I Got 9.9 Lower TTFT on a Real Android Phone by Reusing llama.cpp KV State
Local LLM inference has an expensive habit: It recomputes prefixes it has already seen. A system prompt. A reused RAG document. A few-shot block. A long static context. If the prefix is identical, why pay the prefill cost again? That's the problem I explored with EdgeSync-LLM. The idea The mechanism is simple: Prompt = shared prefix + new suffix On the first request, EdgeSync prefills the prefix and captures its KV cache state. On the next request sharing that exact prefix, it restores the state and decodes only the new suffix. No llama.cpp fork. No patch. The current validated path uses the public: llama_state_seq_get_data and llama_state_seq_set_data APIs. Measured on a real Android ARM64 phone Model: Qwen2.5-0.5B-Instruct Q4_K_M Shared prefix: 123 tokens 40 requests. 4 threads. Release build. Path Mean TTFT p50 p95 Cold 4828 ms 4752 ms 5297 ms KV state reuse 486 ms 476 ms 569 ms 9.9× lower TTFT on cache hits. The warm path was approximately: 363 ms to decode the 10-token suffix 123 ms to restore the state blob Fragment size: 1.64 MB I also measured the same mechanism on x86-64. Cold mean TTFT: 1395 ms Warm mean TTFT: 185 ms That's 7.5× on cache hits. But I almost published a fake 8.8× speedup This was the most important part of the project. My first implementation directly copied raw K/V tensors. It was fast. Very fast. The benchmark reported an 8.8× speedup. There was one problem. It was wrong. llama.cpp tracks more than the K/V tensor values. Cache cells also have position and sequence metadata used to construct the attention mask. Copying tensor values without restoring that bookkeeping produced an inert fragment. The model skipped prefix computation... ...but attention could not actually see the restored prefix. 14 of 24 cache hits reproduced, token for token, the output of a generation with no prefix at all. The “speedup” was dropped context. So I discarded it. Timing is not enough A broken cache can be fast. That's why EdgeSync now runs two correctness chec