server : Smart selection of available slot using Longest Common Prefix

[sasha0552]

In the current implementation, an available slot is selected using LRU (Least Recently Used). This PR adds slot selection by LCS (Longest Common Substring) LCP (Longest Common Prefix) algorithm to select a slot with a prompt that has at least n% similarity to the requested prompt. This reduces prompt processing in multi-user scenarios.

Additionally, this PR:

1. Defers processing if a slot is unavailable and the user has explicitly requested a specific slot.
2. Fixes erase/save/load slot endpoints. In the current implementation, these endpoints use a slot by LRU if the requested slot is not available, which is not correct behavior.

[github-actions]

📈 llama.cpp server for bench-server-baseline on Standard_NC4as_T4_v3 for phi-2-q4_0: 522 iterations 🚀

Expand details for performance related PR only

• Concurrent users: 8, duration: 10m
• HTTP request : avg=8935.67ms p(95)=22613.33ms fails=, finish reason: stop=462 truncated=60
• Prompt processing (pp): avg=98.01tk/s p(95)=403.94tk/s
• Token generation (tg): avg=36.92tk/s p(95)=45.07tk/s
• ggml-org/models/phi-2/ggml-model-q4_0.gguf parallel=8 ctx-size=16384 ngl=33 batch-size=2048 ubatch-size=256 pp=1024 pp+tg=2048 branch=use-slot-by-lcs commit=a8842fdf56dc725b69c19332d46dc8bbf612069e

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[ggerganov]

Need to rebase to latest master and we can merge

[sasha0552]

I'll test again and then mark the PR as ready for review.

[sasha0552]

By the way, should this be on by default? Or is it better to leave it off as it is now?

[ggerganov]

This PR adds slot selection by LCS (Longest Common Substring) algorithm to select a slot with a prompt that has at least n% similarity to the requested prompt. This reduces prompt processing in multi-user scenarios.

The LCS algorithm is an overkill for this purpose. All you need to look for is the longest common prefix, which is much simpler to compute

[sasha0552]

As far as I know, the server can reuse not only the prompt prefix, but also the suffix (llama_kv_cache_seq_rm & llama_kv_cache_seq_add sequence, also known as context shifting).

https://github.com/ggerganov/llama.cpp/blob/2b3389677a833cee0880226533a1768b1a9508d2/examples/server/server.cpp#L1819-L1857

[ggerganov]

Although the llama interface allows various operations with the KV cache, such as shifting the tokens' position and removing tokens, the server only looks for a common prefix when the prompt cache is enabled:

https://github.com/ggerganov/llama.cpp/blob/d5c938cd7716b9a2ace49a43a469dfbffcff4d28/examples/server/server.cpp#L2022-L2025

So for now the slot selection logic is better to follow the prompt caching logic and look just at the prefix

[samikrc]

Would this be adding new parameters to the server command line in order to enable this feature?