Avoid quadratic path copying in prompt trie search - #1607
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Adopts six post-ml-explore#1500 third-party mlx-lm efficiency/correctness PRs, each verified on M5 NAX (see wiki research/efficiency-pr-adoption-2026-07-23.md): ml-explore#1607 de-quadratic PromptTrie.search ml-explore#1571 local_files_only-first _download() ml-explore#1579 fused Metal SSD prefill kernels (complements ml-explore#1586) ml-explore#1584 continuous-batching KV-cache quantization ml-explore#1593 drop import-level sampler mx.compile (server seed bug) ml-explore#1590 never lower RLIMIT_NOFILE on import ml-explore#1555 (Hadamard-rotated KV) was already in-tree — not re-adopted. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Looks like low effort vibe coding without actually understanding the algorithm.
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Summary
PromptTrieinstead of copying the growing path at every visited edgeMotivation
PromptTrie.search()looks for the shortest cached sequence extending a shared prompt prefix. Its DFS currently builds every child path withextra + [tok], so a long branch repeatedly copies its full prefix. This makes the server's prompt-cache lookup quadratic in the length of a long divergent suffix before generation begins.The new traversal keeps a single mutable path and uses stack markers to backtrack. It only copies the path when recording a better match; cache contents and selection semantics are unchanged.
Performance
Measured on an Apple M5 Max through the actual
LRUPromptCache.fetch_nearest_cache()path with four cached branches:The 8K result is the median of seven runs; the 32K result is the median of three runs.
Validation
git diff --checkpass