bound of context set to 8k is capable of ~250 LoC?

Not bad.